[postgis-users] Traverse set distance along a multiline?

[Paul Ramsey]

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 <aren at 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
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