GRASS logo

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


r.findtheriver - Find the stream pixel nearest the input coordinate


raster, hydrology


r.findtheriver --help
r.findtheriver map=name [window=x] [threshold=x] coordinates=east,north [separator=character] [--help] [--verbose] [--quiet] [--ui]


Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


map=name [required]
Name of input upstream accumulation area raster map
The size of the window in pixels to search in for stream pixels. Must be an odd integer. If not supplied, window will be inferred based on raster resolution.
The threshold for distinguishing log(UAA) values of stream and non-stream pixels. If not supplied, threshold will be inferred from minimum and maximum raster values.
coordinates=east,north [required]
Coordinates of outlet point
Field separator
Special characters: pipe, comma, space, tab, newline
Default: space

Table of contents


r.findtheriver finds the nearest stream pixel to a coordinate pair using an upstream accumulating area (UAA) raster map. This is necessary because the coordinates for streamflow gages are often not perfectly registered to the topography represented by a digital elevation model (DEM) map. This presents a problem when trying to derive a watershed contributing area using r.water.outlet; if the streamflow gage does not fall on the stream as represented in the DEM, r.water.outlet can fail to derive the watershed area.

The basic assumption is that the UAA for "stream" pixels will be much higher than for adjacent "non-stream" pixels. r.findtheriver attempts to "snap" the coordinates of the streamflow gage to the "true" stream location by first identifying stream pixels within a search window, and then selecting the stream pixel that is closest (Cartesian distance) to the input gage coordinates. Stream pixels are identified by searching the UAA raster window for pixels that exceed a threshold. This is done by computing the log10 of the UAA value for the pixel corresponding to the gage coordinates and subtracting from it the log10 of each pixel in the window; for a given pixel if this difference is greater than the threshold, the pixel is deemed to be a stream pixel.

r.findtheriver will automatically compute the window and threshold if they are not supplied by the user. The window is determined based on a THRESHOLD_DISTANCE / cell resolution of the UAA map. The threshold is determined by subtracting the log10 of the UAA value at the input gage coordinate from the log10 of the maximum UAA value of the map, and then rounding down to the nearest integer, in other words: threshold = floor( log(maxUAA) - log(gageUAA) ).

The closest stream pixel is printed to standard output. If no stream pixels were found, nothing is printed.




Brian Miles
Updated to GRASS 7 by Huidae Cho


Available at: r.findtheriver source code (history)

Latest change: Saturday Mar 02 21:28:14 2024 in commit: e5ea95658ff194880b6971b9fcd810ecfcb8c7f2

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

Main index | Raster index | Topics index | Keywords index | Graphical index | Full index

© 2003-2023 GRASS Development Team, GRASS GIS 8.2.2dev Reference Manual