GRASS GIS: r.out.mat

NAME

r.out.mat - Exports a GRASS raster to a binary MAT-File.

KEYWORDS

raster, export

SYNOPSIS

r.out.mat
r.out.mat help
r.out.mat [-v] input=name output=string [--verbose] [--quiet]

Flags:

-v: Verbose mode
--verbose: Verbose module output
--quiet: Quiet module output

Parameters:

input=name: Name of input raster map
output=string: Name for the output binary MAT-File

DESCRIPTION

r.out.mat will export a GRASS raster map to a MAT-File which can be loaded into Matlab or Octave for plotting or further analysis. Attributes such as map title and bounds will also be exported into additional array variables.

Specifically, the following array variables are created:

map_data
map_name
map_title (if it exists)
map_northern_edge
map_southern_edge
map_eastern_edge
map_western_edge

In addition, r.out.mat makes for a nice binary container format for transferring georeferenced maps around, even if you don't use Matlab or Octave.

NOTES

r.out.mat exports a Version 4 MAT-File. These files should successfully load into more modern versions of Matlab and Octave without any problems.

Everything should be Endian safe, so the resultant file can be simply copied between different system architectures without binary translation.

As there is no IEEE value for NaN for integer maps, GRASS's null value is used to represent it within these maps. You'll have to do something like this to clean them once the map is loaded into Matlab:

    map_data(find(map_data < -1e9)) = NaN;

Null values in maps containing either floating point or double-precision floating point data should translate into NaN values as expected.

r.out.mat must load the entire map into memory before writing, therefore it might have problems with huge maps. (a 3000x4000 DCELL map uses about 100mb RAM)

GRASS defines its map bounds at the outer-edge of the bounding cells, not at the coordinates of their centroids. Thus, the following Matlab commands may be used to determine the map's resolution information:

    [rows cols] = size(map_data)
    x_range = map_eastern_edge - map_western_edge
    y_range = map_northern_edge - map_southern_edge
    ns_res = y_range/rows
    ew_res = x_range/cols

EXAMPLE

In Matlab, plot with either:

imagesc(map_data), axis equal, axis tight, colorbar

contourf(map_data, 24), axis ij, axis equal, axis tight, colorbar

TODO

Add support for exporting map history, category information, color map, etc.
Option to export as a version 5 MAT-File, with map and support information stored in a single structured array.

AUTHOR

Hamish Bowman
Department of Marine Science
University of Otago
New Zealand

Last changed: $Date: 2005-12-12 06:53:40 -0800 (Mon, 12 Dec 2005) $

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