DESCRIPTION

Module r.stream.snap is a supplementary module for r.stream.extract and r.stream.basins to correct position of outlets or stream init points as they do not lie on the streamlines.
For outlets situation is clear. Points are snapped to nearest point which lies on the streamline. In situation where there can be a small tributuary nearer than main stream accumulation treshold shall be high enough to force program ignoring this tributuary and snap to the main stream. If there is no accumulation map points will be snapped to nearest stream line which in particular situation may be wrong. Bacuase r.stream is prepared to work with MFD accum maps, both stream network and accum map is neccessary to resolve the problem.
While it is assumed accum map is a MFD map, if stream network is not supplied, snap point is calculated in different way: treshold is used to select only these points in search radius which are accum value is greater than treshold. Next mean value of these points is calculated and its value is taken as a new treshold. This procedure guarantee that points are snapped to the center of stream tube. While for inits small tresholds are in use, it is probable than points were snapped to the streamtube border instead of its center.

It is strongly recommended, to use both stream network (even pre-generated with small accum treshold) and accumulation map, than accum or stream map only.

OPTIONS

streams

Stream network created by r.stream.extract or r.watershed. If used, points are snapped to the nearest streamline point which accumulation is greater than treshold. If accumulation is not used point is snapped to the nearest stream.

accum

Accumulation map created with r.watershed and used to generate stream network with r.stream.extract. If stream network is not in use, point is adaptively snapped to the to point where value is greater than mean values of accumulation greater than given treshold in a searcyh radius. See description for details.

radius

Search radius (in cells). If there are no streams in search radius, point is not snapped. If there are no cells with accumulation greater than accumtreshold point also is not snapped.

accumtres

Minimum value of accumulation which cell must have to snap point. This option is added to snap stream inits to the stream tubes and to distinguish between local tributaries and main streams.

input

Vector file containing outlets or inits as vector points. Only point's categories are used Table attached to it is ignored. Every point shall have his own unique category.

OUTPUTS

Vector file containing outlets or inits after snapping. On layer 1 original categories are preserved, on layer 2 there are four categories which mean:

1. skipped (not in use yet)
2. unresolved (points remain unsnapped due to lack of streams in search radius
3. snapped (points snapped to streamlines)
4. correct (points whcich remain on its original position, which was originally corrected

SEE ALSO

AUTHOR

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