Brent and Paul,<div><br></div><div>Thank you for your help!</div><div><br></div><div>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.</div>
<div><br></div><div>I don't think it has a projection because this table's corresponding entry in the <b>geometry_columns</b> table has <b>-1</b> for the <b>srid</b> column.</div><div><br></div><div>This ShapeFile's PRJ file has this:</div>
<div><b><font class="Apple-style-span" face="'courier new', monospace">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]]</font></b></div>
<div><br></div><div>I'm not seeing a clear match between this and any projection.</div><div><br></div><div>Some Google searching suggests this may be 4326, but I'm not sure about this. And if I modify <b>geometry_columns</b> and import the PostGIS table into QGis, I get this QGis error:</div>
<div><div><b><font class="Apple-style-span" face="'courier new', monospace">1 cursor states lost.</font></b></div><div><b><font class="Apple-style-span" face="'courier new', monospace">SQL: CLOSE qgisf0</font></b></div>
<div><b><font class="Apple-style-span" face="'courier new', monospace">Result: 7 (ERROR: current transaction is aborted, commands ignored until end of transaction block</font></b></div><div><b><font class="Apple-style-span" face="'courier new', monospace">)</font></b></div>
</div><div><br></div><div>If I revert that field back to <b>-1</b>, the error goes away on next import.</div><div><br></div><div>When I imported using <b>shp2pgsql</b>, I didn't use the <b>-s</b> switch. I presumed it would catch the projection automatically.</div>
<div><br></div><div>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?</div><div><br></div><div>Aren<br><br><div class="gmail_quote">
On Mon, Dec 27, 2010 at 12:22 AM, Paul Ramsey <span dir="ltr"><<a href="mailto:pramsey@opengeo.org">pramsey@opengeo.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
You need to do your analysis in a projected coordinate system, not geographics.<br>
<br>
CREATE TABLE my_new_texas_roads AS<br>
SELECT ST_Transform(the_geom, 3081) as the_geom, other_attributes<br>
FROM texas_roads;<br>
<br>
EPSG:3081 should be a good coordinate system for working with your Texas data.<br>
<br>
<a href="http://spatialreference.org/ref/epsg/3081/" target="_blank">http://spatialreference.org/ref/epsg/3081/</a><br>
<br>
Not that the units are meters, so perform the appropriate linear<br>
transformations when looking for mile markers.<br>
<font color="#888888"><br>
Paul<br>
</font><div><div></div><div class="h5"><br>
On Sun, Dec 26, 2010 at 4:35 PM, Aren Cambre <<a href="mailto:aren@arencambre.com">aren@arencambre.com</a>> wrote:<br>
> I am trying to determine mile markers along Texas highways. My starting<br>
> point is the ShapeFile TxDOT Roadways 2010<br>
> at <a href="http://www.tnris.state.tx.us/datadownload/download.jsp" target="_blank">http://www.tnris.state.tx.us/datadownload/download.jsp</a>. I've used<br>
> shp2pgsql to get it into a PostGIS 1.52-enabled Postgres 9.01 database.<br>
> I naively thought I could just figure out the number of miles per unit of<br>
> latitude and then traverse each roadway, one mile at a time,<br>
> using ST_Line_Interpolate_Point. However, predictably, the more<br>
> "longitudinal" a route, the more error it shows when I compare my calculated<br>
> mile markers to what Google Maps shows.<br>
> Again, this is because I was using a consistent ratio of degrees to miles,<br>
> so any route E-W component introduces errors.<br>
> So here's the question--does PostGIS allow any way to traverse a route a set<br>
> distance at a time? Specifically, is there a way I can traverse a route a<br>
> mile at a time and then record the points at the end of each mile?<br>
> I reviewed the functions available<br>
> at <a href="http://postgis.refractions.net/documentation/manual-1.5/reference.html" target="_blank">http://postgis.refractions.net/documentation/manual-1.5/reference.html</a> and<br>
> am not seeing anything clear.<br>
> In case it matters, the SHP's PRJ file says NAD83.<br>
> Aren Cambre<br>
</div></div><div><div></div><div class="h5">> _______________________________________________<br>
> postgis-users mailing list<br>
> <a href="mailto:postgis-users@postgis.refractions.net">postgis-users@postgis.refractions.net</a><br>
> <a href="http://postgis.refractions.net/mailman/listinfo/postgis-users" target="_blank">http://postgis.refractions.net/mailman/listinfo/postgis-users</a><br>
><br>
><br>
_______________________________________________<br>
postgis-users mailing list<br>
<a href="mailto:postgis-users@postgis.refractions.net">postgis-users@postgis.refractions.net</a><br>
<a href="http://postgis.refractions.net/mailman/listinfo/postgis-users" target="_blank">http://postgis.refractions.net/mailman/listinfo/postgis-users</a><br>
</div></div></blockquote></div><br></div>