Carly Buch
Exercise 2
Exercise 2:
Basic Terrain Analysis and Flow Path Mapping
Due Date: Feb. 10
Submit your report file in your personal working directory in the GEOG591 course
directory. If your ONYEN ID is smith, name the report file as ex1_smith_report.doc. In
your report, put your histograms and maps which are produced by each process.
Datasets:


Raster Dataset
1) btl_20ft_dem - 20ft Interpolated LIDAR Elevation Data of Battle Creek,
Chapel Hill
Vector Dataset
1) btl_middle_subwatershed – Middle Battle Creek subwatershed
2) btl_upper_subwatershed – Upper Battle Creek subwatershed
3) ch_imperviousness_surface – impervious surface map of Chapel Hill
4) ch_hydroarc – stream network in the area
All datasets of this exercise are in
\\isis.unc.edu\html\courses\2010spring\geog\591\001\exercises_2012\ex2.
Preparation
1. Create a directory named ex2 under your personal working directory, and copy all
files and data in exercises/ex2 to that directory.
2. Open ArcMap.
3. Add btl_20ft_dem raster data on your ArcMap.
4. Overlay all vector data.
Basic Terrain Analysis
5. Compute slope and aspect from btl_20ft_dem raster dataset.
6. Make histograms for btl_20ft_dem and aspect map in Middle and Upper Battle
Creek subwatersheds by selecting each subwatershed and using the zonal
histogram function in Spatial Analyst.
- Paste all four histograms to your reports.
Carly Buch
Exercise 2
Upper Slope Histogram:
\html\courses\2010spring\geog\591\001\students\cbuch\ex2\slope_btl in zones of J:\html\courses\2010spring\geog\591\001\students\cbuch\ex2\btl_upp
VALUE
0.009495846 2.514910642 4.602756305 6.690601968 8.987232197 11.28386243 13.58049266 16.08590746 19.11328367 -
3,500
3,000
2,500
2,000
2.514910641
4.602756304
6.690601967
8.987232196
11.28386242
13.58049265
16.08590745
19.11328366
26.62952805
1,500
1,000
500
0
SUBBA_1268
Upper Aspect:
Histograms of D:\ArcGIS\Default.gdb\Aspect_btl_21 in zones of J:\html\courses\2010spring\geog\591\001\students\cbuch\ex2\btl_upper_subwatershe
VALUE
Flat (-1)
North (0-22.5)
Northeast (22.5-67.5)
East (67.5-112.5)
Southeast (112.5-157.5)
South (157.5-202.5)
Southw est (202.5-247.5)
West (247.5-292.5)
Northw est (292.5-337.5)
North (337.5-360)
2,800
2,600
2,400
2,200
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
0
SUBBA_1268
Middle Slope:
Carly Buch
Exercise 2
html\courses\2010spring\geog\591\001\students\cbuch\ex2\slope_btl in zones of J:\html\courses\2010spring\geog\591\001\students\cbuch\ex2\btl_mid
VALUE
0.009495846 2.514910642 4.602756305 6.690601968 8.987232197 11.28386243 13.58049266 16.08590746 19.11328367 -
2,400
2,200
2,000
1,800
1,600
1,400
1,200
1,000
2.514910641
4.602756304
6.690601967
8.987232196
11.28386242
13.58049265
16.08590745
19.11328366
26.62952805
800
600
400
200
0
SUBBA_1233
Middle Aspect:
Histograms of D:\ArcGIS\Default.gdb\Aspect_btl_21 in zones of J:\html\courses\2010spring\geog\591\001\students\cbuch\ex2\btl_middle_subwatersh
3,000
2,800
2,600
2,400
2,200
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
0
VALUE
Flat (-1)
North (0-22.5)
Northeast (22.5-67.5)
East (67.5-112.5)
Southeast (112.5-157.5)
South (157.5-202.5)
Southw est (202.5-247.5)
West (247.5-292.5)
Northw est (292.5-337.5)
North (337.5-360)
SUBBA_1233
Q1. 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?
The upper watershed has much more very low slopes than the middle watershed. This is
shown by the large amounts of green bars for the upper. This is logical though because
Chapel Hill has fairly low grade slopes over the entire region. Chapel Hill is relatively
flat as part of the piedmont region of North Carolina. This region is characterized by
broad flat areas. The middle watershed though has about equal amounts of low slopes
and high slopes. This region must be characterized by gradual slopes whereas it would
make sense for the upper region to be characterized by low lying valleys based on the
graph of the slope. The most common aspect for the upper watershed is Northeast. The
following most common are East, Southeast, and South about evenly. The lowest ones
are Southwest and West. For the middle watershed, the most common was southeast.
Followed pretty evenly by Northeast, East, and South facing surfaces. These trends are
similar to those of the upper watershed.
Carly Buch
Exercise 2
According to the DEM histogram, the Upper Battle Creek subwatershed has generally
higher elevation than the Middle Battle Creek subwatershed (-2.0).
Flow Direction Map
7. Make a D8 flow direction map
- Go to ArcToolbox  Spatial Analyst Tools  Hydrology  Flow
Direction
- Name your results as “FlowDir_btl”
Flow Accumulation Map
8.
Make a D8 flow specific catchment area map
Carly Buch
Exercise 2
-
Go to ArcToolbox  Spatial Analyst Tools  Hydrology  Flow
Accumulation
Name your results as “FlowAcc_btl”
Stream Network Map with threshold 1000
Carly Buch
Exercise 2
9.
Derive a finite stream network with an accumulation threshold in raster calculator
- Go to Spatial Analyst  Map Algebra  Raster Calculator
- Double click “FlowAcc_btl”, “ >”, and type 1000.
- Hit “Evaluate”. You can get binary stream network map, which is
temporary file with name “Calculation”
- Make this file permanent; right click “Calculation” file  Data  Make
Permanent  Name it as “stream_1000”
Carly Buch
Exercise 2
Filling Pits
10. Fill pits
- Go to ArcToolbox  Spatial Analyst Tools  Hydrology  Fill
- Name your result as “fill_dem_20ft.”
Carly Buch
Exercise 2
11. Redo all processes from 7 to 9 with the pit-filled DEM (fill_dem_20ft). Name
flow direction output with filled DEM as “FlowDir_fill”, flow accumulation
output as “FlowAcc_fill” and stream network with threshold 1000 as
“str_1000_fill.”
Carly Buch
Exercise 2
Carly Buch
Exercise 2
Note: Make sure that you are using different raster output names for this process (e.g.
fill_FlowDir).
12. Repeat the identification of stream channels with a threshold of 500. Name your
defined stream network with threshold 1000 as “str_500_fill.”
Carly Buch
Exercise 2
Q2. Compare the stream network and flow direction maps between the original and
filled DEM. What is the major difference between them?
No connectivity or organized for a stream network map of the original DEM (-2.0).
Carly Buch
Exercise 2
The flow direction is very similar for the original and filled DEM, except where pits exist
(-1.0).
These maps look very similar. This could be because the fill was not extremely large so
that the streams were deep enough to not be filled completely in. (see maps below)
Original Stream network
Carly Buch
Exercise 2
Log based 10 of original
Original Flow Direction
Carly Buch
Exercise 2
Filled Stream network
Log based 10 of filled map
Carly Buch
Exercise 2
Filled Flow Direction
13. Use the Map Algebra to detect locations of all pits by subtracting the original and
pit removed DEM.
- Make the temporary result file permanent (like step 9) and name it
“pit_loc.”
Q3. 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?
To find the pits I used the raster calculator and subtracted the original file from the filled in file. This gave
a positive value to any pits by identifying any raised or filled in areas. I then overlaid the impervious
surfaces file so the pits could be easily identified. These pits appear to be major waterbodies in the area.
Filling in pits affects the flow by makes drainage complete. IF we did not fill the puts we would have
incomplete drainage.
Pit location: Around a main stream (Battle Creek) and roads (-1.0)
Pits near roads: Some culverts may redirect a stream below a road.
Bottom of the Middle Battle Creek subwatershed: By filling the pits, some streams were
rerouted and became more connected.
Note: Overlay vector files given in this exercises to locate the depression cells.
Carly Buch
Exercise 2
Carly Buch
Exercise 2
Q4. How many hectares is the 1000 and the 500 grid cell threshold that defines the
stream heads? Use “FlowAcc_fill”, “str_1000_fill” and “str_500_fill” for the
comparison.
Carly Buch
Exercise 2
Circled stream in red: Stream 1000 layer: 1357
It is easy to tell that the 500 layer is more detailed. More branches are shown by
this layer so we should set the threshold here to effectively view streams.
What about stream_500? (-2.0)
Carly Buch
Exercise 2
Q5. What range of drainage area (in hectares) appears to define a stream head in
this area?
Note: The DEM grid resolution that we used for the analysis is 20ft.
Calculation: 1357cells x 400 ft2= 542,800 hectares  It’s just ft2, not hectares.
(-4.0)
1 square foot = 0.000009290304 hectare
So, the unit conversion should be 1357cells x 400 * 0.000009290304 = ….
(13/25)
Feel free to ask any question to Yuri Kim (yuri513@email.unc.edu.)