Reprojecting a data set creates a new data set with different coordinates.  This can be done for two reasons:

  1. Change the datum.  Taking a data set on NAD27, and changing all the coordinates to NAD83.
  2. Change the projection.  You could take data in a projected coordinate system like UTM, and change the coordinates to geographic lat/long, or do the reverse.

If you want to display the data as a KML file in Google Earth, only one projection and datum are allowed (geographic coordinates with WGS84), and reprojection will be required unless the data already complies with the requirement.

The reversibility of the reprojection depends on the kind of data.

  1. For vector data, you are just taking the coordinates in one reference system and computing them in another.  Within the numerical precision of the computer, this should be completely reversible.
  2. For raster data, a datum shift should be reversible since it only affect the coordinates of a single corner point.  For a change in coordinate system, the location and spacing of the points in the grid will change, which will require interpolation.  This will not be reversible, and hence reprojection will introduce error into the data.

Last revision 9/23/2011

DEM grids for 30 m UTM spacing (triangles) and 1” geographic spacing (squares) for a location in Wyoming. The UTM grid is square, while the geographic grid is almost rectangular. This mismatch varies over the map area, changing slowly in the north-south direction and rapidly east-west. If you reproject from one coordinate system to another, you have to change the locations of the points in the grid, and presumably also their values.  The number of points in the grid will likely change as well.

Reprojecting in MICRODEM

  1. For vector data, MICRODEM only allows the geographic projection in WGS84.  If you have a shapefile with another projection or datum, you must reproject the shapefile before you use it.
  2. For raster DEMs or other grids, the File, Save DEM, Reinterpolate, UTM or lat/long can do a reprojection by selecting the type of projection you want, and the grid spacing.

2006 NLCD, in conic projection.

NLCD2006 reprojected to match the DEM in a geographic projection.  Note the changed orientation of the gaticule. 

Last revision 11/18/2017