Ubuntu Manpage: r.stream.extract - Performs stream network extraction.

NAME

       r.stream.extract  - Performs stream network extraction.

KEYWORDS

       raster, hydrology, stream network

SYNOPSIS

       r.stream.extract
       r.stream.extract --help
       r.stream.extract  elevation=name  [accumulation=name]   [depression=name]  threshold=float
       [d8cut=float]        [mexp=float]         [stream_length=integer]         [memory=integer]
       [stream_raster=name]    [stream_vector=name]    [direction=name]   [--overwrite]  [--help]
       [--verbose]  [--quiet]  [--ui]

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       elevation=name [required]
           Name of input elevation raster map

       accumulation=name
           Name of input accumulation raster map
           Stream extraction will use provided accumulation instead of calculating it anew

       depression=name
           Name of input raster map with real depressions
           Streams will not be routed out of real depressions

       threshold=float [required]
           Minimum flow accumulation for streams
           Must be > 0

       d8cut=float
           Use SFD above this threshold
           If accumulation is larger than d8cut, SFD is used instead of MFD. Applies only  if  no
           accumulation map is given.

       mexp=float
           Montgomery exponent for slope, disabled with 0
           Montgomery:  accumulation  is  multiplied  with pow(slope,mexp) and then compared with
           threshold
           Default: 0

       stream_length=integer
           Delete stream segments shorter than stream_length cells
           Applies only to first-order stream segments (springs/stream heads)
           Default: 0

       memory=integer
           Maximum memory to be used (in MB)
           Cache size for raster rows
           Default: 300

       stream_raster=name
           Name for output raster map with unique stream ids

       stream_vector=name
           Name for output vector map with unique stream ids

       direction=name
           Name for output raster map with flow direction

DESCRIPTION

       r.stream.extract extracts streams in both raster and vector format from a  required  input
       elevation map and optional input accumulation map.

NOTES

NULL (nodata) cells in the input elevation map are ignored, zero and negative values are
valid elevation data. Gaps in the elevation map that are located within the area of
interest must be filled beforehand, e.g. with r.fillnulls, to avoid distortions.

All non-NULL and non-zero cells of depression map will be regarded as real depressions.
Streams will not be routed out of depressions. If an area is marked as depression but the
elevation model has no depression at this location, streams will not stop there. If a flow
accumulation map and a map with real depressions are provided, the flow accumulation map
must match the depression map such that flow is not distributed out of the indicated
depressions. It is recommended to use internally computed flow accumulation if a
depression map is provided.

Option threshold defines the minimum (optionally modified) flow accumulation value that
will initiate a new stream. If Montgomery’s method for channel initiation is used, the
cell value of the accumulation input map is multiplied by (tan(local slope))mexp and then
compared to threshold. If mexp is given than the method of Montgomery and
Foufoula-Georgiou (1993) to initiate a stream with this value. The cell value of the
accumulation input map is multiplied by (tan(local slope))mexp and then compared to
threshold. If threshold is reached or exceeded, a new stream is initiated. The default
value 0 disables Montgomery. Montgomery and Foufoula-Georgiou (1993) generally recommend
to use 2.0 as exponent. mexp values closer to 0 will produce streams more similar to
streams extracted with Montgomery disabled. Larger mexp values decrease the number of
streams in flat areas and increase the number of streams in steep areas. If weight is
given, the weight is applied first.

Option d8cut defines minimum amount of overland flow (accumulation) when SFD (D8) will be
used instead of MFD (FD8) to calculate flow accumulation. Only applies if no accumulation
map is provided. Setting to 0 disables MFD completely.

Option stream_length defines minimum stream length in number of cells for first-order
(head/spring) stream segments. All first-order stream segments shorter than stream_length
will be deleted.

Output direction raster map contains flow direction for all non-NULL cells in input
elevation. Flow direction is of D8 type with a range of 1 to 8. Multiplying values with
45 gives degrees CCW from East. Flow direction was adjusted during thinning, taking
shortcuts and skipping cells that were eliminated by the thinning procedure.

Stream extraction
If no accumulation input map is provided, flow accumulation is determined with a
hydrological analysis similar to r.watershed. The algorithm is MFD (FD8) after Holmgren
1994, as for r.watershed. The threshold option determines the number of streams and detail
of stream networks. Whenever flow accumulation reaches threshold, a new stream is started
and traced downstream to its outlet point. As for r.watershed, flow accumulation is
calculated as the number of cells draining through a cell.

If accumulation is given than the accumulation values of the provided accumulation map are
used and not calculated from the input elevation map. In this case the elevation map must
be exactly the same map used to calculate accumulation. If accumulation was calculated
with r.terraflow, the filled elevation output of r.terraflow must be used. Further on, the
current region should be aligned to the accumulation map. Flow direction is first
calculated from elevation and then adjusted to accumulation. It is not necessary to
provide accumulation as the number of cells, it can also be the optionally adjusted or
weighed total contributing area in square meters or any other unit. When an original flow
accumulation map is adjusted or weighed, the adjustment or weighing should not convert
valid accumulation values to NULL (nodata) values.

In case of getting the error message ERROR: Accumulation raster map is NULL but elevation
map is not NULL the computational region must be carefully adjusted to exclude NULL pixels
in the accumulation raster map prior to stream extraction.

Weighed flow accumulation
Flow accumulation can be calculated first, e.g. with r.watershed, and then modified before
using it as input for r.stream.extract. In its general form, a weighed accumulation map is
generated by first creating a weighing map and then multiplying the accumulation map with
the weighing map using r.mapcalc. It is highly recommended to evaluate the weighed flow
accumulation map first, before using it as input for r.stream.extract.

This allows e.g. to decrease the number of streams in dry areas and increase the number of
streams in wet areas by setting weight to smaller than 1 in dry areas and larger than 1 in
wet areas.

Another possibility is to restrict channel initiation to valleys determined from terrain
morphology. Valleys can be determined with r.param.scale method=crosc (cross-sectional or
tangential curvature). Curvature values < 0 indicate concave features, i.e. valleys. The
size of the processing window determines whether narrow or broad valleys will be
identified (See example below).

Defining a region of interest
The stream extraction procedure can be restricted to a certain region of interest, e.g. a
subbasin, by setting the computational region with g.region and/or creating a MASK. Such
region of interest should be a complete catchment area, complete in the sense that the
complete area upstream of an outlet point is included and buffered with at least one cell.

Stream output
The output raster and vector contains stream segments with unique IDs. Note that these IDs
are different from the IDs assigned by r.watershed. The vector output also contains points
at the location of the start of a stream segment, at confluences and at stream network
outlet locations.

Output stream_raster raster map stores extracted streams. Cell values encode a unique ID
for each stream segment.

Output stream_vector vector map stores extracted stream segments and points. Points are
written at the start location of each stream segment and at the outlet of a stream
network. In layer 1, categories are unique IDs, identical to the cell value of the raster
output. The attribute table for layer 1 holds information about the type of stream
segment: start segment, or intermediate segment with tributaries, and about the stream
network this stream or node belongs to. Columns are cat int,stream_type
varchar(),type_code int,network int. The network attribute is the network ID of the
stream/node. The encoding for type_code is 0 = start, 1 = intermediate. In layer 2,
categories are identical to type_code in layer 1 with additional category 2 = outlet for
outlet points. Points with category 1 = intermediate in layer 2 are at the location of
confluences.

EXAMPLE

       This example is based on the elevation map "elev_ned_30m" in  the  North  Carolina  sample
       dataset  and  uses  valleys  determined  with  r.param.scale  to weigh an accumulation map
       produced with r.watershed.
       # set region
       g.region -p raster=elev_ned_30m@PERMANENT
       # calculate flow accumulation
       r.watershed ele=elev_ned_30m@PERMANENT acc=elev_ned_30m.acc
       # curvature to get narrow valleys
       r.param.scale input=elev_ned_30m@PERMANENT output=tangential_curv_5 size=5 method=crosc
       # curvature to get a bit broader valleys
       r.param.scale input=elev_ned_30m@PERMANENT output=tangential_curv_7 size=7 method=crosc
       # curvature to get broad valleys
       r.param.scale input=elev_ned_30m@PERMANENT output=tangential_curv_11 size=11 method=crosc
       # create weight map
       r.mapcalc "weight = if(tangential_curv_5 < 0, -100 * tangential_curv_5, \
                           if(tangential_curv_7 < 0, -100 * tangential_curv_7, \
                           if(tangential_curv_11 < 0, -100 * tangential_curv_11, 0.000001)))"
       # weigh accumulation map
       r.mapcalc expr="elev_ned_30m.acc.weighed = elev_ned_30m.acc * weight"
       # copy color table from original accumulation map
       r.colors map=elev_ned_30m.acc.weighed raster=elev_ned_30m.acc

       Weight map (spatial subset with lake in the southern half)

       Original flow accumulation map (spatial subset with lake in the southern half)

       Weighed flow accumulation map (spatial subset with lake in the southern half)

       Display both the original and the weighed accumulation map.  Compare them and  proceed  if
       the weighed accumulation map makes sense.
       # extract streams using the original accumulation map
       r.stream.extract elevation=elev_ned_30m@PERMANENT \
                        accumulation=elev_ned_30m.acc \
                        threshold=1000 \
                        stream_rast=elev_ned_30m.streams.noweight
       # extract streams from weighed map
       # note that the weighed map is a bit smaller than the original map
       r.stream.extract elevation=elev_ned_30m@PERMANENT \
                        accumulation=elev_ned_30m.acc.weighed \
                        threshold=1000 \
                        stream_rast=elev_ned_30m.streams

       Now display both stream maps and decide which one is more realistic.

       Extracted streams from original flow accumulation map

       Extracted streams from weighed flow accumulation map

REFERENCES

           •   Ehlschlaeger,  C.  (1989).  Using  the  AT  Search Algorithm to Develop Hydrologic
               Models from  Digital  Elevation  Data,  Proceedings  of  International  Geographic
               Information  Systems  (IGIS) Symposium ’89, pp 275-281 (Baltimore, MD, 18-19 March
               1989). URL: http://faculty.wiu.edu/CR-Ehlschlaeger2/older/IGIS/paper.html

           •   Holmgren, P. (1994). Multiple flow direction algorithms for  runoff  modelling  in
               grid  based elevation models: An empirical evaluation.  Hydrological Processes Vol
               8(4), pp 327-334. DOI: 10.1002/hyp.3360080405

           •   Montgomery,  D.R.,  Foufoula-Georgiou,   E.   (1993).   Channel   network   source
               representation  using  digital  elevation  models.   Water  Resources Research Vol
               29(12), pp 3925-3934.

AUTHOR

       Markus Metz

       Last changed: $Date: 2017-11-12 14:07:55 +0100 (Sun, 12 Nov 2017) $

SOURCE CODE

       Available at: r.stream.extract source code (history)

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