Choice on Raster GIS menu,
migration over time submenu.
- Identify two DEMs:
- Fixed or referenceDEM will be held in place. For a time series, this
should be the earlier DEM.
- DEM to shift will be moved until the best
correlation is obtained, using all points in the reference DEM
and comparing them to the lagged shifted DEM. The shift will be in
whole grid units of the shifting DEM, which might
have different spacing than the reference DEM
(1" versus 3", or UTM versus geographic).
- The computation will use the
elevations at grid nodes in the fixed DEM, and interpolate in the
shifting DEM (if the grids have the same spacing and projection, the
program will use the grid node elevations in both DEMs).
- The user specifies the maximum lag to consider in each direction
(if you are looking at something like dune migration and you know
the direction, it does not make sense to test lags that you know
will not be reasonable).
- The users specifies the size of the analysis region.
- The user specifies the sampling interval, or how far to move the
sampling regions before resampling. This will determine the
speed of the operation
- The shift will be computed as a
integer lag (pixel size) in both the x and y directions, and for each possible
lag the program will compute the correlation coefficient between the
elevations in the two DEMs. The reported lag will have the largest
- The program will create a database, and for each sampling
location, will report the integer lags in the x and y direction, as
well as the floating point lag along the diagonal.
- The database can be used for any desired statistical operations
(average, min, max, lag distribution), and to visually look for any
spatial patterns in the displacements.
- The displacement direction can be computed from the x and y
- The average displacements for the entire region can be computed
from the sampling point data.
- Sub pixel shifts for individual points not allowed, but the
average will be a fractional value.
Lag range to consider (pixels); this should be the smallest value
that you reasonably expect, or you should be prepared to wait a long
time. If you know the lags will be in a particular direction,
say with a migration landform like a dune, this does not have to be
the same in all directions
Search radius (pixels): size of the box in pixels in the fixed or
reference DEM to use for picking the best lag.
Lag sampling (pixels): moves the center point, which determines the
number of regions over which the computation will be done.
Smaller sampling distance, more time to compute
All open DEMs: does the computations for all open DEMs, which of
course will take longer.
Compute: figures the best lag for either the entire DEM or the
Region size; interactive used to assess the effect
of how big a region must be considered
This was initially designed for hole filling SRTM with ASTER data
and Guth, 2008), before the
release of GDEM. It was redone to monitor change in grids,
for instance with dune crest migration. The latest changes were to see if
there are shifts between the
Global Regional DEMs
Example comparing shifts
Last revision 3/29/2022