Reprojecting a data set creates a new data set with different coordinates.
This can be done for two reasons:
- Change the datum. Taking a data set on NAD27, and changing all the
coordinates to NAD83.
- 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
The reversibility of the reprojection depends on the kind of data.
- 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.
- 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.
||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.
Last revision 9/23/2011
Reprojecting in MICRODEM
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.
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