GRASS GIS Manual: r.viewshed.cva.py

NAME

r.viewshed.cva.py

DESCRIPTION

r.viewshed.cva is a module that allows for the construction of "Cumulative Viewshed", or "visualscape" maps from a series of input points stored in a vector points map. The routine is a python wrapper script that iterative loops through each input point, calculating a viewshed map, and then creates an output map that is coded by the number of input locations that can "see" each cell. r.viewshed.cva uses the GRASS addon r.viewshed for the viewshed analysis (instead of the standard module r.los) because r.viewshed is substantially faster, thus allowing for a cumulative viewshed analysis to run in a reasonable amount of time. The final cumulative viewshed map is computed using the "count" method of r.series, rather than with mapcalc, as it better handles the null values in the individual constituent viewshed maps (and allows for interim viewshed maps to be coded in any way)

Options and flags:

r.viewshed.cva requires an input elevation map and an input vector points map with at least three columns of data: one contains a unique identifier for each point (e.g., "cat"), one contains the easting of the point, and one contains the northing of the point. There is currently only one native flag for r.viewshed.cva (-k), which allows you to keep the interim viewshed maps made for each input point. All other flags and options are inherited from r.viewshed (see the r.viewshed help page for more information on these).

NOTES

The input vector points map can be manually digitized (with v.digit) over topographic or cultural features, or can be created as a series of random points (with r.random or v.random). The required "name_column" can be any alpha numeric value, as long as it is unique for each input point. The required "x_column" and "y_column" columns can be added with v.db.addcol, and the eastings and northings uploaded to these columns with v.to.db. Note that using the flag -k allows you to keep any interim viewshed maps created during the analysis. This is also useful for simple creating a large number of individual viewsheds from points in a vector file.

Also note that you must first install the GRASS addon r.viewshed before you can use this module. r.viewshed offers several significant advnatages over r.los, not least a great increase in processing speed. Use of r.los would mean that only a small number of input points could be used. Use of r.viewshed means that a much larger number of points could be used (trials showed that viewsheds were calculated for 100 input points in about 1.5 hrs at a 10m resolution for a region spanning some 40km by 10km).

EXAMPLES

Undertake a cumulative viewshed analysis from a digitized vector points map of prominent peaks in a region:

g.region rast=elevation_10m_dem@PERMANENT -p
v.db.addcol map=prominent_peaks_points@PERMANENT columns=x double, y double
v.to.db map=prominent_peaks_points@PERMANENT option=coor columns=x,y
r.viewshed.cva.py elev=elevation10m_demPERMANENT output=peaks_CVA_map \
  vect=prominent_peaks_points@PERMANENT x_column=x y_column=y \
  name_column=cat obs_elev=0.0 tgt_elev=1.75 max_dist=-1 mem=1500

Undertake a cumulative viewshed analysis from a 10% sample of landscape locations in a region:

g.region rast=elevation_10m_dem@PERMANENT
r.random input=elevation10m_demPERMANENT n=10% vector_output=rand_points_10p
v.db.addcol map=rand_points_10p@PERMANENT columns=x double, y double
v.to.db map=rand_points_10p@PERMANENT option=coor columns=x,y
r.viewshed.cva.py elev=elevation10m_demPERMANENT output=peaks_CVA_map \
  vect=rand_points_10p@PERMANENT x_column=x y_column=y \
  name_column=cat obs_elev=0.0 tgt_elev=1.75 max_dist=-1 mem=1500

AUTHOR

Isaac Ullah

Last changed: $Date$