Help me, cartographers - you're my only hope
Posted 08 April 2008 - 06:38 PM
Posted 08 April 2008 - 09:49 PM
I'm not sure I understand what you are trying to do. So is this the centroid of each zip code polygon? We work with zip code health data. Zip codes are really weird. They can take any shape and can be non-continuous. If it across the country, I'm not sure what your getting that organizing by state wouldn't. Is this a smaller geographic region than country?
I have a list of zip codes with latitude and longitude information,
You want to partition these into square grid polygons, or is any shape OK? Is there any criteria as to how many zip codes or members should be in each partition? If square grids and no criteria, you can create a regular tessellation of square polygons, then intersect the points with the polygons. What software are you using? If ArcGIS, after creating the grid and use a spatial join to assign the zip codes to your grid. You could then create a look-up-table in your database with zip code and region number.
and I'm trying to organize these into (mostly at least) non-overlapping 40 or 50 mile regions, with the end result being a list of the zip codes at the center of those regions.
Maybe I am not understanding your problem? I find it easier to work with what is the end point question and work back than start with what data you have and work forward.
Posted 09 April 2008 - 06:58 AM
A zip code is not a polygon. It is a mail delivery "route", and is generally linear (though, as Philip pointed out, they may not be contiguous) but can also be points (large institutions often have their own zip code). The Census Bureau (and others, I'm sure) have attempted to come up with zip-code related polygons to do similar analysis as you are trying. Ours are called ZCTAs (Zip Code Tabulation Areas), and you can find more on them at the U.S. Census Bureau ZIP Code Tabulation Area FAQs.
An interesting national map of zip codes rendered as points is at Ben Fry's zipdecode page.
US Census Bureau
Posted 09 April 2008 - 08:48 AM
Do you have access to GIS software? This is all doable with ESRI software, but I believe Thiessen polygons require a coverage input, so you would need the full ArcInfo license (I think).
Try googling "thiessen polygon" and see if that's the direction you want to go in. Then find someone with ArcMap.
I hope that helps...
Cartographer, Bureau of Land Management
Oregon State Office
Posted 09 April 2008 - 09:06 PM
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