[postgis-users] Traverse set distance along a multiline?
pramsey at opengeo.org
Sun Dec 26 22:22:18 PST 2010
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
EPSG:3081 should be a good coordinate system for working with your Texas data.
Not that the units are meters, so perform the appropriate linear
transformations when looking for mile markers.
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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