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Note: This addon document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade your GRASS GIS installation, and read the current addon manual page.


r.catchment - Creates a raster buffer of specified area around vector points using cost distances using r.walk.


raster, buffer


r.catchment --help
r.catchment [-kcl] elevation=name [in_cost=name] start_points=name [friction=name] [a=float] [b=float] [c=float] [d=float] [lambda=float] [slope_factor=float] buffer=name [sigma=float] area=integer map_val=integer [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Use knight's move for calculating cost surface (slower but more accurate)
Keep cost surface used to calculate buffers
Show a list of all cost surface values and the area of the catchment that they delimit
Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


elevation=name [required]
Input elevation map (DEM)
Input cost map (This will override the input elevation map, if none specified, one will be created from input elevation map with r.walk)
start_points=name [required]
Name of input vector map
Name of input vector site points map
Optional map of friction costs. If no map selected, default friction=0 making output reflect time costs only
Coefficients for walking energy formula parameters a,b,c,d
Default: 0.72
Default: 6.0
Default: 1.9998
Default: -1.9998
Lambda value for cost distance calculation (for combining friction costs with walking costs)
Default: 1
Slope factor determines travel energy cost per height step
Default: -0.2125
buffer=name [required]
Output buffer map
Slope threshold for mask
area=integer [required]
Area of buffer (Integer value to nearest 100 square map units)
Default: 5000000
map_val=integer [required]
Integer value for output catchment area (all other areas will be Null)
Default: 1

Table of contents


r.catchment is a module that facilitates modeling "catchments" around point locations according to a walking cost function. The module is particularly aimed at "Site Catchment Analysis" for archaeology, but could be potentially useful in any number of appliations where delimiting an area based on walking-costs is desireable. Although defining a catchment based on a threshold in walking-costs (or time) can be undertaken using r.walk or r.cost alone, this addon module allows the user to enter a pre-determined square meterage (option area) for the resultant catchment, which is a different approach. This is useful for applications where the user wants to make a catchment of a particular size (e.g., certain number of square meters needed for farmed fields), and doesn't want to spend time via trial and error experimenting with different cost radii.

Additionally, this module allows the user to enter a slope threshold (option sigma), which will mask out areas of higher slope. This is useful for delimiting catchments that are of generally flat land (e.g., areas where agriculture are likely).

Important: The user must run g.region first to make sure that the region boundaries and the resolution match the input elevation map.

Options and flags:

r.catchment requires an input elevation map, elevation , and an input vector points map of starting locations, start_points. area is also requited, which is an integer value for the size of the desired catchment (in the map units of the defined location/region). The final required parameter is map_val , wich is the integer value to write to the areas defined as part of the catchment in the output map, buffer. The optional value, sigma is the slope threshold cut off value. Slopes above sigma will be masked out during the determination of the catchment configuration. There are two native flags for r.catchment. -c allows you to keep the interim cost surface maps made. -l allows you to show a list of the cost values in that cost map, along with the size of the catchments they delineate. All other flags and options are inherited from r.walk (see ther.walk help page for more information on these).


The module will attempt to find the cost radius that defines an area close to the value of area, but em will likely slightly overestimate the catchement size. The module will display the actual area of the defined catchment in the Command Output. By default, r.catchment will create a friction map of value 0, which, when input into r.walk will yeild a cost surface based on walking times only. The user may optionally create a friction map, however, and, if used, r.walk will consider these costs this as well when determining the cost surface used to determine the catchment. The input vector points map can be manually digitized (with v.digit) over topographic or cultural features, or can be created as a series of random points (with r.random or v.random). In the case of multiple input points, the routine will attempt to equally divide the area (area) between all input points to determine catchments for each point. The total area of all these catchments will sum (close) to area. If two input points are close, their catchments may overlap. In this case, the routine will "meld" the two, and the melded catchment will still be of an area close to area. If truly overlapping catchments are desired, then the routine should be run independantly for each individual start point.


Delimit a catchment of 5,000,000 square meters around a single start point, ignoring areas of slope > 15 degrees:
r.catchment elevation=DEM10m start_points=site buffer=test_catchment 
sigma=15 area=5000000 map_val=1


r.walk, r.cost


Isaac Ullah

Updated for GRASS 7, 23, Feb. 2015.


Available at: r.catchment source code (history)

Latest change: Sunday Apr 24 20:44:20 2022 in commit: 7f52a16ed71bb07f57c554d618ce446d005fdedd

Note: This addon document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade your GRASS GIS installation, and read the current addon manual page.

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