Brent and Paul, Thank you for your help!
So here's my (new) dilemma--my PostGIS table doesn't appear to have a projection specified, and I am not clear how to get to one. I don't think it has a projection because this table's corresponding entry in the *geometry_columns* table has *-1* for the *srid* column. This ShapeFile's PRJ file has this: * GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]] * I'm not seeing a clear match between this and any projection. Some Google searching suggests this may be 4326, but I'm not sure about this. And if I modify *geometry_columns* and import the PostGIS table into QGis, I get this QGis error: *1 cursor states lost.* *SQL: CLOSE qgisf0* *Result: 7 (ERROR: current transaction is aborted, commands ignored until end of transaction block* *)* If I revert that field back to *-1*, the error goes away on next import. When I imported using *shp2pgsql*, I didn't use the *-s* switch. I presumed it would catch the projection automatically. I'm at a loss to know what to do next. I guess I need to figure out what the true SRID of this data is before I can do any re-projections? Aren On Mon, Dec 27, 2010 at 12:22 AM, Paul Ramsey <pram...@opengeo.org> wrote: > 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 >
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