On Fri, May 20, 2011 at 17:12, Rich Shepard <[email protected]> wrote: > On Fri, 20 May 2011, Bryan Keith wrote: > >> I'm not convinced that's a valid wkt description of the coordinate system >> you want. > > Bryan, > > Let me back up and ask the question from a different point. > > Here's the WKT from one of the maps: > > PROJCS["Project", > GEOGCS["GCS_North_American_1927", > DATUM["D_North_American_1927", > SPHEROID["Clarke_1866",6378206.4,294.9786982]], > PRIMEM["Greenwich",0.0], > UNIT["Degree",0.0174532925199433]], > PROJECTION["Transverse_Mercator"], > PARAMETER["False_Easting",500000.0], > PARAMETER["False_Northing",-2000000.0], > PARAMETER["Central_Meridian",-115.583333], > PARAMETER["Scale_Factor",0.9999], > PARAMETER["Latitude_Of_Origin",34.75], > UNIT["Foot_US",0.3048006096012192]] > > From this I created this proj4 description: > > +proj=tmerc +datum=nad27 +ellipse=clrk66 +a=6378206.4 +b=294.9786982 > +x_0=500000.0 +y_0=-2000000.0 +k_0=0.9999 +lat_0=34.75 +lon_0=-115.583333 > +no_defs +units=us-ft > > Is this an accurate description to use in creating the location? Does the > order matter?
Well, let's see how this differs from when I tried to do the same thing: +proj=tmerc +lat_0=34.75 +lon_0=-115.583333 +k=0.9999 +x_0=152400.3048006096 +y_0=-609601.21920243837 +ellps=clrk66 +datum=NAD27 +units=us-ft +no_defs I think a and b are defined by the ellipsoid. Are the a and b you specified the same as what clrk66 uses? I don't know. If you specify both the ellipsoid and a and b, what takes precedence? I don't know. Everything else looks the same except for x and y, the false easting and false northing. In proj.4 those are always specified in meters while in the wkt definition, I think those numbers are in the projection units. That's why I changed the numbers >>> 0.3048006096012192 * 500000 152400.30480060959 >>> 0.3048006096012192 * -2000000 -609601.21920243837 >>> If you use your proj.4 string, does your data end up in the correct location? To answer your other question, I don't think the order matters. These proj.4 questions are outside my area of expertise. Bryan > > Rich > > _______________________________________________ > grass-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/grass-user > _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
