Tribblix: manual page: gdaldem.1

GDALDEM(1) GDAL GDALDEM(1)

NAME

gdaldem - Tools to analyze and visualize DEMs.

SYNOPSIS

gdaldem [--help] [--help-general] <mode> <input> <output> <options>

From any GDAL-supported elevation raster:

Generate a shaded relief map:

gdaldem hillshade <input_dem> <output_hillshade>
[-z <zfactor>] [-s <scale>]
[-az <azimuth>] [-alt <altitude>]
[-alg ZevenbergenThorne] [-combined | -multidirectional | -igor]
[-compute_edges] [-b <Band>] [-of <format>] [-co <NAME>=<VALUE>]... [-q]

Generate a slope map:

gdaldem slope <input_dem> <output_slope_map>
[-p] [-s <scale>]
[-alg ZevenbergenThorne]
[-compute_edges] [-b <band>] [-of <format>] [-co <NAME>=<VALUE>]... [-q]

Generate an aspect map, 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>] [-of format] [-co <NAME>=<VALUE>]... [-q]

Generate a color relief map:

gdaldem color-relief <input_dem> <color_text_file> <output_color_relief_map>
[-alpha] [-exact_color_entry | -nearest_color_entry]
[-b <band>] [-of format] [-co <NAME>=<VALUE>]... [-q]

where color_text_file contains lines of the format "elevation_value red green blue [alpha]". If alpha column is present it can be enabled for use with '-alpha'.

Generate a Terrain Ruggedness Index (TRI) map:

gdaldem TRI input_dem output_TRI_map
[-alg Wilson|Riley]
[-compute_edges] [-b Band (default=1)] [-of format] [-q]

Generate a Topographic Position Index (TPI) map:

gdaldem TPI <input_dem> <output_TPI_map>
[-compute_edges] [-b <band>] [-of <format>] [-co <NAME>=<VALUE>]... [-q]

Generate a roughness map:

gdaldem roughness <input_dem> <output_roughness_map>
[-compute_edges] [-b <band>] [-of <format>] [-co <NAME>=<VALUE>]... [-q]

DESCRIPTION

The 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.

<mode> Where <mode> is one of the seven available modes:

+o hillshade
Generate a shaded relief map from any GDAL-supported
elevation raster.

+o slope
Generate a slope map from any GDAL-supported elevation
raster.

+o aspect
Generate an aspect map from any GDAL-supported elevation
raster.

+o color-relief
Generate a color relief map from any GDAL-supported
elevation raster.

+o TRI
Generate a map of Terrain Ruggedness Index from any
GDAL-supported elevation raster.

+o TPI
Generate a map of Topographic Position Index from any
GDAL-supported elevation raster.

+o roughness
Generate a map of roughness from any GDAL-supported
elevation raster.

The following general options are available:

--help Show this help message and exit

--help-general
Gives a brief usage message for the generic GDAL commandline
options and exit.

<input_dem>
The input DEM raster to be processed

<output_xxx_map>
The output raster produced

-of <format>
Select the output format.

New in version 2.3.0: If not specified, the format is guessed
from the extension (previously was GTiff -- GeoTIFF File
Format). Use the short format name.

-compute_edges
Do the computation at raster edges and near nodata values

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

-co <NAME>=<VALUE>
Many formats have one or more optional creation options that
can be used to control particulars about the file created. For
instance, the GeoTIFF driver supports creation options to
control compression, and whether the file should be tiled.

The creation options available vary by format driver, and some
simple formats have no creation options at all. A list of
options supported for a format can be listed with the
--formats command line option but the documentation for the
format is the definitive source of information on driver
creation options. See Raster drivers 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 3x3 window centered around each source pixel. The
consequence is that there will be a 1-pixel border around each image
set with nodata value.
If -compute_edges is specified, gdaldem will compute values at
image edges or if a nodata value is found in the 3x3 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 :

-alg Horn|ZevenbergenThorne
The literature suggests Zevenbergen & Thorne to be more suited
to smooth landscapes, whereas Horn's formula to perform better
on rougher terrain.

-z <factor>
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, a combination of slope and oblique shading.

-multidirectional
multidirectional shading, a combination of hillshading
illuminated from 225 deg, 270 deg, 315 deg, and 360 deg
azimuth.

New in version 2.2.

-igor shading which tries to minimize effects on other map features
beneath. Can't be used with -alt option.

New in version 3.0.

Multidirectional hillshading applies the formula of
http://pubs.usgs.gov/of/1992/of92-422/of92-422.pdf.

Igor's hillshading uses formula from Maperitive
http://maperitive.net/docs/Commands/GenerateReliefImageIgor.html.

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 :

-alg Horn|ZevenbergenThorne
The literature suggests Zevenbergen & Thorne to be more suited
to smooth landscapes, whereas Horn's formula to perform better
on rougher terrain.

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

-s
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<degree> and 360<degree> representing the azimuth that slopes are
facing. The definition of the azimuth is such that : 0<degree> means
that the slope is facing the North, 90<degree> it's facing the East,
180<degree> it's facing the South and 270<degree> 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 :

-alg Horn|ZevenbergenThorne
The literature suggests Zevenbergen & Thorne to be more suited
to smooth landscapes, whereas Horn's formula to perform better
on rougher terrain.

-trigonometric
Return trigonometric angle instead of azimuth. Thus 0<degree>
means East, 90<degree> North, 180<degree> West, 270<degree>
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.
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

To implement a "round to the floor value" mode, the elevation value
can be duplicate with a new value being slightly above the threshold.
For example to have red in [0,10], green in ]10,20] and blue in
]20,30]:

0 red
10 red
10.001 green
20 green
20.001 blue
30 blue

TRI

This command outputs a single-band raster with values computed from
the elevation. TRI stands for Terrain Ruggedness Index, which
measures the difference between a central pixel and its surrounding
cells.

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

The following option is available:

-alg Wilson|Riley
Starting with GDAL 3.3, the Riley algorithm (see Riley, S.J.,
De Gloria, S.D., Elliot, R. (1999): A Terrain Ruggedness that
Quantifies Topographic Heterogeneity. Intermountain Journal
of Science, Vol.5, No.1-4, pp.23-27) is available and the new
default value. This algorithm uses the square root of the sum
of the square of the difference between a central pixel and
its surrounding cells. This is recommended for terrestrial use
cases.

The Wilson (see Wilson et al 2007, Marine Geodesy 30:3-35)
algorithm uses the mean difference between a central pixel and
its surrounding cells. This is recommended for bathymetric
use cases.

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.

C API

This utility is also callable from C with GDALDEMProcessing().

New in version 2.1.

AUTHORS

Matthew Perry perrygeo@gmail.com, Even Rouault
even.rouault@spatialys.com, 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.

AUTHOR

Matthew Perry <perrygeo@gmail.com>, Even Rouault
<even.rouault@spatialys.com>, Howard Butler <hobu.inc@gmail.com>,
Chris Yesson <chris.yesson@ioz.ac.uk>

COPYRIGHT

1998-2025

January 8, 2025 GDALDEM(1)