Dune Geomorphometry

Dune migration for asymmetric dunes.  All methods require:

1. Two grids of the same area at different times
2. Select on Migration over time submenu on the Raster GIS menu

Method 1, using linear dune crests

1. Create a slope grid.
2. Identify slip faces by slope.  Set all slopes less than the threshold (50% might be a good initial selection) as missing.
3. Find edges with a directional filter.  The edge direction to pick is the opposite of the migration direction.
4. To display on the base map, export the edge grids as a database and plot them.
5. Pick Migration over time, Feature migration, grids with linear features on the Raster GIS
6. Resulting table will have the motion at each point on the crest that can be matched in the two grids.

Crests from two time periods.  The first is in green, and is derived from the DEM on which both are overlaid.

Method 2, using grid correlations

1. Algorithm requires three parameters:
   • interval between computation points,
   • size of the regions (which can overlap).  Regions must be large enough to contain a significant proportion of dunes, as this will not work in flat interdune regions. 
   • maximum distance to consider for shifting the two grids.
2. Algorithm shifts one grid relative to the other one grid node at a time, computes the correlation coefficient the elevation in the stationary grid and the shifted grid. The offsets for the highest correlation define the computed migration.

Method 3, using slip face objects

1. Create slip face objects.
   • Create slope map
   • Mark as missing all slopes less than threshold (e.g. 50%)
   • ID features to get grid with each slip face having a unique code.

Method 4, using slip face point clouds

1. For both time periods:
   • Create slip face objects
   • Open grid with slip face objects
   • You might have to change the display to elevation so they appear (will not effect results, but good verification)
   • If missing data appears as 0, set it to missing.
   • Open LAS files for point cloud
   • Extract valid grid, to create a LAS file with just the points on the slip faces
2. Open the two LAS points clouds in CloudCompare.  You will have to recenter them both (use "Yes to all").  If you are going to export the cloud-cloud distances, you might want to skip importing the scalar fields.
3. Select (highlight) the two point clouds.
4. Tools, distances, Cloud/cloud dist.  Pick the reference and compared cloud. You will get distances for every point in the compared cloud, as scalar fields.
5. On the Distance computation form, check "split x,y,z components"
6. Pick compute
7. Pick OK to close window
8. Highlight one cloud.
9. Set the active scalar field.  There should be four choices for the shift between the two clouds, the absolute distances and x, y, z componnents.
10. Tools, statistics, compute stat params. 
11. File, Save As, and pick the ASCII export.
12. If you have skipped the scalar fields when you imported the LAS files, you can Import in MICRODEM.  This file may well be too big for the normal import; you can thin the file.

Last revision 10/7/2013