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@@ -40,7 +40,7 @@ For example, the two expressions below are equivalent inputs to <em>r.watershed<
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<dd>Without this flag set, the entire analysis is run in memory
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maintained by the operating system. This can be limiting, but is
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relatively fast. Setting the flag causes the program to manage memory
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-on disk which allows larger maps to be processes but is considerably
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+on disk which allows larger maps to be processed but is considerably
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slower.
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<dt><em>memory</em>
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@@ -51,6 +51,7 @@ speeds up the processes.
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<dt><em>-s</em>
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<dd>Use single flow direction (SFD) instead of multiple flow direction (MFD).
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+MFD is enabled by default.
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<dt><em>convergence</em>
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@@ -192,19 +193,19 @@ by 100 for the GRASS output map layer.
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<h4>A<sup>T</sup> least-cost search algorithm</h4>
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<em>r.watershed</em> uses an A<sup>T</sup> least-cost search algorithm
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-(see <a href="#references">REFERENCES</a> section) designed to minimize the
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-impact of DEM data errors. This algorithm works slower than
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-<em>r.terraflow</em> but provides more accurate results in
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-areas of low slope as well as DEMs constructed with techniques that
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-mistake canopy tops as the ground elevation. Kinner et al. (2005), for
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-example, used SRTM and IFSAR DEMs to compare <em>r.watershed</em>
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-against <em>r.terraflow</em> results in Panama. <em>r.terraflow</em> was
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-unable to replicate stream locations in the larger valleys while
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-<em>r.watershed</em> performed much better. Thus, if forest canopy exists
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-in valleys, SRTM, IFSAR, and similar data products will cause major
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-errors in <em>r.terraflow</em> stream output. Under similar conditions,
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-<em>r.watershed</em> will generate better <b>stream</b> and <b>half_basin</b>
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-results. If watershed divides contain flat to low slope, <em>r.watershed</em>
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+(see <a href="#references">REFERENCES</a> section) designed to minimize
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+the impact of DEM data errors. Compared to <em>r.terraflow</em>, this
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+algorithm provides more accurate results in areas of low slope as well
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+as DEMs constructed with techniques that mistake canopy tops as the
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+ground elevation. Kinner et al. (2005), for example, used SRTM and IFSAR
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+DEMs to compare <em>r.watershed</em> against <em>r.terraflow</em>
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+results in Panama. <em>r.terraflow</em> was unable to replicate stream
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+locations in the larger valleys while <em>r.watershed</em> performed
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+much better. Thus, if forest canopy exists in valleys, SRTM, IFSAR, and
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+similar data products will cause major errors in <em>r.terraflow</em>
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+stream output. Under similar conditions, <em>r.watershed</em> will
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+generate better <b>stream</b> and <b>half_basin</b> results. If
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+watershed divides contain flat to low slope, <em>r.watershed</em>
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will generate better basin results than <em>r.terraflow</em>.
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(<em>r.terraflow</em> uses the same type of algorithm as ESRI's ArcGIS
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watershed software which fails under these conditions.) Also, if watershed
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@@ -216,11 +217,13 @@ The algorithm produces results similar to those obtained when running
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<em><a href="r.drain.html">r.drain</a></em> on every cell on the map.
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<h4>Multiple flow direction (MFD)</h4>
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-<p><em>r.watershed</em> has experimental support for multiple flow
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-direction (MFD). Water flow is distributed to all neighbouring cells with
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-lower elevation, using slope towards neighbouring cells as a weighing
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-factor for proportional distribution. The A<sup>T</sup> least-cost path
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-is always included and assigned the maxmimum observed weighing factor.
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+
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+<em>r.watershed</em> offers two methods to calculate surface flow:
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+single flow direction (SFD, D8) and multiple flow direction (MFD). With
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+MFD, water flow is distributed to all neighbouring cells with lower
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+elevation, using slope towards neighbouring cells as a weighing factor
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+for proportional distribution. The A<sup>T</sup> least-cost path is
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+always included and assigned the maxmimum observed weighing factor.
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As a result, depressions and obstacles are overflown with a gracefull
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flow convergence before the overflow. The convergence factor causes flow
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accumulation to converge more strongly with higher values. The supported
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@@ -230,7 +233,7 @@ to a higher value can reduce the amount of small sliver basins.
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<br>See example below with the North Carolina dataset for using MFD mode.
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<h4>In-memory mode and disk swap mode</h4>
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-<p>There are two versions of this program: <em>ram</em> and <em>seg</em>.
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+There are two versions of this program: <em>ram</em> and <em>seg</em>.
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<em>ram</em> is used by default, <em>seg</em> can be used by setting
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the <em>-m</em> flag.
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<br>
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@@ -244,12 +247,14 @@ current geographic region is huge.
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Due to memory requirements of both programs, it is quite easy to run out of
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memory when working with huge map regions. If the <em>ram</em> version runs
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out of memory and the resolution size of the current geographic region
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-cannot be increased, either more memory needs to be added to the computer,
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+cannot be increased, either more memory needs to be added to the computer,
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or the swap space size needs to be increased. If <em>seg</em> runs out of
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memory, additional disk space needs to be freed up for the program to run.
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+The <em>r.terraflow</em> module was specifically designed with huge
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+regions in mind and may be useful here as an alternative.
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<h4>Large regions with many cells</h4>
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-<p>In some situations, the region size (number of cells) may be too large for
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+In some situations, the region size (number of cells) may be too large for
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the amount of time or memory available. Running <em>r.watershed</em> may
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then require use of a coarser resolution. To make the results more closely
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resemble the finer terrain data, create a map layer containing the
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@@ -266,7 +271,7 @@ using the values from the <em>neighborhood</em> output map layer that
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represents the minimum elevation within the region of the coarser cell.
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<h4>High-resolution elevation maps with floating point values</h4>
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-<p>To get better results with high resolution elevation maps with
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+To get better results with high resolution elevation maps with
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floating point values, it may be necessary to multiply the original map
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with e.g. 100 to change elevation units from meters to cm, because
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<em>r.watershed</em> reads input elevation maps as integer. This allows
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@@ -277,7 +282,7 @@ output accordingly. See example below for using <em>r.mapcalc</em> to
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convert elevation from meter to millimeter.
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<h4>Basin threshold</h4>
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-<p>The minimum size of drainage basins, defined by the <em>threshold</em>
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+The minimum size of drainage basins, defined by the <em>threshold</em>
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parameter, is only relevant for those watersheds with a single stream
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having at least the <em>threshold</em> of cells flowing into it.
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(These watersheds are called exterior basins.)
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@@ -413,7 +418,8 @@ Display output in a nice way
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<br>
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<i>This example uses the North Carolina sample dataset.</i>
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-<p>Using MFD mode on a LIDAR elevation dataset:
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+<p>
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+Using MFD mode on a LIDAR elevation dataset:
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<br>Convert elevation values of elev_lid972_1m from meter to millimeter
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then compare MFD mode with default medium flow convergence to SFD mode.
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<div class="code"><pre>
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Hamish Bowman