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.
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