This Tech Tip provides an overview of many different ways to compute slopes with RiverTools 3.0, and explains the advantages and limitations of each method.
RiverTools 3.0 provides many different tools and methods for computing slopes of channels and hillslopes. In order to get the best results for a particular application, it is important to understand the underlying issues and various limitations of commonly-used elevation data sets. Slopes as measured between adjacent grid cells are
notorious for being inaccurate when the grid cells are
in the bottom of a valley. This is especially true
when elevations have been rounded to the nearest meter
or foot (as with many commonly-used DEMs), because then
the minimum (nonzero) slope is given by 1/cellsize.
For example, DEMs with grid cell sizes of 10 meters and
30 meters have minimum resolvable nonzero slopes of 0.1
and 0.0333, even though real channel slopes are often
much smaller, say between 10local^{-7} and 10^{-3}.
This represents an enormous error that is unacceptable for
most applications. Notice that the minimum resolvable slope
actually as cellsize decreases if
the vertical resolution is 1 meter. Due to this constraint,
cell-to-cell slopes in valley bottoms are often zero for a
considerable distance and then too steep when there is
finally a 1-meter drop in elevation. The key then to
getting a more accurate measurement of channel or valley
slope is to consider a reach that spans enough grid cells
to get around the vertical resolution issue but few enough
cells that a single slope value for the reach is reasonable.
increasesThe , and
these have the compound extensions "_links.rtv" and
"_streams.rtv". These files store numerous attributes for
every link and Horton-Strahler stream in an extracted river
network or set of networks. Straight-line and along-channel
slopes are among this set of attributes, and since links and
Horton-Strahler streams typically span multiple grid cells,
the associated slopes are often much more representative of
actual channel slopes than cell-to-cell slopes. These
channel slopes are optional attributes that can be exported
to an ESRI shapefile using the streamfileExport Vector > Channels
dialog in the File menu. They are also used within
RiverTools by the various tools in the Analyze menu.
In the Strahler Streams menu, for example, statistics
for channel slopes are listed in the Data Summary
under the headings "Straight-line Slope" and "Along-channel
Slope". A convenient way to explore the relationship between
channel slope and contributing area is to use the Strahler
Streams > Horton Plots dialog with an X-axis choice of
"Drainage area" and a Y-axis choice of "Along-channel slope".
RiverTools 3.0 also offers a unique interactive window
tool in the In addition to the In order to automatically create a slope grid that has
more accurate channel slopes, RIVIX has developed a new
algorithm for "profile smoothing" in support of the
Despite their limitations for computing channel slopes,
cell-to-cell or local methods typically give good results
when used to compute slopes on hillslopes or in low-order
basins. RiverTools 3.0 offers many different methods for
computing local slopes in the The preceding discussion has been focused on slope as
defined as a nondimensional ratio of lengths, or "rise over
run". Some applications instead require a "slope angle grid",
which can be computed as the arctangent of the nondimensional
slope. Such a grid can easily be computed using the Grid
Calculator in RiverTools 3.0, which is accessed by choosing
If you have any additional questions or comments, please contact Rivix Technical Support for assistance. Copyright (C) RIVIX, LLC. 1998-2016 |