The green lines are the original rivers. The black lines are the
rivers as traced downhill.
Profile files have 6 columns:
- distance downstream (meters)
- elevation (after fixing)
- raw elevation
- contributing area (number of cells)
The proffix2.aml profile-tracing algorithm
Profiles were traced with the ARC/INFO macro proffix2.aml, written by
Harvey Greenberg and based on a macro by Rolf Aalto. The macro addresses
the major difficulties in tracing river profiles from DEMs.:
Geographic projections are not equidistant, but they will will lose
information if converted to equidistant projection.
This macro can work on a geographic projection, using different distance
values for vertical, horizontal, and diagonal lines.
Profiles through DEMs (at least in the ARC/INFO environment) yield a
bilinear interpolation of the four lattice points nearest to the sample
points. Therefore river profiles include points partway up the river
This macro steps (slowly) from lattice point to lattice point, always
getting the lowest elevation of the represented stream bed.
DEMs frequently contain "sinks" from which water cannot flow.
These are usually artifacts.
Depending on the algorithm used in DEM creation, the low values tend to
be correct, while the high values that block flow are the result of
narrow channels "falling through the cracks" of a coarse DEM. The usual
solution is to flag the real sinks, then fill the rest. Flowidrection,
flowaccumulation, and other functions are performed on the filled DEM.
This macro outputs the elevation of the raw DEM.
Most algorithms trace flowdirection across a filled sink, ignoring the
Proffix2.aml,when it encounters a filled sink, will interrogate the raw
DEM and trace the path of steepest descent. When it reaches a low point,
it assigns weights to paths of shallowest ascent and to the direct path to
to sink's pour point, and works its way uphill to that pour point.
Any profile containing raw elevations will have uphill segments at sinks.
When proffix2.aml reaches an uphill part of the profile, it will assign
a user-designated minimum slope to the river. Both raw and fixed profiles
are output. Also, the altered values are saved to a different files so that
they can be used to alter the DEM itself.