r.surf.contourgrass - Online in the Cloud

PROGRAM:

NAME

r.surf.contour - Generates surface raster map from rasterized contours.

KEYWORDS

raster, surface, interpolation

SYNOPSIS

r.surf.contour
r.surf.contour --help
r.surf.contour input=name output=name [--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 containing contours

output=name [required]
Name for output raster map

DESCRIPTION

r.surf.contour creates a raster elevation map from a rasterized contour map. Elevation
values are determined using procedures similar to a manual methods. To determine the
elevation of a point on a contour map, an individual might interpolate its value from
those of the two nearest contour lines (uphill and downhill).

r.surf.contour works in a similar way. Initially, a vector map of the contour lines is
made with the elevation of each line as an attribute. When the program v.to.rast is run
on the vector map, continuous "lines" of rasters containing the contour line values will
be the input for r.surf.contour. For each cell in the input map, either the cell is a
contour line cell (which is given that value), or a flood fill is generated from that spot
until the fill comes to two unique values. So the r.surf.contour algorithm linearly
interpolates between contour lines. The flood fill is not allowed to cross over the
rasterized contour lines, thus ensuring that an uphill and downhill contour value will be
the two values chosen. r.surf.contour interpolates from the uphill and downhill values by
the true distance.

Parameters:
input=name
Name of an existing raster map that contains a set of initial category values (i.e.,
some cells contain known elevation values (denoting contours) while the rest contain
NULL values).

output=name
Name to be assigned to new output raster map that represents a smooth (e.g.,
elevation) surface generated from the known category values in the input raster map
layer.

An existing MASK raster map is respected for both reading input and writing output.

NOTES

r.surf.contour works well under the following circumstances: 1) the contour lines extend
to the the edge of the current region, 2) the program is run at the same resolution as
that of the input map, 3) there are no disjointed contour lines, and 4) no spot elevation
data BETWEEN contour lines exist. Spot elevations at the tops of hills and the bottoms of
depressions, on the other hand, improve the output greatly. Violating these constraints
will cause non-intuitive anomalies to appear in the output map. Run r.slope.aspect
on r.surf.contour results to locate potential anomalies.

The running of r.surf.contour is very sensitive to the resolution of rasterized vector
map. If multiple contour lines go through the same raster, slight anomalies may occur.
The speed of r.surf.contour is dependent on how far "apart" the contour lines are from
each other (as measured in raster cells). Since a flood fill algorithm is used, the
program’s running time will grow exponentially with the distance between contour lines.

EXAMPLE

Example to create contour lines from elevation model, then recreating DEM from these
contour lines along with differences analysis (North Carolina sample data set):
g.region raster=elevation -p
# get minimum elevation value
r.univar elevation
# generate vector contour lines
r.contour input=elevation output=contours_5m step=5 minlevel=50
# rasterize contour lines
v.info -c contours_5m
v.to.rast input=contours_5m output=contours_5m use=attr attribute_column=level
# generate DEM from rasterized contour lines
r.surf.contour input=contours_5m output=elevation_from_cont5m
# calculate difference map
r.mapcalc "diff = elevation - elevation_from_cont5m"
r.colors diff color=differences
# analyze differences statistically
r.univar diff

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