GTOPO30 Documentation

Table of Contents:

 1.0  Introduction

 2.0  Data Set Characteristics

 3.0  Data Format

    3.1  DEM File

    3.2  Header File

    3.3  World File

    3.4  Statistics File

    3.5  Projection File

    3.6  Shaded Relief Image

    3.7  Source Map

    3.8  Source Map Header File

 4.0  Data Distribution

    4.1  Procedures for Obtaining Data

    4.2  File Sizes

 5.0  Notes and Hints for GTOPO30 Users

 6.0  Data Set Development

    6.1  Data Sources

       6.1.1  Digital Terrain Elevation Data

       6.1.2  Digital Chart of the World

       6.1.3  USGS Digital Elevation Models

       6.1.4  Army Map Service Maps

       6.1.5  International Map of the World

       6.1.6  Peru Map

       6.1.7  New Zealand DEM

       6.1.8  Antarctic Digital Database

    6.2  Data Processing

       6.2.1  Raster Source Processing

       6.2.2  Vector Source Processing

       6.2.3  DEM Merging

       6.2.4  Global Product Assembly

 7.0  Accuracy

 8.0  GTOPO30 Caveats

    8.1  Grid Spacing and Resolution

    8.2  Topographic Detail and Accuracy

    8.3  Production Artifacts

 9.0  Summary

10.0  References

11.0  Disclaimers

1.0  Introduction

GTOPO30 is a global digital elevation model (DEM) resulting from a

collaborative effort led by the staff at the U.S. Geological Survey's

EROS Data Center in Sioux Falls, South Dakota.  Elevations in GTOPO30 are

regularly spaced at 30-arc seconds (approximately 1 kilometer).  GTOPO30

was developed to meet the needs of the geospatial data user community for

regional and continental scale topographic data.  This release represents

the completion of global coverage of 30-arc second elevation data that

have been available from the EROS Data Center beginning in 1993.   Several

areas have been updated and the entire global data set has been repackaged,

so these data supersede the previously released continental data sets.

Comments from users of GTOPO30 are welcomed and encouraged.  Please send

your comments to Dean Gesch at gesch@edcmail.cr.usgs.gov or to Sue Greenlee

at sgreenlee@edcmail.cr.usgs.gov.

2.0  Data Set Characteristics

GTOPO30 is a global data set covering the full extent of latitude from 90

degrees south to 90 degrees north, and the full extent of longitude from

180 degrees west to 180 degrees east.  The horizontal grid spacing is

30-arc seconds (0.008333333333333 degrees), resulting in a DEM having

dimensions of 21,600 rows and 43,200 columns.  The horizontal coordinate

system is decimal degrees of latitude and longitude referenced to WGS84.

The vertical units represent elevation in meters above mean sea level.

The elevation values range from -407 to 8,752 meters.  In the DEM, ocean

areas have been masked as "no data" and have been assigned a value of -9999.

Lowland coastal areas have an elevation of at least 1 meter, so in the

event that a user reassigns the ocean value from -9999 to 0 the land

boundary portrayal will be maintained.  Due to the nature of the raster

structure of the DEM, small islands in the ocean less than approximately

1 square kilometer will not be represented.

3.0  Data Format

To facilitate electronic distribution, GTOPO30 has been divided into 33

smaller pieces, or tiles.  The area from 60 degrees south latitude to

90 degrees north latitude and from 180 degrees west longitude to 180

degrees east longitude is covered by 27 tiles, with each tile covering 50

degrees of latitude and 40 degrees of longitude.  Antarctica (90 degrees

south latitude to 60 degrees south latitude and 180 degrees west longitude

to 180 degrees east longitude) is covered by 6 tiles, with each tile covering

30 degrees of latitude and 60 degrees of longitude.  The tiles names refer

to the longitude and latitude of the upper-left (northwest) corner of the

tile.  For example, the coordinates of the upper-left corner of tile

E020N40 are 20 degrees east longitude and 40 degrees north latitude.

There is one additional tile that covers all of Antarctica with data in

a polar stereographic projection.  The following table lists the name,

latitude and longitude extent, and elevation statistics for each tile.

             Latitude          Longitude                  Elevation

 Tile    Minimum  Maximum   Minimum  Maximum   Minimum  Maximum  Mean  Std.Dev.

-------  ----------------   ----------------   --------------------------------

W180N90     40       90       -180    -140         1      6098    448     482

W140N90     40       90       -140    -100         1      4635    730     596

W100N90     40       90       -100     -60         1      2416    333     280

W060N90     40       90        -60     -20         1      3940   1624     933

W020N90     40       90        -20      20       -30      4536    399     425

E020N90     40       90         20      60      -137      5483    213     312

E060N90     40       90         60     100      -152      7169    509     698

E100N90     40       90        100     140         1      3877    597     455

E140N90     40       90        140     180         1      4588    414     401

W180N40    -10       40       -180    -140         1      4148    827     862

W140N40    -10       40       -140    -100       -79      4328   1321     744

W100N40    -10       40       -100     -60         1      6710    375     610

W060N40    -10       40        -60     -20         1      2843    212     168

W020N40    -10       40        -20      20      -103      4059    445     298

E020N40    -10       40         20      60      -407      5825    727     561

E060N40    -10       40         60     100         1      8752   1804    1892

E100N40    -10       40        100     140       -40      7213    692     910

E140N40    -10       40        140     180         1      4628    549     715

W180S10    -60      -10       -180    -140         1      2732    188     297

W140S10    -60      -10       -140    -100         1       910     65     124

W100S10    -60      -10       -100     -60         1      6795   1076    1356

W060S10    -60      -10        -60     -20         1      2863    412     292

W020S10    -60      -10        -20      20         1      2590   1085     403

E020S10    -60      -10         20      60         1      3484    893     450

E060S10    -60      -10         60     100         1      2687    246     303

E100S10    -60      -10        100     140         1      1499    313     182

E140S10    -60      -10        140     180         1      3405    282     252

W180S60    -90      -60       -180    -120         1      4009   1616    1043

W120S60    -90      -60       -120     -60         1      4743   1616     774

W060S60    -90      -60        -60       0         1      2916   1866     732

W000S60    -90      -60          0      60         1      3839   2867     689

E060S60    -90      -60         60     120         1      4039   2951     781

E120S60    -90      -60        120     180         1      4363   2450     665

ANTARCPS   -90      -60       -180     180         1      4748   2198    1016

The 27 tiles that individually cover 50 degrees of latitude and 40 degrees

of longitude each have 6,000 rows and 4,800 columns.  The 6 Antarctica tiles

that individually cover 30 degrees of latitude and 60 degrees of longitude

each have 3,600 rows and 7,200 columns.  There is no overlap among the tiles

so the global data set may be assembled by simply abutting the adjacent

tiles.

The tile named ANTARCPS includes the same data as the 6 geographic tiles

covering Antarctica, but is presented in a polar stereographic projection.

The horizontal grid spacing is 1,000 meters, and the tile has 5,400 rows

and 5,400 columns.  The projection parameters used for the polar

stereographic projection are: 0 degrees for the longitude of the central

meridian, 71 degrees south for the latitude of true scale, and 0 for the

false easting and false northing.

Data for each tile are provided in a set of 8 files.  The files are named

with the tile name and a file name extension indicating the contents of the

file.  The following extensions are used:

Extension               Contents

---------               --------

   DEM        digital elevation model data

   HDR        header file for DEM

   DMW        world file

   STX        statistics file

   PRJ        projection information file

   GIF        shaded relief image

   SRC        source map

   SCH        header file for source map

The simple format should allow for easy ingest into most popular image

processing and geographic information systems packages.  Further information

on the contents of the files is provided below.

3.1  DEM File (.DEM)

The DEM is provided as 16-bit signed integer data in a simple binary raster.

There are no header or trailer bytes imbedded in the image.  The data are

stored in row major order (all the data for row 1, followed by all the data

for row 2, etc.).

3.2  Header File (.HDR)

The DEM header file is an ASCII text file containing size and coordinate

information for the DEM.  The following keywords are used in the header file:

BYTEORDER      byte order in which image pixel values are stored 

                  M = Motorola byte order (most significant byte first)

LAYOUT         organization of the bands in the file

                  BIL = band interleaved by line (note: the DEM is a single

                  band image)

NROWS          number of rows in the image

NCOLS          number of columns in the image

NBANDS         number of spectral bands in the image (1 for a DEM)

NBITS          number of bits per pixel (16 for a DEM)

BANDROWBYTES   number of bytes per band per row (twice the number of columns

                  for a 16-bit DEM)

TOTALROWBYTES  total number of bytes of data per row (twice the number of

                  columns for a single band 16-bit DEM)

BANDGAPBYTES   the number of bytes between bands in a BSQ format image

                  (0 for a DEM)

NODATA         value used for masking purposes

ULXMAP         longitude of the center of the upper-left pixel (decimal degrees)

ULYMAP         latitude  of the center of the upper-left pixel (decimal degrees)

XDIM           x dimension of a pixel in geographic units (decimal degrees)

YDIM           y dimension of a pixel in geographic units (decimal degrees)

Example header file (W100N40.HDR):

BYTEORDER      M

LAYOUT       BIL

NROWS         6000

NCOLS         4800

NBANDS        1

NBITS         16

BANDROWBYTES         9600

TOTALROWBYTES        9600

BANDGAPBYTES         0

NODATA        -9999

ULXMAP        -99.99583333333334

ULYMAP        39.99583333333333

XDIM          0.00833333333333

YDIM          0.00833333333333

3.3  World File (.DMW)

The world file is an ASCII text file containing coordinate information.  It

is used by some packages for georeferencing of image data.  The following is

an example world file (W100N40.DMW) with a description of each record:

     0.00833333333333     x dimension of a pixel (decimal degrees)

     0.00000000000000     rotation term (will always be zero)

     0.00000000000000     rotation term (will always be zero)

    -0.00833333333333     negative y dimension of a pixel (decimal degrees)

   -99.99583333333334     longitude of the center of the upper-left pixel

    39.99583333333333     latitude of the center of the upper-left pixel

3.4  Statistics File (.STX)

The statistics file is an ASCII text file which lists the band number,

minimum value, maximum value, mean value, and standard deviation of the

values in the DEM data file.

Example statistics file (W100N40.STX):

1 -9999 6710 -6078.8 5044.2

3.5  Projection File (.PRJ)

The projection information file is an ASCII text file which describes the

projection of the DEM and source map image.

Example projection file (W100N40.PRJ):

Projection    GEOGRAPHIC

Datum         WGS84

Zunits        METERS

Units         DD

Spheroid      WGS84

Xshift        0.0000000000

Yshift        0.0000000000

Parameters

3.6  Shaded Relief Image (.GIF)

A shaded relief image is provided as an overview of the data in each tile.

The images were derived from a generalized version of GTOPO30 with a

horizontal grid spacing of 240-arc seconds (approximately 8 kilometers), so

many small islands and features will not be visible.  The images are meant

to provide a convenient way for users to view the general topographic

features portrayed in each tile.  The shaded relief images are provided as

GIF images which can be displayed by many popular image display programs and

World Wide Web browsers.  An image size of 750 rows by 600 columns is used

for the tiles covering 50 degrees of latitude by 40 degrees of longitude. 

An image size of 450 rows by 900 columns is used for the Antarctica tiles

which cover 30 degrees of latitude by 60 degrees of longitude each.  The

Antarctica polar stereographic tile is portrayed by a shaded relief image

having 675 rows by 675 columns.

3.7  Source Map (.SRC)

The source map is a simple 8-bit binary image which has values that indicate

the source used to derive the elevation for every cell in the DEM.  The

source map is the same resolution and has the same dimensions and coordinate

system as the DEM.  Like the DEM, it has no header or trailer bytes and is

stored in row major order.  These codes are used in the source map image:

Value               Source

-----               ------

  0       Ocean

  1       Digital Terrain Elevation Data

  2       Digital Chart of the World

  3       USGS 1-degree DEM's

  4       Army Map Service 1:1,000,000-scale maps

  5       International Map of the World 1:1,000,000-scale maps

  6       Peru 1:1,000,000-scale map

  7       New Zealand DEM

  8       Antarctic Digital Database

More information on each of these sources is provided in section 6.1

(Data Sources).  The cells with value 0 (ocean) in the source map can

be used as an ocean mask (the ocean cells match exactly all the cells

masked as "no data" in the DEM with a value of -9999).  Likewise, the cells

with values 1-8 together constitute a global land mask.  Every cell in the

DEM with an elevation has a corresponding cell in the source map with a

value in the range 1-8.

3.8  Source Map Header File (.SCH)

The source map header file is an ASCII text file containing size and

coordinate information, similar to the DEM header file.  The following

keywords are used in the source map header file:

BYTEORDER      byte order in which image pixel values are stored 

                  M = Motorola byte order (most significant byte first)

LAYOUT         organization of the bands in the file

                  BIL = band interleaved by line (note: the source map is

                  a single band image)

NROWS          number of rows in the image

NCOLS          number of columns in the image

NBANDS         number of spectral bands in the image (1 for the source map)

NBITS          number of bits per pixel (8 for the source map)

BANDROWBYTES   number of bytes per band per row (the number of columns for

                  an 8-bit source map)

TOTALROWBYTES  total number of bytes of data per row (the number of columns

                  for a single band 8-bit source map)

BANDGAPBYTES   the number of bytes between bands in a BSQ format image

                  (0 for the source map)

NODATA         value used for masking purposes

ULXMAP         longitude of the center of the upper-left pixel (decimal degrees)

ULYMAP         latitude  of the center of the upper-left pixel (decimal degrees)

XDIM           x dimension of a pixel in geographic units (decimal degrees)

YDIM           y dimension of a pixel in geographic units (decimal degrees)

Example source map header file (W100N40.SCH):

BYTEORDER      M

LAYOUT       BIL

NROWS         6000

NCOLS         4800

NBANDS        1

NBITS         8

BANDROWBYTES         4800

TOTALROWBYTES        4800

BANDGAPBYTES         0

NODATA        -9999

ULXMAP        -99.99583333333334

ULYMAP        39.99583333333333

XDIM          0.00833333333333

YDIM          0.00833333333333

4.0  Data Distribution

Data for each GTOPO30 tile are distributed electronically as a compressed tar

file.  The 8 files for each tile have been combined into one file with the

Unix "tar" command, and the tar file has been compressed with GNU "gzip"

utility.  To use the GTOPO30 data files, the tar file must first be

decompressed and then the individual data files extracted from the tar file.

For example, the following Unix command could be used:

   gunzip < w100n40.tar.gz | tar xvf -

If you do not have access to gzip, you can leave off the .gz extension and

the FTP server will decompress the tar file as it is downloaded.  However,

you will still have to run the tar command to extract separate files. 

Please note that a decompressed file is typically many times larger than

the compressed version and therefore will take much longer to transmit.  If

you would like to obtain the gzip or tar programs they are available via

anonymous FTP from the following sites:

 Unix gzip:  

        ftp://prep.ai.mit.edu/pub/gnu 

        ftp://wuarchive.wustl.edu/systems/gnu

 Macintosh gzip and tar:   

        ftp://mirrors.aol.com/pub/mac/util/compression

            macgzip0.3b2.sit.hqx

            suntar2.03.cpt.hqx

 DOS gzip and tar: 

        ftp://prep.ai.mit.edu/pub/gnu

            gzip-1.2.4.tar 

        ftp://ftp.uu.net/systems/ibmpc/msdos/pcroute

            tar.exe

4.1  Procedures for Obtaining Data

GTOPO30 is available electronically through an Internet anonymous File

Transfer Protocol (FTP) account at the EROS Data Center (at no cost). 

To access this account:

   1. FTP to edcftp.cr.usgs.gov

   2. Enter "anonymous" at the Name prompt.

   3. Enter your email address at the Password prompt.

   4. Change ("cd") to the  "/pub/data/gtopo30/global" subdirectory.

   5. Files are named according to the longitude and latitude coordinates

      of the upper-left corner of the tile, followed by the extension

      ".tar.gz".

   6. Enter "binary" to set the transfer type.

   7. Use "get" or "mget" to retrieve the desired files.

   8. For the Antarctica polar stereographic data, change ("cd") to the

      "/pub/data/gtopo30/antarctica" subdirectory and use "get" to

      retrieve the file antarcps.tar.gz.

Click here to place an order for the data on CD-ROM or 8 mm high density

tar tape.

For assistance and information contact:

   EDC DAAC User Services

   EROS Data Center 

   Sioux Falls, SD 57198 USA

   Tel: 605-594-6116 (7:30 am to 4:00 pm CT)  

   Fax: 605-594-6963 (24 hours) 

   Internet: edc@eos.nasa.gov (24 hours)

Data distributed on CD-ROM and 8 mm tape are provided as sets of files for

each tile as described above in section 3.0 (Data Format).  They are not

combined into one tar file or compressed as they are for electronic

distribution.

4.2  File Sizes

After decompression and extraction from the tar files, the following file

sizes are present for each of the 3 sizes of tiles:

    Tile size                File         Size (bytes)

    ---------                ----         ------------

50 degrees latitude           DEM           57600000

         by

40 degrees longitude       Source map       28800000

30 degrees latitude           DEM           51840000

         by

60 degrees longitude       Source map       25920000

Antarctica polar              DEM           58320000

stereographic data

(5,400 km x 5,400 km)      Source map       29160000

For each tile, the total for all the other file types (HDR, DMW, STX, PRJ,

GIF, and SCH) is well under 1 megabyte.

The global 16-bit DEM (21,600 rows by 43,200 columns) has a size of 1.74

gigabytes.  The global 8-bit source map of the same dimensions has a size of

889.9 megabytes.

Through the use of the gzip compression utility the total size of the global

data set is reduced about 90% from almost 2.72 gigabytes to under 290

megabytes.  The list below shows the compressed size for each tile.  The

sizes range from less than 1 megabyte to about 25 megabytes, with the average

at about 8 megabytes.  Decompressed, the tar file for each tile can be as

large as 84 megabytes.

      File        Size (bytes)

      ----        ------------

antarcps.tar.gz     10538463

 e020n40.tar.gz     26124072

 e020n90.tar.gz     16992230

 e020s10.tar.gz      8262946

 e060n40.tar.gz     17935016

 e060n90.tar.gz     22402428

 e060s10.tar.gz       113591

 e060s60.tar.gz      5308336

 e100n40.tar.gz     14175303

 e100n90.tar.gz     24994154

 e100s10.tar.gz      4361555

 e120s60.tar.gz      6131365

 e140n40.tar.gz      1140685

 e140n90.tar.gz      9222752

 e140s10.tar.gz      4059027

 w000s60.tar.gz      5080091

 w020n40.tar.gz     16938044

 w020n90.tar.gz      8844434

 w020s10.tar.gz      2927056

 w060n40.tar.gz      3721100

 w060n90.tar.gz      6820815

 w060s10.tar.gz      6738966

 w060s60.tar.gz      3558292

 w100n40.tar.gz     11330238

 w100n90.tar.gz     15656539

 w100s10.tar.gz      9575882

 w120s60.tar.gz      5677801

 w140n40.tar.gz      6497682

 w140n90.tar.gz     17031379

 w140s10.tar.gz        89706

 w180n40.tar.gz       131975

 w180n90.tar.gz      5477564

 w180s10.tar.gz       116231

 w180s60.tar.gz      3500153

5.0  Notes and Hints for GTOPO30 Users

Because the DEM data are stored in a 16-bit binary format, users must be

aware of how the bytes are addressed on their computers.  The DEM data are

provided in Motorola byte order, which stores the most significant byte

first ("big endian").  Systems such as Sun SPARC and Silicon Graphics

workstations use the Motorola byte order.  The Intel byte order, which

stores the least significant byte first ("little endian"), is used on DEC

Alpha systems and most PCs.  Users with systems that address bytes in the

Intel byte order may have to "swap bytes" of the DEM data unless their

application software performs the conversion during ingest.  The statistics

file (.STX) provided for each tile gives the range of values in the DEM

file, so users can check if they have the correct DEM values stored on

their system.

Users of ARC/INFO or ArcView can display the DEM data directly after simply

renaming the file extension from .DEM to .BIL.  However, if a user needs

access to the actual elevation values for analysis in ARC/INFO the DEM must

be converted to an ARC/INFO grid with the command IMAGEGRID.  IMAGEGRID does

not support conversion of signed image data, therefore the negative 16-bit

DEM values will not be interpreted correctly.  After running IMAGEGRID, an

easy fix can be accomplished using the following formula in Grid:

   out_grid = con(in_grid >= 32768, in_grid - 65536, in_grid)

The converted grid will then have the negative values properly represented,

and the statistics of the grid should match those listed in the .STX file.

If desired, the -9999 ocean mask values in the grid could then be set to

NODATA with the SETNULL function.

6.0  Data Set Development

GTOPO30, completed in late 1996, was developed over a 3 year period through

a collaborative effort led by staff at the U.S. Geological Survey's EROS Data

Center (EDC).  The following organizations participated by contributing

funding or source data: the National Aeronautics and Space Administration

(NASA), the United Nations Environment Programme/Global Resource Information

Database (UNEP/GRID), the U.S. Agency for International Development (USAID),

the Instituto Nacional de Estadistica Geografica e Informatica (INEGI) of

Mexico, the Geographical Survey Institute (GSI) of Japan, Manaaki Whenua

Landcare Research of New Zealand, and the Scientific Committee on Antarctic

Research (SCAR).

6.1  Data Sources

GTOPO30 is based on data derived from 8 sources of elevation information,

including vector and raster data sets.  The following table lists the

percentage of the global land surface area derived from each source (a full

description of each source is provided below):

                Source                                 % of global land area

                ------                                 ---------------------

Digital Terrain Elevation Data                                 50.0

Digital Chart of the World                                     29.9

USGS 1-degree DEM's                                             6.7

Army Map Service 1:1,000,000-scale maps                         1.1

International Map of the World 1:1,000,000-scale maps           3.7

Peru 1:1,000,000-scale map                                      0.1

New Zealand DEM                                                 0.2

Antarctic Digital Database                                      8.3

6.1.1  Digital Terrain Elevation Data

Digital Terrain Elevation Data (DTED) is a raster topographic data base with

a horizontal grid spacing of 3-arc seconds (approximately 90 meters) produced

by the National Imagery and Mapping Agency (NIMA) (formerly the Defense

Mapping Agency).  DTED was used as the source for most of Eurasia and large

parts of Africa, South America, Mexico, Canada, and Central America.  DTED

coverage for Mexico was provided by INEGI.

6.1.2  Digital Chart of the World

Digital Chart of the World (DCW) is a vector cartographic data set based on

the 1:1,000,000-scale Operational Navigation Chart (ONC) series, which is the

largest scale base map source with global coverage (Danko, 1992).  The DCW

and the ONC series are products of NIMA.

The topographic information of interest for generating DEM's is contained in

several DCW hypsography layers.  The primary contour interval on the source

ONC's is 1,000 feet (305 meters), and supplemental contours at an interval of

250 feet (76 meters) are shown in areas below 1,000 feet in elevation.  In

limited areas of higher elevation there supplemental contours at 500-foot

(152-meter) intervals.  The DCW drainage layers were also used as input to

the DEM generation process; this information included stream networks, lake

shorelines, lake elevations, and ocean coastlines.  The DCW was used as the

primary source for filling gaps in the DTED coverage, including all of

Australia, most of Greenland, and large areas of Africa, South America, and

Canada.

6.1.3  USGS Digital Elevation Models

USGS 1-degree DEM's with a horizontal grid spacing of 3-arc seconds

(approximately 90 meters) were used as the source data for the continental

United States, Alaska, and Hawaii.  The topographic information content is

similar to that of DTED.  The "1-degree" designation refers to the unit of

data distribution.

6.1.4  Army Map Service Maps

Paper maps at a scale of 1:1,000,000 produced by the Army Map Service (AMS),

a predecessor of DMA and NIMA, were acquired and digitized by GSI of Japan.

Contours (with intervals of 100, 150, 300, and 500 meters), spot heights,

drainage lines , and coastlines for some islands of southeast Asia and some

small areas in South America were delivered to EDC as digital vector

cartographic data sets.

6.1.5  International Map of the World

Paper maps from the 1:1,000,000-scale International Map of the World (IMW)

series were digitized by GSI to provide source data for the Amazon basin. 

The International Map of the World includes national maps produced to a

United Nations specified standard for 1:1,000,000-scale mapping.  The maps

used for this project had a 100-meter contour interval.

6.1.6  Peru Map

Small areas of a 1:1,000,000-scale map from the Peruvian government were

digitized to fill gaps in source data for South America.  The map had a

contour interval of 1,000 meters.

6.1.7  New Zealand DEM

Manaaki Whenua Landcare Research contributed a DEM with a 500-meter

horizontal grid spacing for New Zealand.  The DEM was derived from elevation

information on 1:63,360-scale maps with a 100-foot (30-meter) contour

interval.

6.1.8  Antarctic Digital Database

The Antarctic Digital Database (ADD) was produced under the auspices of the

Scientific Committee on Antarctic Research.  Digital contours and coastlines

from the ADD were used as source material for Antarctica.  The ADD vector

data were compiled from maps ranging in scale from 1:200,000 to 1:5,000,000.

The detail, density, and interval of the contours in the ADD vary widely,

with the more detailed data near the coastline and very generalized data in

the interior of the continent.  Detailed metadata provided in the ADD

identifies the map scale from which each contour line was extracted.

6.2  Data Processing

GTOPO30 was developed over a 3 year period during which continental and

regional areas were produced individually.  As such, processing techniques

were developed and refined throughout the duration of the project.  Although

the techniques used for the various continental areas are very similar,

there were some differences in approach due to varying source material. 

More details about data development for several of the continental areas are

reported by Verdin and Greenlee (1996), Bliss and Olsen (1996), and Gesch and

Larson (1996).

Data processing was accomplished using commercially available geographic

information system software, public domain image processing software,

vector-to-raster gridding software, and utilities developed specifically for

this project.  To more efficiently handle the numerous input data sets and to

standardize the proper sequence of processing steps, the production

procedures were automated to a great extent by employing preset parameter

values, scripted command files, and consistent naming schemes for input and

output data files.

6.2.1  Raster Source Processing

Processing of the raster source data, including DTED, USGS DEM's, and the New

Zealand DEM, involved generalizing the higher resolution data to the 30-arc

second horizontal grid spacing.  Because the DTED and USGS DEM's were already

in a geographic "projection" they only required a sampling of the full

resolution 3-arc second data.  One representative elevation value was selected

to represent the area covered by 100 full resolution cells (a 10 by 10

matrix).  As the project progressed, several methods of generalization were

used.  Selection of the representative 30-arc second value was accomplished

by systematic subsampling for North and South America, by calculation of the

median value for Eurasia, and by the breakline emphasis approach (Gesch and

Larson, 1996) for Africa.  The 500-meter New Zealand DEM was generalized to

30-arc seconds by reprojecting it from the New Zealand National Grid

projection to geographic coordinates using bilinear resampling.

6.2.2  Vector Source Processing

The topographic information from the vector cartographic sources, including

the DCW, the ADD, and the Army Map Service, International Map of the World,

and Peru 1:1,000,000-scale maps, was converted into elevation grids through

a vector-to-raster gridding approach.  Contours, spot heights, stream lines,

lake shorelines, and ocean coastlines were input to the ANUDEM surface

gridding program developed at the Australian National University (Hutchinson,

1989).  ANUDEM, specifically designed for creating DEM's from digital contour,

spot height, and stream line data, employs an approach known as drainage

enforcement to produce raster elevation models that represent more closely

the actual terrain surface and contain fewer artifacts than those produced

with more general purpose surface interpolation routines.  Drainage

enforcement was performed for all areas covered by vector source data except

Antarctica and Greenland.

A significant amount of preprocessing was required to prepare and format the

vector source data for input to ANUDEM.  This processing included editing and

updating the vector stream lines so that the direction of each was oriented

downstream (a requirement of ANUDEM).  Further preprocessing involved

detection and correction of erroneous contour and point elevations (Larson,

1996).  Ocean coastlines were assigned an elevation of zero for input as

contours.  Also, shorelines of lakes for which the DCW included elevations

were tagged and used as contour input.  The output from ANUDEM was an

elevation model grid referenced in the same horizontal coordinate system as

the generalized raster source data.  The output grid spacing of 30-arc

seconds has been shown to be appropriate for the information content present

in the DCW hypsography layers (Hutchinson, 1996; Shih and Chiu, 1996).

6.2.3  DEM Merging

Prior to merging with the generalized raster data, lakes for which the DCW

did not indicate an elevation were updated on the DCW grid with the lowest

grid cell elevation found along the shoreline.  When each of the vector

sources was gridded, an overlap area with the adjacent raster sources was

included so that smoothing could be performed to minimize the elevation

discrepancies among the sources.  Also, additional point control was input

into the ANUDEM gridding process so interpolated elevations in the overlap

region would more closely match the raster source elevations.  The additional

control was derived from the generalized raster sources within a 1-degree

buffer surrounding the vector source areas.

Merging of the generalized raster sources and the gridded vector sources was

accomplished by mosaicking the data sets.  The generalized raster sources

had the highest priority so coverage of the data with the greater

topographic detail and accuracy was maximized.  The grid derived from DCW

data had the highest priority among the vector sources, and the other

digitized map data was used when DCW hypsography was unavailable.  The

merging procedure including blending of the generalized raster sources and

the vector-derived grids within an approximate 1-degree overlap area along

the irregular boundaries.  The blending algorithm computes a weighted average

with the weights for each data source determined on a cell-by-cell basis

according to the cell's proximity to the edges of the overlap area (Franke,

1982).

A final processing step performed on the mosaicked and blended product

involved "clipping out" the land (as defined by vector coastline data) and

setting the ocean areas to a constant background value.  Use of vector

coastline data resulted in a more consistent portrayal of the land/ocean

interface, especially in areas where raster source data (which had an implied

coastline) met with vector source data.  The DCW coastline was used to clip

the following areas: Africa, Eurasia, South America, Australia, New Zealand,

Greenland, and isolated ocean islands.  The World Vector Shoreline (WVS), a

vector shoreline data set from NIMA, was used for North America, including

Hawaii, the Caribbean islands, and Central America.  The islands of Borneo

and Sulawesi in southeast Asia were clipped with the coastline digitized

from the 1:1,000,000-scale map source.  Antarctica was defined by the

coastline as portrayed in the ADD.

6.2.4  Global Product Assembly

The global product was assembled from the continental and regional DEM's. 

Several areas of overlap due to different production stages of the project

were addressed and eliminated, most notably between the Africa and Eurasia

data sets.  The global source map was generated from masks of source data

coverage, and was verified to register with the DEM precisely.  Finally, the

entire data set was packaged into tiles for easier electronic distribution.

7.0  Accuracy

The absolute vertical accuracy of GTOPO30 varies by location according to

the source data.  Generally, the areas derived from the raster source data

have higher accuracy than those derived from the vector source data.  The

full resolution 3-arc second DTED and USGS DEM's have a vertical accuracy

of + or - 30 meters linear error at the 90 percent confidence level (Defense

Mapping Agency, 1986; U.S. Geological Survey, 1993).  If the error

distribution is assumed to be Gaussian with a mean of zero, the statistical

standard deviation of the errors is equivalent to the root mean square error

(RMSE).  Under those assumptions, vertical accuracy expressed as + or - 30

meters linear error at 90 percent can also be described as a RMSE of 18

meters.  The areas of GTOPO30 derived from DTED and USGS DEM's retain that

same level of accuracy because through generalization a representative

elevation value derived from the full resolution cells is chosen to represent

the area of the reduced resolution cell (although the area on the ground

represented by that one elevation value is now much larger than the area

covered by one full resolution cell).

The absolute vertical accuracy of the DCW, the vector source with the largest

area of coverage, is stated in its product specification as + or - 650 meters

linear error at the 90% confidence level (Defense Mapping Agency, 1990). 

Experience has shown that the grids derived from DCW data should in many

areas be much more accurate than the 650-meter specification.  To better

characterize the accuracy of the areas of GTOPO30 derived from DCW vector

hypsography, the DCW grid was compared to 30-arc second DTED, which had been

aggregated by averaging.  By aggregating, the comparison could be done at the

30-arc second cell size of the DCW grid.  The comparison was done for

portions of southern Europe and the Mideast, and all of Africa.  Eliminated

from the comparison were those areas of the DCW grid for which supplemental

DTED point control had been included in the gridding process.  If the

averaged DTED are thought of as the reference data set, the RMSE of the DCW

grid is 95 meters.  To get an idea of the overall absolute accuracy of the

DCW grid, the relative error between the DCW and DTED can be combined with

the known error of the DTED itself in a sum of squares.  The root of that sum

of squares is 97 meters.  Using the assumptions about the error distribution

cited above, a RMSE of 97 meters can be expressed as + or - 160 meters linear

error at 90 percent confidence.  This number compares favorably with an

expected vertical accuracy (linear error at 90 percent) of one-half of the

primary contour interval of 1,000 feet (305 meters) for the topographic maps

on which the DCW is based.

The accuracy of the areas of GTOPO30 based on the other sources can only be

estimated based on that which is known about each source.  Using certain

assumptions, the vertical accuracy of each source (and the derived 30-arc

second grid) can be estimated from the contour interval.  One assumption is

that the original map sources meet the commonly used accuracy standard which

states that 90% of the map elevations are within + or - one-half of the

contour interval.  It is unknown if any of these maps actually meet this

standard.  Also, map digitizing and elevation surface interpolation errors

are unknown and therefore not included.  The table below lists the estimated

absolute vertical accuracy for the areas of GTOPO30 derived from each

source, with the method of estimating the accuracy also identified.  The

RMSE numbers were calculated using the assumptions about the error

distribution cited above (a Gaussian distribution with a mean of zero).

            Vertical accuracy (meters)

 Source        L.E. at 90%     RMSE                Estimation method

 ------     --------------------------             -----------------

DTED                30          18         product specification

DCW                160          97         calculated vs. DTED

USGS DEM            30          18         product specification

AMS maps           250         152         estimated from 500-meter interval

IMW maps            50          30         estimated from 100-meter interval

Peru map           500         304         estimated from 1,000-meter interval

N.Z. DEM            15           9         estimated from 100-foot interval

ADD               highly variable          wide range of scales and intervals

Local differences among DEM grid cells are often analyzed to calculate slope

and other land surface parameters.  The relative vertical accuracy (or

point-to-point accuracy on the surface of the elevation model), rather than

the absolute accuracy, determines the quality of such parameters derived from

local differencing operations.  Although not specified for this data set,

for many areas the relative accuracy is probably better than the estimated

absolute accuracy.

8.0  GTOPO30 Caveats

As with all digital geospatial data sets, users of GTOPO30 must be aware of

certain characteristics of the data set (resolution, accuracy, methods of

production and any resulting artifacts, etc.) in order to better judge its

suitability for a specific application.  A characteristic of GTOPO30 that

renders it unsuitable for one application may have no relevance as a

limiting factor for its use in a different application.  Because only the end

user can judge the applicability of the data set, it is the responsibility

of the data producer to describe the characteristics of the data as fully as

possible, so that an informed decision can be made by the user.

8.1  Grid Spacing and Resolution

For any application, the horizontal grid spacing (which limits the

resolution) and the vertical accuracy of GTOPO30 must be considered.  The

30-arc second grid spacing equates to about 1 kilometer, although that

number decreases in the east/west (longitudinal) direction as latitude

increases,  The table below lists the approximate distance covered by 30-arc

seconds at different latitudes.  Thus, at high latitudes there is an

unavoidable redundancy of data in order to keep the 30-arc second spacing

consistent for the global data set.  This is particularly true for the

geographic version of Antarctica where the ground distance for 30-arc seconds

of longitude converges to zero at the South Pole.

Latitude      Ground distance (meters)

(degrees)         E/W        N/S

---------     ------------------------

Equator           928        921

  10              914        922

  20              872        923

  30              804        924

  40              712        925

  50              598        927

  60              465        929

  70              318        930

  73              272        930

  78              193        930

  82              130        931

The variation in ground dimensions for one 30-arc second cell should be

especially considered for any application that measures area of or distance

across a group of cells.  Derivative products, such as slope maps, drainage

basin areas, and stream channel length, will be more reliable if they are

calculated from a DEM that has been first projected from geographic

coordinates to an equal area projection, so that each cell, regardless of

latitude, represents the same ground dimensions and area as every other cell.

Users should maintain the distinction between grid spacing and resolution. 

Even though the global data set has a consistent 30-arc second grid spacing,

not all topographic features that one would expect to be resolved at that

spacing will be represented.  The level of detail of the source data

determines whether the 30-arc second sampling interval is truly appropriate

for resolving the important topographic features represented in the source. 

Certainly, a 30-arc second grid spacing is appropriate for the areas derived

from higher resolution DEM's (DTED, USGS DEM's, and the New Zealand DEM), and

30-arc seconds has been shown to be suitable as the cell spacing for grids

derived from DCW hypsography (Hutchinson, 1996; Shih and Chiu, 1996). 

However, coverage of DCW contours is not complete, and there are areas for

which elevations were interpolated based only on very sparse DCW point data

and/or distant contours.  Small areas of this nature are located in Africa,

South America, and islands of southeast Asia, while Australia and Greenland

contain larger such areas.  Also, the quality of the contours from the ADD

for the interior of Antarctica does not realistically support a 30-arc second

(or even 1-kilometer) grid spacing, although such data are provided for

completeness and consistency of the global product.

8.2  Topographic Detail and Accuracy

Differences in topographic detail among the sources are evident in GTOPO30. 

This change in level of topographic information is especially evident at the

boundary between areas derived from DTED and DCW in regions of higher relief.

The mosaicking techniques that were used resulted in a smoothing of the

transition areas, but the change in detail between the two sources remains

very noticeable.  Even if the same topographic feature (ridge, stream valley,

lake, etc.) is represented in the data derived from the two sources, the

elevations across the feature may change somewhat abruptly due to the varying

accuracy of the sources.  Derived products, such as slope maps, for the

source transition areas also emphasize the differences in topographic

information derived from the varying sources.

Users are reminded that the accuracy levels described above are estimates,

and that the accuracy for specific locations within the overall area derived

from any one source can vary from the estimate.  For instance, approximately

30% of the DTED 1-degree by 1-degree tiles (the production and distribution

unit for full resolution DTED) have an absolute vertical accuracy worse than

the product specification of + or - 30 meters at 90% confidence.  Also, the

actual accuracy for some areas derived from the vector contour sources may

be better or worse than the estimate.  When the map source had multiple

contour intervals, the largest interval was used for a conservative estimate.

In contrast, some areas may be worse than the estimate because no contour

coverage was available for those specific locations.

8.3  Production Artifacts

Artifacts due to the production method are apparent in some areas of

GTOPO30.  While the magnitude of the artifacts in a local area are usually

well within the estimated accuracy for the source, users are nonetheless

made aware because the effects are plainly visible and they may affect some

applications of the DEM.  Some areas derived from DTED, especially in Africa

and the Mideast, exhibit a striping artifact, most likely due to the

production method of the DTED.  The artifact is very evident in the full

resolution data, but remains noticeable even in the generalized 30-arc

second version.  Generally, the pattern is more noticeable in low relief

areas, while in higher relief areas it is masked by the actual terrain

variation.  Another pattern seen in some areas derived from DTED is a blocky

appearance, which is reflection of the 1-degree tiling structure of the full

resolution DTED.  These areas derived from contiguous DTED 1-degree tiles

appear blocky because of vertical offsets among the tiles in the original

full resolution DTED.  The artifacts in the DTED areas may or may not be

visible, depending on the method used to display the data.  For instance,

when viewing the DEM data as an image either in shades of gray or color, the

artifacts may be hidden, depending on the number of shades or colors used. 

If the data are displayed as a shaded relief image the appearance of the

artifacts will vary depending on the direction of illumination, vertical

exaggeration applied, and the scale of the display.  Generally, none of the

artifacts will be visible on a small scale portrayal of the global data set.

Some production artifacts are also present in the areas derived from the

vector sources.  Small artificial mounds and depressions may be present in

localized areas, particularly where steep topography is adjacent to

relatively level areas, and the hypsography data were sparse.  Additionally,

a "stair step" (or terracing) effect may be seen in profiles of some areas,

where the transition between contour line elevations does not slope

constantly across the area but instead is covered by a flat area with

sharper changes in slope at the locations of the contour lines.  When a

histogram of elevations is presented there are sharp peaks at elevations

that are multiples of the contour interval of the source.  This effect is

common in DEM's produced by gridding of contour data in which the

interpolation process favors elevations at or near the contour values, thus

leading to a greater frequency of those elevations.  Every effort to reduce

these effects has been made by careful selection of parameters for the

interpolation process, but some level of these conditions inevitably remain

due to the nature of vector-to-raster surface generation.

9.0  Summary

GTOPO30 provides a new level of detail in global topographic data.

Previously, the best available global DEM was the ETOPO5 data set, and its

successor TerrainBase, with a horizontal grid spacing of 5-arc minutes

(approximately 10 kilometers) (Row, Hastings, and Dunbar, 1995).  GTOPO30

data are suitable for many regional and continental applications, such as

climate modeling, continental-scale land cover mapping, extraction of

drainage features for hydrologic modeling (Danielson, 1996; Verdin and

Greenlee, 1996), and geometric and atmospheric correction of medium and coarse

resolution satellite image data (Gesch, 1994; Jet Propulsion Laboratory,

1997).

10.0  References

Bliss, N.B., and Olsen, L.M., 1996. Development of a 30-arc-second digital

elevation model of South America. In: Pecora Thirteen, Human Interactions

with the Environment - Perspectives from Space, Sioux Falls, South Dakota,

August 20-22, 1996. 

Danielson, J.J., 1996. Delineation of drainage basins from 1 km African 

digital elevation data. In: Pecora Thirteen, Human Interactions with the

Environment - Perspectives from Space, Sioux Falls, South Dakota, August

20-22, 1996. 

Danko, D.M., 1992. The digital chart of the world. GeoInfo Systems, 2:29-36.

Defense Mapping Agency, 1986. Defense Mapping Agency product specifications

for digital terrain elevation data (DTED) (2d ed.). Defense Mapping Agency

Aerospace Center, St. Louis, Missouri, 26 p.

Defense Mapping Agency, 1990. Digitizing the future (3d ed.). Defense

Mapping Agency, Washington, D.C., 105 p.

Franke, R., 1982. Smooth interpolation of scattered data by local thin plate

splines. Computing & Mathematics with Applications, 8:273-281.

Gesch, D.B., 1994. Topographic data requirements for EOS global change

research. U.S. Geological Survey Open-File Report 94-626, 60 p.

Gesch, D.B., and Larson, K.S., 1996. Techniques for development of global

1-kilometer digital elevation models. In: Pecora Thirteen, Human Interactions

with the Environment - Perspectives from Space, Sioux Falls, South Dakota,

August 20-22, 1996. 

Hutchinson, M.F., 1989. A new procedure for gridding elevation and stream

line data with automatic removal of spurious pits. Journal of Hydrology,

106:211-232.

Hutchinson, M.F., 1996. A locally adaptive approach to the interpolation

of digital elevation models. In: Proceedings, Third International

Conference/Workshop on Integrating GIS and Environmental Modeling, Santa

Fe, New Mexico, January 21-26, 1996. National Center for Geographic

Information and Analysis, Santa Barbara, California.

Jet Propulsion Laboratory, 1997. DEM auxiliary datasets preparation plan:

digital elevation mapping support to the EOS/AM1 platform - JPL D13508,

Release 2. Jet Propulsion Laboratory, California Institute of Technology,

Pasadena, California, 65 p.

Larson, K.S., 1996. Error detection and correction of hypsography layers.

In: Proceedings, Sixteenth Annual ESRI User Conference, May 20-24, 1996.

Environmental Systems Research Institute, Inc., Redlands, California.

Row, L.W., Hastings, D.A., and Dunbar, P.K., 1995. TerrainBase Worldwide

Digital Terrain Data - Documentation Manual, CD-ROM Release 1.0. National

Geophysical Data Center, Boulder, Colorado.

Shih, T.Y., and Chiu, Y.C., 1996. On the quality of DCW hypsographic data,

a study for Taiwan. In: Technical Papers, ASPRS/ACSM Annual Convention and

Exhibition, April 22-25, 1996, Baltimore, Maryland. American Society for

Photogrammetry and Remote Sensing, Bethesda, Maryland, Volume III, p. 248-257.

U.S. Geological Survey, 1993. Digital elevation models, data user guide 5.

Reston, Virginia, 50 p.

Verdin, K.L., and Greenlee, S.K., 1996. Development of continental scale

digital elevation models and extraction of hydrographic features. In:

Proceedings, Third International Conference/Workshop on Integrating GIS and

Environmental Modeling, Santa Fe, New Mexico, January 21-26, 1996. National

Center for Geographic Information and Analysis, Santa Barbara, California.

11.0  Disclaimers

Any use of trade, product, or firm names is for descriptive purposes only

and does not imply endorsement by the U.S. Government.

Please note that some U.S. Geological Survey (USGS) information contained

in this data set and documentation may be preliminary in nature and

presented prior to final review and approval by the Director of the USGS.

This information is provided with the understanding that it is not guaranteed

to be correct or complete and conclusions drawn from such information are the

sole responsibility of the user.