You didn't say exactly what type of lat-long DEM you started with, but if it was rectangular tiles what you're seeing is the difference between a rectangular grid in lat-long and a grid in the UTM projection.
If you visualize a globe or look at Google Earth you'll see that the longitude lines converge toward the poles, so that the width (east-west extent) of the grid would be less at the top than at the bottom (in the northern hemisphere). The transverse Mercator projection maps the ellipsoid onto a horizontal cylinder. This causes the scale factor of the projection to change with distance from the central meridian (longitude of the center of the UTM zone), which also distorts your lat-long grid somewhat. The answer is to get the UTM coordinates you want for the tile corners, convert those to lat-long, extract the corresponding area from the original data, and then transform that non-rectangular area back into UTM. Hope this is helpful. Chris J. Chris McGlone, Ph.D., CP Technical Fellow SAIC 14668 Lee Rd. Chantilly, VA 20151 703-676-9228 703-537-9233 (blackberry) 703-676-9300 (fax) [EMAIL PROTECTED] 2008/8/7 Tueller, Shayne R Civ USAF AFMC 519 SMXS/MXDEC < [EMAIL PROTECTED]> > Using the GDAL tools, I've built a mosaic of terrain tiles that I'm > stitching together using osgdem to create a terrain database. Each terrain > tile uses a UTM mapping that is reprojected from a lat/long mapping. > Unfortunately when I do the reprojection for each tile, the terrain tile > gets slightly shifted so that when I stitch them all together using osgdem, > I have discontinuities at each tile boundary in the finished terrain > database. > > > > My question is, is there a way to avoid this nasty artifact using either > GDAL or osgdem? Perhaps I need to resort to another tool to massage the data > before handing it over to osgdem? > > > > Thanks for any input in advance? > > -Shayne _______________________________________________ osg-users mailing list [email protected] http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
