[postgis-users] Selecting geocoded postal codes within a given distance ( miles for example )

[Paul Ramsey]

You have a couple options.

One is to take your whole data set and translate it into a planar 
projection. If you have a contrained area, that might be a good thing to 
do, because it makes the whole problem simpler.

Two is to essentially two-stage the problem. Problem (a) is subsetting 
your complete data set to just those codes which *might* be within your 
radius. Problem (b) is explicitly testing those codes. To do (b) on a 
lat/lon data set, you need to know the ratio of miles/degrees longitude 
and miles/degrees latitude for your area. You can calculate these with a 
couple selects using length_spheroid and two-point LINESTRINGS 
constructed in our search area. Then you use these distances to 
construct a query box, and used distance_spheroid() to parse out just 
those codes which fall in your search area.

You can see why option one is simpler :)

Hugh W. O'Brien wrote:
> Dear List,
> 
> How would one go about selecting the geocoded postal codes within given 
> a distance ( say in miles ) of another geocoded postal code.  Finding 
> the long/lat of the postal code we are searching around is simple ( no 
> help is needed there ). I have read/seen the faq for postgis.  The 
> example given  there using a bounding box doesn't really fit what I am 
> trying to do exactly because I will be dealing with degrees 
> longitude/latitude and miles ( the example in the faq uses meters 
> exclusively ).
> 
> Thanks in advance for any help you can give.
> 
> -- Hugh W. O'Brien
> 
> 
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users


-- 
       __
      /
      | Paul Ramsey
      | Refractions Research
      | Email: pramsey at refractions.net
      | Phone: (250) 885-0632
      \_