DESCRIPTION

r.import imports a map or selected bands from a GDAL raster datasource into the current location and mapset. If the projection of the input does not match the projection of the location, the input is reprojected into the current location. If the projection of the input does match the projection of the location, the input is imported directly with r.in.gdal.

NOTES

Resolution

r.import reports the estimated target resolution for each input band. The estimated resolution will usually be some floating point number, e.g. 271.301. In case option resolution is set to estimated (default), this floating point number will be used as target resolution. Since the target resolution should be typically the rounded estimated resolution, e.g. 250 or 300 instead of 271.301, flag -e can be used first to obtain the estimate without importing the raster bands. Then the desired resolution is set with option resolution_value and option resolution=value. For latlong locations, the resolution might be set to arc seconds, e.g. 1, 3, 7.5, 15, and 30 arc seconds are commonly used resolutions.

Resampling methods

When reprojecting a map to a new spatial reference system, the projected data is resampled with one of four different methods: nearest neighbor, bilinear, bicubic iterpolation or lanczos.

In the following common use cases:

nearest is the simplest method and the only possible method for categorical data.

bilinear does linear interpolation and provides smoother output than nearest. bilinear is recommended when reprojecting a DEM for hydrological analysis or for surfaces where overshoots must be avoided, e.g. precipitation should not become negative.

bicubic produces smoother output than bilinear, at the cost of overshoots.

lanczos produces the smoothest output of all methods and preserves contrast best. lanczos is recommended for imagery. Both bicubic and lanczos preserve linear features. With nearest or bilinear, linear features can become zigzag features after reprojection.

For explanation of the -l flag, please refer to the r.in.gdal manual.

When importing whole-world maps the user should disable map-trimming with the -n flag. For further explanations of -n flag, please refer the to r.proj manual.

EXAMPLE

Import of a subset from Bioclim data set, to be reprojected to current location projection (North Carolina sample dataset). While normally the full raster map is imported, we spatially subset using the extent parameter:

# download selected Bioclim data
wget http://biogeo.ucdavis.edu/data/climate/worldclim/1_4/grid/cur/bio_2-5m_bil.zip

# extract BIO1 from package:
unzip bio_2-5m_bil.zip bio1.bil bio1.hdr

# set computational region to North Carolina, 4000 m target pixel resolution
g.region -d res=4000 -ap

# subset to current region and reproject on the fly to current location projection,
# using -n since whole-world map is imported:
r.import input=bio1.bil output=bioclim01 resample=bilinear \
         extent=region resolution=region -n

r.info bioclim01
r.univar -e bioclim01

KNOWN ISSUES

The option extent=region only works when the dataset has a different projection than the current location (i.e., internally r.proj is invoked).

AUTHORS

Markus Metz
Improvements: Martin Landa, Anna Petrasova

Last changed: $Date: 2015-01-20 20:52:27 +0100 (Tue, 20 Jan 2015) $