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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | Re: GISList: converting a grid into square polygon |
| Date: |
07/31/2002 03:19:22 PM |
| From: |
Quantitative Decisions |
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At 09:13 PM 7/23/02 +0800, Shih-Liang Chan wrote:
>I'm looking for a tool to convert every single cell of a grid theme into
>square polygons in shapefile format. I'll be highl appreciated if someone
>can provide me with any suggestion. Thanks,
The terminology used in this question indicates you are using Spatial Analyst.
Populate a grid with random integers and then convert that to a shapefile.
The Map Calculator expression is
(grid.MakeRandom * 1000000).int
Provided no two neighboring cells have the same value, the result will be
perfect. It is highly unlikely that two neighboring cells will have the
same value, so if you get fewer shapes than you expect, simply try again.
(In any grid of dimensions M by N, there will be 2*M*N - M - N pairs of
neighboring cells, and therefore the chance of all pairs having distinct
random values is approximately 0.999999^(2*M*N-M-N), which is about
0.999998 raised to the number of grid cells. This chance is close to 100%
until the number of grid cells gets into the hundreds of thousands. One
could increase the chance of success even further by using random integers
with a range greater than 1,000,000, but limitations in Spatial Analyst
preclude that.)
This method will not transfer the grid theme attributes to the shapefile
polygons. One way to get the attributes is to add the centers of the
shapefile squares as a theme in the view (compute the x, y coordinates of
the polygon centers, then add the table as a point event theme). Convert
the original grid to a shapefile in the usual way. Perform a spatial join
of that shapefile to the polygon centers. The rest is easy.
--Bill Huber
www.quantdec.com
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