[R] masking a regular lat/lon grid to extract map boundaries

Greg Snow Greg.Snow at imail.org
Wed Oct 8 05:06:21 CEST 2008

Another approach that will probably be more work up front, but may be better in the long run if you are going to be doing a lot of this is to use the maptools and sp packages.  Create a spatialpointsdataframe from your data, read in a us map (there are shapefiles that you can get from the US census bureau website or other places) as a spatialpolygondataframe and use the overlay methods to subset the data.

You may want to post future questions on this topic to the R-sig-geo list for more specific help.

Hope this helps,

Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of claudia tebaldi
> Sent: Tuesday, October 07, 2008 4:09 PM
> To: r-help at r-project.org
> Subject: [R] masking a regular lat/lon grid to extract map boundaries
> Dear R-helpers,
> I have lat/lon coordinates of regularly spaced grid points, about 4Km
> apart, covering the entire US continental region.
> I would like to mask this rectangular grid in order to extract all and
> only the grid points within a specific region.  Today I want to
> extract Montana, say, from this grid, and I am hoping to somehow use
> the returned value of the function map("state",region="montana").
> I'm not espoused to using the "maps" library database, it's just the
> way I thought one could go about it (one smarter than I am...) and it
> would certainly be useful to exploit the many political boundaries
> provided by that library, when I move on from Montana to something
> else...
> Any suggestion for an algorithm to do this?
> Of course if someone out there has a high-res mask for the US states
> I'll happily forget about using map(): I'll take it and interpolate
> (or knn1) away to my grid!
> thank you in advance
> claudia
> --
> Claudia Tebaldi
> Research Scientist, Climate Central
> http://www.climatecentral.org
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