Josh C. Hickman wrote: > Hello listers,
> I'm having a bit of a GIS brain fart... When a Map Projection for > Pennsylvania says Albers Conical Equal Area, which projection does > that relate to in MapInfo? I chose the Regional Equal-Area > Projections from the drop down list and then selected the Equal-Area > Projection (United States). Is this the same as Albers Conical Equal > Area?? The problem I am experiencing is that when I load DOQQ data > in that is in UTM 17, the vector data dosn't stretch to match the raster. > This is a good indicator that the projection system I chose is wrong. Josh: The short answer is, you're going to have to go to whoever supplied your vector data (or dig through its documentation) and get all of the data's projection information. "Albers Conic" is simply not enough information. The long answer is, terminology is a little confused here, since people frequently use the word "projection" to mean "coordinate system". When you select a "projection" from a dialog box in MapInfo, you're really selecting from a list of coordinate systems, despite the fact that many of the coordinate systems call themselves "projections". A projection is an abstract set of mathematical formulae for transforming latitudes and longitude into a coordinate system. A coordinate system associates coordinate pairs (i.e. (X, Y) values) with points in some space. In MapInfo, this space is usually the surface of the Earth. When you put vector data in a map, and the data has a different coordinate system from the one associated with the map, MapInfo is able to use the definitions of the two coordinate systems to draw the vector data in map's coordinate system. To make a specific coordinate system from a particular projection, you need to know which plug the values of several parameters into the formulae (besides latitudes and longitudes). In the case of the Albers Conic projection, you need: - Datum - coordinate units - Latitude and longitude of origin - the latitudes of two standard parallels - a "false easting" and a "false northing" MapInfo's "Equal-Area Projection (United States)" is really a coordinate system. I suspect it uses the Albers Conic Projection, since that's the best equal-area projection to use for a map of the entire United States. But the parameters used to make this coordinate system are probably not the same as the ones for the coordinate system your vector data came from. So, you'll have to find out those parameter values. Appendix H of the MapInfo User's Guide explains all of these concepts. Hope this helps Spencer --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 11293
