David: You could do a self-consistency check by doing the procedure in reverse...
Start with the second map, create new polygons which closely follow the ones you copied from the first map, create a polygon file like before (this should have the projection info from the second map) and import them into the first map. Only if both the forward and reverse projection transformations are correct will the pairs of polygons in both maps maintain the same spatial relationships with each other... This is not absolute proof that the transformations are accurate, just self-consistent. _______________ Geosoft Inc. Stephen Cheesman [EMAIL PROTECTED] (905) 315-8207 Software and services for effective earth science decision-making. Free Oasis montaj interface now available at http://www.geosoft.com -----Original Message----- From: David Humphrey [mailto:[EMAIL PROTECTED]] Sent: June 25, 2002 10:17 AM To: '[EMAIL PROTECTED]' Subject: [geonet]: Mixing data from different projections on a map The more I work with geodesy, I realize how much more I don't know about it. A project I am currently working on is a case in point. We have a Geosoft map from another source that was constructed with the following parameters: datum: NAD27 ellipsoid: Clarke 1866 Projection: UTM Zone 16N local datum: [NAD27] MEAN CONUS This map contains some polygons that I want to put on a second map. This second map was constructed with the following parameters: datum: WGS 84 ellipsoid: WGS 84 Projection UTM Zone 16N local datum: [WGS 84] World What is the best process that I can use so that I am sure that these polygons are posted in the right place on this second map? One thing that I have done is: 1. In the first map, use Mapping->Get Polygons from a Group. I find that this creates a polygon file with some header records containing projection information. 2. In the second map, use Mapping->Draw from Polygon File. Indeed, the polygons appear on the map. Are they in the right position? I have reason to believe that Oasis montaj uses the header information to do coordinate transformations on-the-fly to insure that the points are properly placed, but want to check with some authorities in the Geosoft community to be sure. Are there simple experiments one can run that will demonstrate this? Regards, David Humphrey Director, Technical Services Bell Geospace, Inc _______________________________________________________ More mailing list info http://www.geosoft.com/support/listserv/index.html _______________________________________________________ More mailing list info http://www.geosoft.com/support/listserv/index.html
