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

[Aren Cambre]

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 <pramsey at 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 <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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> > postgis-users at postgis.refractions.net
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> >
> >
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