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Building Customized Map Projections

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/.


Note
ENVI's UTM projection uses the North American 1927 datum and Clarke 1866 ellipsoid.

  1. Select Map  Customized Map Projection, or click New in any dialog where map projection selection is available.

    2.  

  2. When the Customized Map Projection Definition dialog appears, either enter a new projection name and parameters, or select Projection  Load Existing Projection to select from a list of "standard" projections contained in the file map_proj.txt located in the ENVI directory structure (see ENVI Map Projections File).

    3.  

    When loaded, all of the parameters can be edited.

     

  3. Enter or modify the projection name in the Projection Name: text box, if desired.

    4.  

  4. Select the Projection Type from the list of supported projections (see Map Projections).

    5.  

  5. Click the arrow toggle button to select Projection Ellipsoid: or Projection Datum and select from the following options:

    6.  

    If a different ellipsoid is desired:

    1. Select User Defined from the list.

      2.  

    2. Enter the desired A and B values to define the ellipsoid (See Snyder, 1982).

      3.  

      To change the datum:

       

    3. Select the desired datum from the scrolling list.

      4.  

      The ellipsoid that corresponds with the selected datum is shown next to the Ellipsoid label.

       

    4. Toggle back to the Projection Ellipsoid list and select this ellipsoid.

     

  6. If a false easting and northing is desired (usually used to keep map coordinates from being negative), enter the values in the corresponding text boxes.

    7.  

  7. Enter the Projection Origin latitude and longitude in degrees, minutes, and seconds or select Options  DMS <-> DD and enter latitude and longitude in decimal-degrees. You can also select Options  Extract Projection Origin From Image to set the latitude/longitude to the center pixel location of an existing georeferenced image.

    8.  

  8. Enter the projection-type-specific parameters in the available text boxes on the right side of the dialog.

    9.  

    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.

     

  9. Select Projection  Add New Projection to add the projection to the list of projections used by ENVI.

    10.  

    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.

     

    The file 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.

     

  10. Select File  Cancel to exit the function.

  ENVI Online Help (August 12, 2005)