[postgis-users] Traverse set distance along a multiline?
pramsey at opengeo.org
Mon Dec 27 16:15:26 PST 2010
Right, use 4269, that's a good NAD83-geographic-coordinates number.
Import with shp2pgsql -s 4269 and go from there.
On Mon, Dec 27, 2010 at 3:54 PM, Aren Cambre <aren at arencambre.com> wrote:
> 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:
> 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?
> On Mon, Dec 27, 2010 at 12:22 AM, Paul Ramsey <pramsey at opengeo.org> wrote:
>> You need to do your analysis in a projected coordinate system, not
>> 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
>> 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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