Ed, Depends of your region of interest and which analysis (mostly database or just user drawing on a map) and how much of it you will be doing. Also what kind of tool you will be using to render your maps.
If you are covering the whole globe - you might be best to leave your data in long lat and transform to an appropriate UTM zone or other regional zone as needed or have your mapping app do the transformation for you to the base map layer projection. (a lot of mapping software can do this) For display purposes long lat (4326, 4269) looks squashed so I usually go with US National Atlas (2163 which provides a nice balance of look and measurement but really only works for continental US) or Mercator/Google Mercator (which sucks for measurement but looks decent on a map and covers the globe) . If you are just dealing with overlaying points on a map (and not polygons, lines etc then the issue of the skewedness of polygons etc is a non-issue so you can keep in 4326) but you'll probably want to maintain a functional index on the based on transform of the UTMzone. By far a state plane zone or UTM zone is preferable for any kind of intense measurement analysis and for presentation on a map. You could transform on the fly but if you are doing enough of that, it will affect performance a little and is an added hassle since you will also want to keep a functional index on that transformation. Anyrate 2 resources I always recommend if you haven't looked at them already 1) http://spatialreference.org - lists both well-known and user contributed projections and for each provides the PostGIS sql insert state to insert into the spatial_ref_sys table if the spatial ref doesn't already exist in PostGIS. 2) http://postgis.refractions.net/support/wiki/index.php?plpgsqlfunctions - the UTMZone function which given a point geometry will return the SRID of the UTM zone the point falls in. Hope that helps, Regina -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Edward Q. Bridges Sent: Friday, June 27, 2008 8:26 PM To: postgis-users@postgis.refractions.net Subject: [postgis-users] Selecting an SRID Hi, I'm relatively new to PostGIS and to GIS concepts in general, and I'm working on a project which has a goal of being able to identify points within regions (basically). In enabling a geometry column I'm stuck on how to decide on which SRID to use. The data will come into the system as lon/lat; and it will be used as both lon/lat (i.e. to show system users points within regions) and in meters (i.e. to tell users how far away one point is from another). Other sorts of analyses will be what regions are near a region, what points are near a point, and what regions and points are enclosed by another region. Will my choice of SRID affect how efficiently these analyses can be done? Is it possible to use one SRID for all of these different usage scenarios? Thanking in advance for any help, or useful ideas. Best regards, Ed _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
