You need to do your analysis in a projected coordinate system, not geographics.
CREATE TABLE my_new_texas_roads AS SELECT ST_Transform(the_geom, 3081) as the_geom, other_attributes FROM texas_roads; EPSG:3081 should be a good coordinate system for working with your Texas data. http://spatialreference.org/ref/epsg/3081/ Not that the units are meters, so perform the appropriate linear transformations when looking for mile markers. Paul On Sun, Dec 26, 2010 at 4:35 PM, Aren Cambre <a...@arencambre.com> wrote: > I am trying to determine mile markers along Texas highways. My starting > point is the ShapeFile TxDOT Roadways 2010 > at http://www.tnris.state.tx.us/datadownload/download.jsp. I've used > shp2pgsql to get it into a PostGIS 1.52-enabled Postgres 9.01 database. > I naively thought I could just figure out the number of miles per unit of > latitude and then traverse each roadway, one mile at a time, > using ST_Line_Interpolate_Point. However, predictably, the more > "longitudinal" a route, the more error it shows when I compare my calculated > mile markers to what Google Maps shows. > Again, this is because I was using a consistent ratio of degrees to miles, > so any route E-W component introduces errors. > So here's the question--does PostGIS allow any way to traverse a route a set > distance at a time? Specifically, is there a way I can traverse a route a > mile at a time and then record the points at the end of each mile? > I reviewed the functions available > at http://postgis.refractions.net/documentation/manual-1.5/reference.html and > am not seeing anything clear. > In case it matters, the SHP's PRJ file says NAD83. > Aren Cambre > _______________________________________________ > 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
