# gdaldem

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##### gdaldem

R wrapper for gdaldem: Tools to analyze and visualize DEMs. (since GDAL 1.7.0)

##### Usage
gdaldem(mode, input_dem, output, of, compute_edges, alg, b, co, q, z, s, az,
alt, combined, p, trigonometric, zero_for_flat, color_text_file, alpha,
exact_color_entry, nearest_color_entry, output_Raster = FALSE,
ignore.full_scan = TRUE, verbose = FALSE)
##### Arguments
mode

input_dem

Character. The input DEM raster to be processed.

output

Character. The output raster produced.

of

Character. Select the output format. The default is GeoTIFF (GTiff). Use the short format name.

compute_edges

Logical. (GDAL >= 1.8.0) Do the computation at raster edges and near nodata values.

alg

Character. "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

Numeric. Select an input band to be processed. Bands are numbered from 1.

co

Character. (GDAL >= 1.8.0) Passes a creation option ("NAME=VALUE") to the output format driver. Multiple -co options may be listed. See format specific documentation for legal creation options for each format.

q

Logical. Suppress progress monitor and other non-error output.

z

Numeric. (mode=="hillshade") vertical exaggeration used to pre-multiply the elevations.

s

Numeric. (mode=="hillshade" | mode=="slope) 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

Numeric. (mode=="hillshade") 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

Numeric. (mode=="hillshade") altitude of the light, in degrees. 90 if the light comes from above the DEM, 0 if it is raking light.

combined

p

Logical. (mode=="slope") if specified, the slope will be expressed as percent slope. Otherwise, it is expressed as degrees.

trigonometric

Logical. (mode=="aspect") return trigonometric angle instead of azimuth. Thus 0deg means East, 90deg North, 180deg West, 270deg South.

zero_for_flat

Logical. (mode=="aspect") 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_text_file

Character. (mode=="color-relief") text-based color configuration file (see Description).

alpha

Logical. (mode=="color-relief") add an alpha channel to the output raster.

exact_color_entry

Logical. (mode=="color-relief") 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

Logical. (mode=="color-relief") use the RGBA quadruplet corresponding to the closest entry in the color configuration file.

output_Raster

Logical. Return output dst_dataset as a RasterBrick?

ignore.full_scan

Logical. If FALSE, perform a brute-force scan if other installs are not found. Default is TRUE.

verbose

Logical. Enable verbose execution? Default is FALSE.

##### Details

This is an R wrapper for the 'gdaldem' function that is part of the Geospatial Data Abstraction Library (GDAL). It follows the parameter naming conventions of the original function, with some modifications to allow for more R-like parameters. For all parameters, the user can use a single character string following, precisely, the gdalinfo format (http://www.gdal.org/gdaldem.html), or, in some cases, use R vectors to achieve the same end.

This function assumes the user has a working GDAL on their system. If the "gdalUtils_gdalPath" option has been set (usually by gdal_setInstallation), the GDAL found in that path will be used. If nothing is found, gdal_setInstallation will be executed to attempt to find a working GDAL.

The user can choose to (optionally) return a RasterBrick of the output file (assuming raster/rgdal supports the particular output format).

##### Value

NULL or if(output_Raster), a RasterBrick.

##### References

http://www.gdal.org/gdaldem.html

• gdaldem
##### Examples
# NOT RUN {
# We'll pre-check to make sure there is a valid GDAL install
# and that raster and rgdal are also installed.
# Note this isn't strictly neccessary, as executing the function will
# force a search for a valid GDAL install.
gdal_setInstallation()
valid_install <- !is.null(getOption("gdalUtils_gdalPath"))
if(require(raster) && require(rgdal) && valid_install)
{
# We'll pre-check for a proper GDAL installation before running these examples:
gdal_setInstallation()
if(!is.null(getOption("gdalUtils_gdalPath")))
{
input_dem  <- system.file("external/tahoe_lidar_highesthit.tif", package="gdalUtils")
plot(raster(input_dem),col=gray.colors(256))

# Command-line gdaldem call:

# Slope:
# Command-line gdaldem call:
# gdaldem slope tahoe_lidar_highesthit.tif output_slope.tif -p
output_slope <- gdaldem(mode="slope",input_dem=input_dem,
output="output_slope.tif",p=TRUE,output_Raster=TRUE,verbose=TRUE)
plot(output_slope,col=gray.colors(256))

# Aspect:
# Command-line gdaldem call:
# gdaldem aspect tahoe_lidar_highesthit.tif output_aspect.tif
output_aspect <- gdaldem(mode="aspect",input_dem=input_dem,
output="output_aspect.tif",output_Raster=TRUE,verbose=TRUE)
plot(output_aspect,col=gray.colors(256))
}
}
# }

Documentation reproduced from package gdalUtils, version 2.0.1.7, License: GPL (>= 2)

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