gdaldem Man page

gdaldem General Commands Manual gdaldem

NAME

gdaldem – gdaldem Tools to analyze and visualize DEMs. (since GDAL
1.7.0)

SYNOPSIS

– To generate a shaded relief map from any GDAL-supported elevation raster :
gdaldem hillshade input_dem output_hillshade
[-z ZFactor (default=1)] [-s scale* (default=1)]”
[-az Azimuth (default=315)] [-alt Altitude (default=45)] [-alg ZevenbergenThorne] [-combined] [-compute_edges] [-b Band (default=1)] [-of format] [-co “NAME=VALUE”]* [-q]

– To generate a slope map from any GDAL-supported elevation raster :
gdaldem slope input_dem output_slope_map”
[-p use percent slope (default=degrees)] [-s scale* (default=1)] [-alg ZevenbergenThorne] [-compute_edges] [-b Band (default=1)] [-of format] [-co “NAME=VALUE”]* [-q]

– To generate an aspect map from any GDAL-supported elevation raster
Outputs a 32-bit float raster with pixel values from 0-360 indicating azimuth :
gdaldem aspect input_dem output_aspect_map”
[-trigonometric] [-zero_for_flat] [-alg ZevenbergenThorne] [-compute_edges] [-b Band (default=1)] [-of format] [-co “NAME=VALUE”]* [-q]

– To generate a color relief map from any GDAL-supported elevation raster
gdaldem color-relief input_dem color_text_file output_color_relief_map
[-alpha] [-exact_color_entry | -nearest_color_entry] [-b Band (default=1)] [-of format] [-co “NAME=VALUE”]* [-q] where color_text_file contains lines of the format “elevation_value red green blue”

– To generate a Terrain Ruggedness Index (TRI) map from any GDAL-supported elevation raster:
gdaldem TRI input_dem output_TRI_map
[-compute_edges] [-b Band (default=1)] [-of format] [-q]

– To generate a Topographic Position Index (TPI) map from any GDAL-supported elevation raster:
gdaldem TPI input_dem output_TPI_map
[-compute_edges] [-b Band (default=1)] [-of format] [-q]

– To generate a roughness map from any GDAL-supported elevation raster:
gdaldem roughness input_dem output_roughness_map
[-compute_edges] [-b Band (default=1)] [-of format] [-q]

Notes :
gdaldem generally assumes that x, y and z units are identical. If x (east-west)
and y (north-south) units are identical, but z (elevation) units are different, the
scale (-s) option can be used to set the ratio of vertical units to horizontal. For
LatLong projections near the equator, where units of latitude and units of
longitude are similar, elevation (z) units can be converted to be compatible
by using scale=370400 (if elevation is in feet) or scale=111120 (if elevation is in
meters). For locations not near the equator, it would be best to reproject your
grid using gdalwarp before using gdaldem..fi

DESCRIPTION

This utility has 7 different modes :

hillshade
to generate a shaded relief map from any GDAL-supported elevation
raster

slope
to generate a slope map from any GDAL-supported elevation raster

aspect
to generate an aspect map from any GDAL-supported elevation raster

color-relief
to generate a color relief map from any GDAL-supported elevation
raster

TRI
to generate a map of Terrain Ruggedness Index from any GDAL-
supported elevation raster

TPI
to generate a map of Topographic Position Index from any GDAL-
supported elevation raster

roughness
to generate a map of roughness from any GDAL-supported elevation
raster

The following general options are available :

input_dem:
The input DEM raster to be processed

output_xxx_map:
The output raster produced

-of format:
Select the output format. The default is GeoTIFF (GTiff). Use the
short format name.

-compute_edges:
(GDAL >= 1.8.0) Do the computation at raster edges and near nodata
values

-alg ZevenbergenThorne:
(GDAL >= 1.8.0) Use Zevenbergen & Thorne formula, instead of Horn’s
formula, to compute slope & aspect. The litterature suggests
Zevenbergen & Thorne to be more suited to smooth landscapes,
whereas Horn’s formula to perform better on rougher terrain.

-b band:
Select an input band to be processed. Bands are numbered from 1.

-co ‘NAME=VALUE’:
Passes a creation option to the output format driver. Multiple -co
options may be listed. See format specific documentation for legal
creation options for each format.

-q:
Suppress progress monitor and other non-error output.

For all algorithms, except color-relief, a nodata value in the target
dataset will be emitted if at least one pixel set to the nodata value
is found in the 3×3 window centered around each source pixel. The
consequence is that there will be a 1-pixel border around each image
set with nodata value. From GDAL 1.8.0, if -compute_edges is specified,
gdaldem will compute values at image edges or if a nodata value is
found in the 3×3 window, by interpolating missing values.

Modes
hillshade
This command outputs an 8-bit raster with a nice shaded relief effect.
It’s very useful for visualizing the terrain. You can optionally
specify the azimuth and altitude of the light source, a vertical
exaggeration factor and a scaling factor to account for differences
between vertical and horizontal units.

The value 0 is used as the output nodata value.

The following specific options are available :

-z zFactor:
vertical exaggeration used to pre-multiply the elevations

-s scale:
ratio of vertical units to horizontal. If the horizontal unit of
the source DEM is degrees (e.g Lat/Long WGS84 projection), you can
use scale=111120 if the vertical units are meters (or scale=370400
if they are in feet)

-az azimuth:
azimuth of the light, in degrees. 0 if it comes from the top of the
raster, 90 from the east, … The default value, 315, should rarely
be changed as it is the value generally used to generate shaded
maps.

-alt altitude:
altitude of the light, in degrees. 90 if the light comes from above
the DEM, 0 if it is raking light.

-combined combined shading:
(starting with GDAL 1.10) a combination of slope and oblique
shading.

slope
This command will take a DEM raster and output a 32-bit float raster
with slope values. You have the option of specifying the type of slope
value you want: degrees or percent slope. In cases where the horizontal
units differ from the vertical units, you can also supply a scaling
factor.

The value -9999 is used as the output nodata value.

The following specific options are available :

-p :
if specified, the slope will be expressed as percent slope.
Otherwise, it is expressed as degrees

-s scale:
ratio of vertical units to horizontal. If the horizontal unit of
the source DEM is degrees (e.g Lat/Long WGS84 projection), you can
use scale=111120 if the vertical units are meters (or scale=370400
if they are in feet)

aspect
This command outputs a 32-bit float raster with values between 0° and
360° representing the azimuth that slopes are facing. The definition of
the azimuth is such that : 0° means that the slope is facing the North,
90° it’s facing the East, 180° it’s facing the South and 270° it’s
facing the West (provided that the top of your input raster is north
oriented). The aspect value -9999 is used as the nodata value to
indicate undefined aspect in flat areas with slope=0.

The following specifics options are available :

-trigonometric:
return trigonometric angle instead of azimuth. Thus 0° means East,
90° North, 180° West, 270° South

-zero_for_flat:
return 0 for flat areas with slope=0, instead of -9999

By using those 2 options, the aspect returned by gdaldem aspect should
be identical to the one of GRASS r.slope.aspect. Otherwise, it’s
identical to the one of Matthew Perry’s aspect.cpp utility.

color-relief
This command outputs a 3-band (RGB) or 4-band (RGBA) raster with values
are computed from the elevation and a text-based color configuration
file, containing the association between various elevation values and
the corresponding wished color. By default, the colors between the
given elevation values are blended smoothly and the result is a nice
colorized DEM. The -exact_color_entry or -nearest_color_entry options
can be used to avoid that linear interpolation for values that don’t
match an index of the color configuration file.

The following specifics options are available :

color_text_file:
text-based color configuration file

-alpha :
add an alpha channel to the output raster

-exact_color_entry :
use strict matching when searching in the color configuration file.
If none matching color entry is found, the ‘0,0,0,0’ RGBA
quadruplet will be used

-nearest_color_entry :
use the RGBA quadruplet corresponding to the closest entry in the
color configuration file.

The color-relief mode is the only mode that supports VRT as output
format. In that case, it will translate the color configuration file
into appropriate LUT elements. Note that elevations specified as
percentage will be translated as absolute values, which must be taken
into account when the statistics of the source raster differ from the
one that was used when building the VRT.

The text-based color configuration file generally contains 4 columns
per line : the elevation value and the corresponding Red, Green, Blue
component (between 0 and 255). The elevation value can be any floating
point value, or the nv keyword for the nodata value.. The elevation can
also be expressed as a percentage : 0% being the minimum value found in
the raster, 100% the maximum value.

An extra column can be optionally added for the alpha component. If it
is not specified, full opacity (255) is assumed.

Various field separators are accepted : comma, tabulation, spaces, ‘:’.

Common colors used by GRASS can also be specified by using their name,
instead of the RGB triplet. The supported list is : white, black, red,
green, blue, yellow, magenta, cyan, aqua, grey/gray, orange, brown,
purple/violet and indigo.

Since GDAL 1.8.0, GMT .cpt palette files are also supported
(COLOR_MODEL = RGB only).

Note: the syntax of the color configuration file is derived from the
one supported by GRASS r.colors utility. ESRI HDR color table files
(.clr) also match that syntax. The alpha component and the support of
tab and comma as separators are GDAL specific extensions.

For example :

3500 white
2500 235:220:175
50% 190 185 135
700 240 250 150
0 50 180 50
nv 0 0 0 0

TRI
This command outputs a single-band raster with values computed from the
elevation. TRI stands for Terrain Ruggedness Index, which is defined as
the mean difference between a central pixel and its surrounding cells
(see Wilson et al 2007, Marine Geodesy 30:3-35).

The value -9999 is used as the output nodata value.

There are no specific options.

TPI
This command outputs a single-band raster with values computed from the
elevation. TPI stands for Topographic Position Index, which is defined
as the difference between a central pixel and the mean of its
surrounding cells (see Wilson et al 2007, Marine Geodesy 30:3-35).

The value -9999 is used as the output nodata value.

There are no specific options.

roughness
This command outputs a single-band raster with values computed from the
elevation. Roughness is the largest inter-cell difference of a central
pixel and its surrounding cell, as defined in Wilson et al (2007,
Marine Geodesy 30:3-35).

The value -9999 is used as the output nodata value.

There are no specific options.

AUTHORS
Matthew Perry perrygeo@gmail.com, Even Rouault even.rouault@mines-
paris.org, Howard Butler hobu.inc@gmail.com, Chris Yesson
chris.yesson@ioz.ac.uk

Derived from code by Michael Shapiro, Olga Waupotitsch, Marjorie
Larson, Jim Westervelt : U.S. Army CERL, 1993. GRASS 4.1 Reference
Manual. U.S. Army Corps of Engineers, Construction Engineering Research
Laboratories, Champaign, Illinois, 1-425.

See also
Documentation of related GRASS utilities :

http://grass.osgeo.org/grass64/manuals/html64_user/r.slope.aspect.html

http://grass.osgeo.org/grass64/manuals/html64_user/r.shaded.relief.html

http://grass.osgeo.org/grass64/manuals/html64_user/r.colors.html

GDAL Tue Apr 5 2016 gdaldem

Ils en parlent aussi

Creating Hill Shaded Tile Overlays | Alastair Aitchison