|ENVI User's Guide: Map Tools|
ENVI includes many standard map projections such as Universal Transverse Mercator (UTM) and Gauss-Kruger. For more information about ENVI's supported map projections, see ENVI Map Projections. Use Customized Map Projection to enter other known map projections and to create your own customized map projections such as UTM projections with other datum or ellipsoids. You have complete control over all of the map projection parameters including projection type (Transverse Mercator, Lamberts Conformal Conic, Lambert Azimuthal Equal Area, Oblique Mercator (A and B), Stereographic, Albers Equal Area, Polyconic, and others), Ellipsoid, Datum, False Easting and Northing. Details of specific map projections can be found in Snyder, 1982, USGS Bulletin 1532, "Map Projections Used by the U. S. Geological Survey." Details on datums can be found on the following internet web pages.
http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html and http://www.connect.net/jbanta/.
ENVI's UTM projection uses the North American 1927 datum and Clarke 1866 ellipsoid.
map_proj.txtlocated in the ENVI directory structure (see ENVI Map Projections File).
When loaded, all of the parameters can be edited.
If a different ellipsoid is desired:
To change the datum:
The ellipsoid that corresponds with the selected datum is shown next to the Ellipsoid label.
The parameters vary depending upon the projection selected. For example, for transverse mercator, the central meridian and base latitude are required along with a scaling constant (this constant is the scale on the central meridian, used so that the mean scale of the entire map is more correct). Polyconic requires the projection origin. Lamberts Conformal Conic and Albers Equal Area projections require the projection origin and two standard parallels. See Snyder, 1982 for details.
The available projections are modified for the current ENVI session and you will be asked if you want to save the changes to the
map_proj.txt file when you close the dialog (see ENVI Map Projections File).
To save the new or modified projection information, select File Save Projections.
map_proj.txt, located in the ENVI directory structure, is modified to contain the new projection. You can edit this file using any text editor as an alternative to the interactive definition above.
ENVI Online Help (August 12, 2005)