Hi Everyone,
I hope you have a simple answer to my hopefully simple question. I have a
file with around 1000 polygons. I need to have displayed or selected only
those polygons that are less than 1544 acres. This file exists in vector
and raster format. What command in GRASS would be used to accomplish this?
I am running GRASS 4.13 on a Sun Sparc 10. Thanks to anyone who can help.
kathy@nrcsga.ga.nrcs.usda.gov
On Tue, 4 Feb 1997, Kathy Burroughs wrote:
Hi Everyone,
I hope you have a simple answer to my hopefully simple question. I have a
file with around 1000 polygons. I need to have displayed or selected only
those polygons that are less than 1544 acres. This file exists in vector
and raster format. What command in GRASS would be used to accomplish this?
I am running GRASS 4.13 on a Sun Sparc 10. Thanks to anyone who can help.kathy@nrcsga.ga.nrcs.usda.gov
Hi, Kathy
In the raster realm (where GRASS has all of its power), you can create a
reclass file to reclass the polygons by their area by running something
like the following command:
r.stats -az in=allfld | awk '{ acre=$2 * 0.000247 ; printf("%d = %d\n",
$1, acre) }' > reclass_rules
(all of the above should be entered on one line)
Then, use it with r.reclass:
r.reclass i=polygon_file o=polygon_areas < reclass_rules
Last, use r.mapcalc to find your polygons under 1544 acres:
r.mapcalc small_areas='if(polygon_areas<=1544, polygon_areas)'
Voila!
Please double-check my square-meter to acres conversion in the awk command.
-Malcolm
--
Malcolm D. Williamson - GIS Specialist E-mail: malcolm@cast.uark.edu
Center for Advanced Spatial Technologies Telephone: (501) 575-6159
Ozark Rm. 12 Fax: (501) 575-5218
University of Arkansas
Fayetteville, AR 72701