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NAME

r.local.relief.py - Creates a local relief model from elevation map.

KEYWORDS

raster, elevation, terrain, relief, LRM, visualization

SYNOPSIS

r.local.relief.py
r.local.relief.py --help
r.local.relief.py [-ivngf] input=string output=string [neighborhood_size=integer] [color_table=string] [shaded_output=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

Flags:

-i
Save intermediate maps
-v
Use bspline interpolation to construct the surface
Uses v.surf.bspline cubic interpolation instead of r.fillnulls cubic interpolation.
-n
Invert colors in the color table
-g
Logarithmic scaling of the color table
-f
Do not perform histogram equalization on the color table
--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=string [required]
Name of the input elevation raster map
output=string [required]
Name for output local relief map
neighborhood_size=integer
Smoothing neighborhood size
Neighborhood size used when smoothing the elevation model
Options: 0-
Default: 11
color_table=string
Color table for the local relief model raster map
If not provided, grey is used for output and differences is used for the shaded_output
Options: grey, differences
shaded_output=name
Local relief combined with shaded relief
Local relief model combined with shaded relief of the original elevation

Table of contents

DESCRIPTION

r.local.relief generates a local relief model (LRM) from lidar-derived high-resolution DEMs. Local relief models enhance the visibility of small-scale surface features by removing large-scale landforms from the DEM.

Generating the LRM is accomplished in 7 steps (Hesse 2010:69):

  1. Creation of the DEM from the LIDAR data. Buildings, trees and other objects on the earth's surface should be removed.
  2. Apply a low pass filter to the DEM. The low pass filter approximates the large-scale landforms. The neighborhood size of the low pass filter determines the scale of features that will be visible in the LRM. A default neighborhood size of 11 is used.
  3. Subtract the low-pass filter result from the DEM to get the local relief.
  4. Extract the zero contour lines from the difference map.
  5. Extract the input DEM's elevation values along the zero contour lines.
  6. Create a purged DEM by interpolating the null values between the rasterized contours generated in the previous step. This layer represents the large-scale landforms that will be removed to expose the local relief in the final step.
  7. Subtract the purged DEM from the original DEM to get the local relief model.

The interpolation step is performed by the r.fillnulls module by default (using cubic interpolation). If this is not working on your data, you can use -v flag to use v.surf.bspline cubic interpolation instead (this might be slower on some types of data).

OUTPUT

The final local relief model is named according to the output parameter. When the -i flag is specified, r.local.relief creates additional output files representing the intermediate steps in the LRM generation process. The names and number of the intermediate files vary depending on whether r.fillnulls (default) or v.surf.bspline (specified by using the -v flag) is used for interpolation. The intermediate maps are composed of the user-specified output parameter and suffixes describing the intermediate map.

Without using the -v flag (r.fillnulls interpolation), intermediate maps have the following suffixes:

With using the -v flag (v.surf.bspline interpolation), intermediate maps have the following suffixes:

The module sets equalized gray scale color table for local relief model map and for the elevation difference (subtracted elevations). The color tables of other raster maps are set to the same color table as the input elevation map has.

EXAMPLE

Basic example using the default neighborhood size of 11: 
r.local.relief input=elevation output=lrm11
Example with a custom neighborhood size of 25: 
r.local.relief input=elevation output=lrm25 neighborhood_size=25
Example using the default neighborhood size of 11 and saving the intermediate maps: 
r.local.relief -i input=elevation output=lrm11
Example using the default neighborhood size of 11 with bspline interpolation and saving the intermediate maps: 
r.local.relief -i -v input=elevation output=lrm11
Example in NC sample location (area of Raleigh downtown): 
# set the computational region to area of interest
g.region n=228010 s=223380 w=637980 e=644920 res=10

# compute local relief model
r.local.relief input=elevation output=elevation_lrm

# show the maps, e.g. using monitors
d.mon wx0
d.rast elevation
d.rast elevation_lrm

# try alternative red (negative values) and blue (positive values) color table
# color table shows only the high values which hides small streets
# for non-unix operating systems use file or interactive input in GUI
# instead of rules=- and EOF syntax
r.colors map=elevation_lrm@PERMANENT rules=- <<EOF
100% 0:0:255
0 255:255:255
0% 255:0:0
nv 255:255:255
default 255:255:255
EOF
Local relief in NC location (gray scale) Local relief in NC location (red blue)

Figure: Local relief model of downtown Raleigh area created from elevation raster map in NC sample location with the default (gray scale) color table and custom red (negative values) and blue (positive values) color table

SEE ALSO

r.relief, r.shaded.pca, r.skyview

REFERENCES

AUTHORS

Vaclav Petras, NCSU OSGeoREL,
Eric Goddard

SOURCE CODE

Available at: r.local.relief source code (history)

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