r.reliefgrass - Online in the Cloud

PROGRAM:

NAME

r.relief - Creates shaded relief map from an elevation map (DEM).

KEYWORDS

raster, elevation, relief, terrain, hillshade

SYNOPSIS

r.relief
r.relief --help
r.relief input=name output=name [altitude=float] [azimuth=float] [zscale=float]
[scale=float] [units=string] [--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:
input=name [required]
Name of input raster map

output=name [required]
Name for output shaded relief map
Name for output raster map

altitude=float
Altitude of the sun in degrees above the horizon
Options: 0-90
Default: 30

azimuth=float
Azimuth of the sun in degrees to the east of north
Options: 0-360
Default: 270

zscale=float
Factor for exaggerating relief
Default: 1

scale=float
Scale factor for converting meters to elevation units
Default: 1

units=string
Elevation units (overrides scale factor)
Options: intl, survey
intl: international feet
survey: survey feet

DESCRIPTION

r.relief creates a raster shaded relief map based on current resolution settings and on
sun altitude, azimuth, and z-exaggeration values entered by the user. If no output
shademap name is given, the new shaded relief map is named <input_map >.shade. The map is
assigned a grey-scale color table.

The parameters controlling the shading are:

1 A raster map layer to provide elevation values for the shaded relief map.
Typically, this would be a map layer of elevation; however, any raster map layer
can be named.

2 The altitude of the sun in degrees above the horizon (a value between 0 and 90
degrees).

3 The azimuth of the sun in degrees to the east of north (a value between 0 and 360
degrees; clockwise from north).

4 The scaling parameter, which compensates for a different horizontal scale than
vertical scale. If scale is a number, then the ewres and nsres are multiplied by
that scale to calculate the shading. (Default=1.0 for equivalent horizontal and
vertical scales.)

5 The zscale exaggeration factor that changes the apparent relief for the shaded
relief map. This can be any positive (or negative) floating point value.
(Default=1.0)

6 Horizontal distances are calculated in meters, using geodesic distances for a
latitude-longitude projection. With an elevation map measured in feet, the units
option can be set to automatically convert meters to international feet (0.3048
meters = 1 foot) or survey feet (1200 / 3937 meters = 1 foot). The units parameter
overrides the scale parameter.

r.relief assigns a grey-scale color table to the new shaded relief map.

NOTES

To visually improve the result of shade maps from low resolution elevation models, use
r.resamp.interp with bilinear or bicubic method to resample the DEM at higher resolution.
r.relief is then run on the resampled DEM.

EXAMPLES

Shaded relief map
In this example, the aspect map in the North Carolina sample dataset location is used to
hillshade the elevation map:
g.region raster=elevation -p
r.relief input=elevation output=elevation_shade
r.relief: shaded elevation map (subset)

Colorizing a shaded relief map
Color can be added later using r.shade:
r.shade shade=elevation_shade color=elevation output=elevation_shaded

Using the scale factor in Latitude-Longitude
In Latitude-Longitude locations (or other non-metric locations), the scale factor has to
be used:
# Latitude-Longitude example
g.region raster=srtm -p
r.relief input=srtm output=srtm_shaded scale=111120

Exporting shaded relief maps to GeoTIFF
The data range of shaded relief maps usually does not permit exporting the map to GeoTIFF
format along with its associated color table due to limitations in the GeoTIFF format.

The most simple way to export it while even reducing the file size is to export as palette
byte map. This requires a conversion done in r.mapcalc, using the # operator to convert
map category values to their grey scale equivalents:
# using the map created above
# create new map from map category values
r.mapcalc "elevation_shade_byte = #elevation_shade"
# verify data range
r.info elevation_shade_byte
# assign grey color table
r.colors elevation_shade_byte color=grey
# export (optionally: createopt="COMPRESS=DEFLATE,BIGTIFF=YES")
r.out.gdal input=elevation_shade_byte createopt="COMPRESS=DEFLATE" \
output=elevation_shade.tif
# add overview images in GeoTIFF file for faster zooming
gdaladdo --config GDAL_CACHEMAX 2000 elevation_shade.tif 2 4 8 16

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