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1
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&args DEMgrid
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2
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3
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/* Calculates a measure of the noisiness of the surface.
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4
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/* John Gallant 8 Jan 2009
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5
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/*
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6
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/* Modified from noisemag on 15 Oct 2010 to work at 3 second resolution
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7
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/*
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8
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/* The principle is that noise is characterised by short-range variation.
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9
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/* The difference from the local mean will vary erratically over a short
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10
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/* distance in the presence of noise. Terrain features are generally smoother
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11
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/* so do not vary as erratically. The noise is measured as the local standard
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12
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/* deviation of the difference from the mean.
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13
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/*
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14
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/* The local mean is calculated using an annulus so that local peaks and
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15
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/* holes are further accentuated.
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16
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/*
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17
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/* The raw noise magnitude is smoothed using two steps of median filtering
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18
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/* to give the median value over a 25-cell circle.
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19
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/*
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20
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/* The calculated values can be used as the standard deviation of the
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21
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/* noise of the surface.
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22
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/*
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23
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/* One flaw is that higher relief terrain is considered "noisy" by this method.
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24
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/* (Lower relief terrain is not.)
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25
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/*
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26
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/* Could remove the effect of steeper terrain by using a fitted polynomial
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27
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/* rather than a simple mean as the basis for calculating the difference.
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28
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/* Not pursued at this time because the application is to control where
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29
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/* vegetation edges are visible in SRTM data, and high relief areas also
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30
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/* obscure the veg edges, so including the high relief as "high noise"
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31
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/* is not a problem.
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32
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33
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setwindow %DEMgrid%
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34
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setcell %DEMgrid%
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35
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36
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noisemean = focalmean(%DEMgrid% , annulus, 1, 2)
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37
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noisediffmn = %DEMgrid% - noisemean
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38
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noiseraw = focalstd(noisediffmn, circle, 2)
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39
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/* setcell minof
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40
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/* noisemag2 = focalmedian(aggregate(noiseraw, 2, median), circle, 2)
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41
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/* setcell %DEMgrid%
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42
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/* noisemag = resample(noisemag2, #, bilinear)
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43
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44
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/* testing rejection of terrain signal by capturing longer-range differences from mean
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45
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/* noisemag_d3 looks particularly good - terrain effects almost completely absent, for both small and extended features
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46
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47
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noisemndf = focalmean(noisediffmn, circle, 2) /* calculate mean difference from mean
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48
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noisemndf_abs = sqrt(1 + sqr(noisemndf)) - 1 /* soft absolute value of that
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49
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noisemndf_mdn = focalmedian(noisemndf_abs, circle, 3) /* smooth, this is the terrain signal
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50
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noiseraw_rdc = noiseraw / (1 + 2 * mndiffmn2bfmd) /* reduce noiseraw by the terrain signal
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51
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52
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setcell minof
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53
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noisemag_2_2 = focalmedian(aggregate(noiseraw_rdc, 2, median), circle, 2)
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54
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setcell %DEMgrid%
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55
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noisemag = resample(noisemag_2_2, #, bilinear)
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