Scaling DEMs and flowdirection
No, this page is not about how measures like slope vary with scale.
It describes techniques for scaling DEMs and flowdirection rasters
to preserve the maximum hydrological flow information from the high-resolution
raster. For example, one might want to use 2-meter or 10-meter DEMs to build
150-meter DEMs and flowdirections for the DHSVM model, or use 3-second DEMs
to build 5-minute cells for the VIC model.
DEMs
Information will be added soon.
Flowdirection grids
This sections refers to the ESRI method of encoding a unique flow direction
from a grid cell to one of its eight neighbors.
Instead of building a course DEM, filling sinks, and calculating a flow
direction grid, and it possible to track the high-resolution flow
(e.g. of water) over the boundaries of the large cells, and to reflect it
in the large cells. The result is a DEM that may not correspond exactly
to any DEM, but reflects actual flows with high accuracy.
flowscale.py is internally documented for now.
I will add better instructions.
I will also build a pushbutton version that incorporates
calls to ESRI libraries.
Direction tweaking
One way of tweaking flowdirection grids in arcmap is to
- Download and unzip fixarrows.zip
- Open fixarrows.lyr. You will see points IDs in the table of contents
represented by arrow symbols. In the catalog, select fixarrows.shp,
and make sure that the projection matches your flowdirection raster.
- In arcmap, execute "streamline" with your flowdirection file as
both inputs. This will create a dense flowdirection tree.
- Lay this over other reference layers such as mapped basins and streams,
or orthophotos.
- Open fixarrows for editing. If you wish to alter a flowdirection, select
the appropriate arrow from the create_features window and click to add
a point. Take care not to create infinite loops.
- When you are satisfied, save your edits to fixarrows.shp, convert in to
a raster matching the cell size of your flowdirection grid, and snapped
to that grid.
- Mosaic that raster over your original flowdirection raster.
Harvey Greenberg
hgreeness49@gmail.com
University of Washington
Department of Earth and Space Sciences
Box 351310
Seattle WA 98195-1310