Hi David, Your method for displaying the polygons is correct. The polygon file with projection information in the header will be reprojected on-the-fly to the projection of the map which you are displaying it in. You can use the reference cursor to compare locations between the two maps.
Another way to accomplish this, perhaps as a back-up or simple test, is to copy the polygon group from the map in NAD27, and paste using Paste Georeferenced on the second map in WGS 84. If you are in doubt, or really keen to check the reprojection, then you can load the coordinates of the polygon into a database, set the projection in the database and then reproject the x and y coordinate channels. Plotting the results on the second map, as symbols from the database or from a new polygon file, should put your mind at ease. Another thing to be aware of is that you can select a more accurate Local Datum Transform than the "MEAN CONUS" to refine your reprojection to the area that you are working in. The East or West of the Mississippi local datum are more accurate, and the NADCON option is more accurate still. If you have any problems, please let me know. Regards, Natalie Green Technical Support [EMAIL PROTECTED] Geosoft Latinoam�rica Ltda. Av. Rio Branco, 156 Gr. 2615 - Centro Rio de Janeiro - RJ - BRASIL - 20043-900 Tel: 55 21 2532-0140 Fax: 55 21 2532-7197 Knowledge from data. Free Oasis montaj interface with public-domain Data Access Protocol (DAP) for accessing high-volume spatial data over the Internet available for download at www.geosoft.com/pinfo/free/downloadfree.html -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of David Humphrey Sent: ter�a-feira, 25 de junho de 2002 11:17 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
