You have encounter "the Mapinfo canadian problem" Mapinfo has a problem converting data from NAD27 to NAD83 and vice versa IN CANADA. It's a known Mapinfo limitation. Personnaly, I've been asking to solve since version 5,5 or so...
The NAD83 conversion used in Mapinfo is made using equations for AMERICAN standards. In Canada, we use a set of reference points called NTV2 and equations made to use to reference point to respect Canadian standards. The results of using the Mapinfo internal transformation that are not Canadian based will create errors that can be a few meters to 20-30 meters off! (and maybe more...) You have to use an external tool to do so like the FME from SAFE. This tool allows the usage of a transformation grid. You could then set it to use the NTV2 grid file or other more local grid file (some province have local, more dense grid files) Like I said, Mapinfo is aware of this situation but never supported the Canadian standards. I asked before every upgrade of Mapinfo Pro without any results... It's a real pain but if you want to have precise data to transform them, you have to use an external software at this point. FME from safe (www.safe.com) can be downloaded and active for a few days as evaluation. I'm not sure if the NTV2 file is provided with it. You may have to get it from Natural Resources Canada... Hope this help GH -----Original Message----- From: Susan Yu [mailto:[EMAIL PROTECTED] Sent: Tuesday, February 22, 2005 09:32 To: [email protected] Subject: MI-L NAD27 to NAD83 coordinate conversion Hello, I have a bunch of coordinates in NAD27. I would like to convert them to NAD83. So what I've done is create points, setting the projection at Longitude/Latitude (NAD27 for Canada). Then I saved the table as Longitude/Latitude NAD83. To retrieve the new NAD83 coordinates, I used update column's "centroidx" and "centroidy" functions. This seemed to work okay except for coordinates with longitude less than -100.00000. In fact, anything less than -100.00000, the new NAD83 defaults to -99.9999... like it can't calculate past that boundary. All of my NAD27 coordinates are everywhere in Canada from BC to the Atlantic provinces. My knowledge of datums and projections is not the strongest, so I was hoping someone could shed some light on why this occurs. Thanks, Susan --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 15382 --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 15384
