OK, now you have your DTM, what the heck do you
do with it? The following discussion will focus on DEMs,
as TINs are limited in their modelling capabilities.
First, we need to worry about basic dem errors (spurious
peaks, pits, and holes). We discussed methods of correcting these
errors.
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Slope angle calculations. These are
probably the most common calculation from a DEM. They
are used for all sorts of applications, however, users tend
to just accept what the computer calculates. Even though there
are huge differences in the different methods of calculation.
If I were you, I'd carefully read the Dunn and Hickey paper
(2nd set of readings) prior to the exams, as this is an important
topic. We went through, in detail, three different methods
for calculating slope: averaging techniques (there are many,
we only used one), maximum slope, and maximum downhill slope.
If I were you, I would be ready to calculate slope on the
final exam.
No class on Wed. Veteran's Day.
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Aspect (flowdirection) calculations.
Aspect and flowdirection are often defined the same way.
The way I visualise it, you are standing in the middle of
a cell and looking directly downhill. That direction
is both the aspect of that bit of slope AND the direction
in which water will flow. GIS programs often calculate
this as a 0 - 360 degrees type answer, however, for any sort
of modelling, the answer will be 1 - 8, as those are the maximum
number of adjacent cells. As with the slope angle calculations,
there are a number of different algorithms for calculating
aspect. As usual, beware. The code that I have
developed for Arc/Info GRID and IDRISI calculate flowdirection
as a 1 - 8 value in a direction that exactly corresponds to
the maximum downhill slope angle. As you might expect,
I like my slope and aspect calculations to be done in the
same manner and having overlapping characteristics.
Fortunately, I suppose, aspect calculations are not as commonly
used, nor are they typically as important as slope angle.
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Slope length calculations: The length
of a slope from a ridge to a stream (roughly). These are done
as an input to erosion models. Simply put, the farther
water flows down a slope, the faster it flows. This
is obvious in any roadcut - the gullies are deeper at the
bottom of the slope than higher up. The only code currently
available for these calculations within Arc/Info or IDRISI
is downloadable from my home page. It is important to
note that the slope length is often reset where there is a
significant decrease in slope angle. This is done because
as the slope decreases, flow speeds decrease, and you get
deposition rather than erosion. Therefore, the longer
slope argument no longer applies....
We also discussed the universal soil loss equation
in considerable detail. A=RKLSCP. Be sure you know what all the
factors represent, how the computer calculates them (or where
you get the data), and the value of the output.