R help - Oct 2008 - 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

On 07/10/2008 6:08 PM, claudia tebaldi wrote:
> 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!

I believe this should work:

montana <- map("state", "montana", fill=TRUE, plot=FALSE)
inmontana <- !is.na(map.where(montana, x, y))

(where x and y are the longitude and latitude coordinates of your points).

Duncan Murdoch

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
801.408.8111

> -----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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