--- raster/r.watershed/front/r.watershed.html 2008-12-12 11:55:39.000000000 +0100
+++ raster/r.watershed.mfd/front/r.watershed.html 2008-12-13 09:37:23.000000000 +0100
@@ -39,8 +39,8 @@
Without this flag set, the entire analysis is run in memory
maintained by the operating system. This can be limiting, but is
-relatively fast. Setting the flag causes the program to manage memory
-on disk which allows larger maps to be processes but is considerably
+very fast. Setting this flag causes the program to manage memory
+on disk which allows larger maps to be processes but is somewhat
slower.
memory
@@ -51,6 +51,7 @@
-s
Use single flow direction (SFD) instead of multiple flow direction (MFD).
+MFD is enabled by default.
convergence
@@ -192,19 +193,19 @@
AT least-cost search algorithm
r.watershed uses an AT least-cost search algorithm
-(see REFERENCES section) designed to minimize the
-impact of DEM data errors. This algorithm works slower than
-r.terraflow but provides more accurate results in
-areas of low slope as well as DEMs constructed with techniques that
-mistake canopy tops as the ground elevation. Kinner et al. (2005), for
-example, used SRTM and IFSAR DEMs to compare r.watershed
-against r.terraflow results in Panama. r.terraflow was
-unable to replicate stream locations in the larger valleys while
-r.watershed performed much better. Thus, if forest canopy exists
-in valleys, SRTM, IFSAR, and similar data products will cause major
-errors in r.terraflow stream output. Under similar conditions,
-r.watershed will generate better stream and half_basin
-results. If watershed divides contain flat to low slope, r.watershed
+(see REFERENCES section) designed to minimize
+the impact of DEM data errors. Compared to r.terraflow, this
+algorithm provides more accurate results in areas of low slope as well
+as DEMs constructed with techniques that mistake canopy tops as the
+ground elevation. Kinner et al. (2005), for example, used SRTM and IFSAR
+DEMs to compare r.watershed against r.terraflow
+results in Panama. r.terraflow was unable to replicate stream
+locations in the larger valleys while r.watershed performed
+much better. Thus, if forest canopy exists in valleys, SRTM, IFSAR, and
+similar data products will cause major errors in r.terraflow
+stream output. Under similar conditions, r.watershed will
+generate better stream and half_basin results. If
+watershed divides contain flat to low slope, r.watershed
will generate better basin results than r.terraflow.
(r.terraflow uses the same type of algorithm as ESRI's ArcGIS
watershed software which fails under these conditions.) Also, if watershed
@@ -216,11 +217,12 @@
r.drain on every cell on the map.
Multiple flow direction (MFD)
-r.watershed has experimental support for multiple flow
-direction (MFD). Water flow is distributed to all neighbouring cells with
-lower elevation, using slope towards neighbouring cells as a weighing
-factor for proportional distribution. The AT least-cost path
-is always included and assigned the maxmimum observed weighing factor.
+
r.watershed offers two methods to calculate surface flow:
+single flow direction (SFD, D8) and multiple flow direction (MFD). With
+MFD, water flow is distributed to all neighbouring cells with lower
+elevation, using slope towards neighbouring cells as a weighing factor
+for proportional distribution. The AT least-cost path is
+always included and assigned the maxmimum observed weighing factor.
As a result, depressions and obstacles are overflown with a gracefull
flow convergence before the overflow. The convergence factor causes flow
accumulation to converge more strongly with higher values. The supported