Ubuntu Manpage: r.path - Traces paths from starting points following input directions.

NAME

       r.path  - Traces paths from starting points following input directions.

KEYWORDS

       raster, hydrology, cost surface

SYNOPSIS

       r.path
       r.path --help
       r.path     [-can]    input=name    format=string     [values=name]      [raster_path=name]
       [vector_path=name]                         [start_coordinates=east,north[,east,north,...]]
       [start_points=name[,name,...]]   [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -c
           Copy input cell values on output

       -a
           Accumulate input values along the path

       -n
           Count cell numbers along the path

       --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:
       input=name [required]
           Name of input direction
           Direction in degrees CCW from east, or bitmask encoded

       format=string [required]
           Format of the input direction map
           Options: auto, degree, 45degree, bitmask
           Default: auto
           auto: auto-detect direction format
           degree: degrees CCW from East
           45degree: degrees CCW from East divided by 45 (e.g. r.watershed directions)
           bitmask: bitmask encoded directions (e.g. r.cost -b)

       values=name
           Name of input raster values to be used for output
           Name of input raster map

       raster_path=name
           Name for output raster path map
           Name for output raster map

       vector_path=name
           Name for output vector path map
           Name for output vector map

       start_coordinates=east,north[,east,north,...]
           Coordinates of starting point(s) (E,N)

       start_points=name[,name,...]
           Name of starting vector points map(s)
           Or data source(s) for direct OGR access

DESCRIPTION

r.path traces a path from starting points following input directions. Such a movement
direction map can be generated with r.walk, r.cost, r.slope.aspect, r.watershed, or
r.fill.dir, provided that the direction is in degrees, measured counterclockwise from
east.

Alternatively, bitmask-encoded directions can be provided where each bit position
corresponds to a specific neighbour. A path will continue to all neighbours with their bit
set. This means a path can split and merge. Such bitmasked directions can be created with
the -b flag of r.cost and r.walk.
Direction encoding for neighbors of x
135 90 45 7 8 1
180 x 360 6 x 2
225 270 315 5 4 3
degrees bit positions
CCW from East
A path stops when the direction is zero or negative, indicating a stop point or outlet.

The output raster map will show one or more least-cost paths between each user-provided
location(s) and the target points (direction ≤ 0). By default, the output will be an
integer CELL map with the id of the start points along the least cost path, and null cells
elsewhere.

With the -c (copy) flag, the values raster map cell values are copied verbatim along the
path. With the -a (accumulate) flag, the accumulated cell value from the starting point up
to the current cell is written on output. With either the -c or the -a flags, the
raster_path map is created with the same cell type as the values raster map (integer,
float or double). With the -n (number) flag, the cells are numbered consecutively from
the starting point to the final point. The -c, -a, and -n flags are mutually
incompatible.

The start_coordinates parameter consists of map E and N grid coordinates of a starting
point. Each x,y pair is the easting and northing (respectively) of a starting point from
which a path will be traced following input directions. The start_points parameter can
take multiple vector maps containing additional starting points.

NOTES

       The  directions  are  recorded  as  degrees CCW from East, the Knight’s move of r.cost and
       r.walk is considered:
              112.5     67.5
       157.5  135   90  45   22.5
              180   x   0
       202.5  225  270  315  337.5
              247.5     292.5
       i.e. a cell with the value 135 means the next cell is to the North-West, and a  cell  with
       the value 157.5 means that the next cell is to the West-North-West.

EXAMPLES

   Hydrological path
       We  are using the full North Carolina sample dataset.  First we create the two points from
       a text file using v.in.ascii module (here the text file is  CSV  and  we  are  using  unix
       here-file syntax with EOF, in GUI just enter the values directly for the parameter input):
       v.in.ascii input=- output=start format=point separator=comma <<EOF
       638667.15686275,220610.29411765
       638610.78431373,220223.03921569
       EOF
       We  need  to supply a direction raster map to the r.path module.  To get these directions,
       we use the r.watershed module:
       r.watershed elevation=elev_lid792_1m accumulation=accum drainage=drain_dir
       The directions are categorical and we convert them to degrees using raster algebra:
       r.mapcalc "drain_deg = if(drain_dir != 0, 45. * abs(drain_dir), null())"
       Now we are ready to extract the drainage paths starting at the two points.
       r.path input=drain_deg raster_path=drain_path vector_path=drain_path start_points=start
       Before we visualize the result, we set a color table for the elevation we  are  using  and
       create a shaded relief map:
       r.colors map=elev_lid792_1m color=elevation
       r.relief input=elev_lid792_1m output=relief
       We visualize the input and output data:
       d.shade shade=relief color=elev_lid792_1m
       d.vect map=drain_path color=0:0:61 width=4 legend_label="drainage paths"
       d.vect map=start color=none fill_color=224:0:0 icon=basic/circle size=15 legend_label=origins
       d.legend.vect -b
       Figure: Drainage paths from two points where directions from r.watershed were used

   Least-cost path
       We compute bitmask encoded movement directions using r.walk:
       g.region swwake_30m -p
       # create friction map based on land cover
       r.recode input=landclass96 output=friction rules=- << EOF
       1:3:0.1:0.1
       4:5:10.:10.
       6:6:1000.0:1000.0
       7:7:0.3:0.3
       EOF
       # without Knight’s move
       r.walk -b elevation=elev_ned_30m friction=friction output=walkcost \
           outdir=walkdir start_coordinates=635576,216485
       r.path input=walkdir start_coordinates=640206,222795 \
           raster_path=walkpath vector_path=walkpath
       # with Knight’s move
       r.walk -b -k elevation=elev_ned_30m friction=friction output=walkcost_k \
           outdir=walkdir_k start_coordinates=635576,216485
       r.path input=walkdir_k start_coordinates=640206,222795 \
           raster_path=walkpath_k vector_path=walkpath_k
       # without Knight’s move and without bitmask encoding (single direction)
       r.walk elevation=elev_ned_30m friction=friction output=walkcost_s \
           outdir=walkdir_s start_coordinates=635576,216485
       r.path input=walkdir_s start_coordinates=640206,222795 \
           raster_path=walkpath_s vector_path=walkpath_s
       The  extracted  least-cost  path  splits and merges on the way from the start point to the
       stop point (start point for r.walk). Note the gaps in  the  raster  path  when  using  the
       Knight’s move.
       Figure:  Comparison of shortest paths using single directions and multiple bitmask encoded
       directions without and with Knight’s move

AUTHOR

       Markus Metz
       Multiple path directions sponsored by mundialis

SOURCE CODE

       Available at: r.path source code (history)

       Accessed: Tuesday Jun 27 11:13:03 2023

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       © 2003-2023 GRASS Development Team, GRASS GIS 8.3.0 Reference Manual