Casey Stern
2.10.2012
Exercise 2: Basic Terrain Analysis and Flow Path Mapping
Histograms:
[Note: although the instruction say create a histogram for only aspect and DEM, Dr. Band said we should make one for slope as well – otherwise, there would have been no point in computing the slope in the first place]
1. Differentiate between terrain characteristics in each of the subwatershed areas based on the DEM and aspect distributions. How are DEM and aspect distributed in each of the two subwatersheds?
Upper Battle Creek / Middle Battle CreekDEM / · Relatively flat
· According to the histogram, most pixels have similar elevations.
· Overall, the elevation in the upper portion of the basin is higher than the middle portion. / · Varied elevation values.
· Looking at the histogram above, while most pixels had an elevation of approximately 260 ft., the wide distribution of heights suggests a more varied terrain.
SLOPE / · For the most part, the slope observed was much lower than that of the Middle Battle Creek. The low slopes make sense if the pixels have similar elevations. / · The uneven (and variable) terrain of the middle basin means that there is a greater distribution of slopes.
· On average, the slopes of the region are steeper in this area.
ASPECT / · The upper watershed primarily faces the Northeast / · The middle water shed primarily faces the Southeast
2. Compare the stream network and flow direction maps between the original and filled DEM. What is the major difference between them?
Before using the fill function of the spatial analyst toolbar, the stream network lacks connectivity. The connectivity of the DEM is disrupted by pits, low elevation areas in DEMs that are surrounded by higher terrain that disrupts the flow path. For that reason, the un-filled stream network appears to flow towards the pit (compare pit locations to the flow direction map), rather than flowing off the edge of the grid. After performing the fill, the streams appear connected and the flow follows the natural elevation gradients (see filled DEM map).
3. Comment on the spatial distribution of depression cells. Where can you find major pits? How does filling pits affect the flow paths of this area? Why?
Major pits are typically found behind large roadways, such as Franklin Street, Cameron Avenue and South Road. The pits interfere with the connectivity of the stream network, and water appears to flow into areas where water ought to backup. Lakes appear to form in areas where the non-pit-filled stream network cannot recognize culverts running below (or aside) the roadway. By filling the pits, the stream is able to follow its natural course given the elevation gradients. Put differently, the stream appears to run through (or over or under) the roadway as if there was a section cut out of the roadway.
4. How many hectares is the 1000 and the 500 grid cell threshold that defines the stream heads?
5. What range of drainage area (in hectares) appears to define a stream head in this area?
Stream Type / >500 / >1000Fill Pixel Count / 601 / 1346
Drainage Area / 240400 ft^2 / 538400 ft^2
Hectares / 2.233 hectares / 5.002 hectares
(25/25)