Geography Reference
In-Depth Information
11.
By looking at the T/C entry for FlowDir, and by clicking a few cells with the Identify tool, verify
that the cells contain the codes for each direction: east (1), southeast (2), south (4), southwest
(8), and so on. Revisit Figure 8-21 for the complete definition of the code values.
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You may be
able to note visually that much of the water in the study area tends to drain toward the west
(16), with other directions being toward the southwest, and northwest (values of 8 and 32,
respectively). You can also note that, in small areas, water flows in lots of different directions.
12.
Open the attribute table of FlowDir. Run Statistics on the Count column. How many cells are there
total? ________. Sort the Count column into descending order. Select the first three records,
which indicate flows to the west (16), southwest (8), and northwest (32). Look at the map. Run
Statistics. How many cells fall into these three categories? ________. Clear all selections.
As mentioned previously, water needs to ultimately flow to the edge of the raster for this model to work
properly. If it flows to internal cells from which it cannot exit because the surrounding cells are all of
greater elevation, the model gives wrong answers.
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You can check for this situation by applying the Sink
tool to the Flow Direction dataset, to make the raster AnySinks.
13.
Identify sinks:
Turn off all layers. Run the Sink tool on FlowDir. Call the Output raster AnySinks.
Once it is added to the map, you will be able to see many spots where sinks have been
identified. Open its attribute table. How many sinks have been identified? ________
Only if the dataset consisted entirely of NoData values would you be assured that there were no sinks in
the study area. But, unfortunately, this is not the case. Maybe there are real sinks here. Or perhaps
the surface generated by the Interpolate to Raster has sinks in it where none exist in reality. Or maybe the
elevation data was wrong. In any event, you need a sink-free surface to proceed.
To fill sinks, you would normally use more extensive Spatial Analyst or Hydrologic Modeling tools. This
is a time-consuming process and requires knowledge you may not have at this point. Filling sinks is an
iterative process, meaning that filling one set of sinks may generate others. That is, there are ways of
filling sinks effectively, but they are beyond the scope of this text. So you will use a “repaired” layer, called
ElevSurface2, which has all the sinks filled for you.
14.
Start a new map. From the
___IGIS-Arc_
YourInitials
\Spatial_Analyst_Data\Hydrology_Data_SA
folder add the raster ElevSurface2. Re-create the flow direction dataset with ElevSurface2,
making the output FlowDir2. Run the Sink tool on FlowDir2, naming the resulting dataset
AnySinksNow. The raster should consist entirely of NoData. The attribute table for AnySinksNow
should be empty. Remove the AnySinksNow dataset.
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I feel like apologizing for the assortment of codes you have to cope with to determine direction. You have degrees
(starting at both north and east), Euclidean direction (1, 2, 3 … 8), and now you have, powers of two (1, 2, 4, 8, …, 128).
However, I didn't write the software. I'm just the person communicating the convoluted message. If you needed proof
that computer software just develops in unfortunate ways that become “standards” you have it here. But it's not just
computers. Think about the absurd layout of letters on a “standard” keyboard. Or key layout on phone vs. calculator.
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One other problem in modeling the environment rears its head. In the real world, there are such things as sink holes,
which connect the surface water with the under surface water (ground water). Here, we choose to ignore this complication.
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