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.
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?
From what I see, lower elevations appear in the middle watershed based on the
DEM. This direction leads into the upper watershed, which based on the DEM and the
name of the file, is at an upper elevation. Aspects for both watersheds look similar except
for the apparent directional change in the East of the middle watershed. Also, for both
watersheds, higher elevations are in the North and South areas that reach a lower elevation
towards the middle of the watersheds.
Here are the histograms of upper and middle watersheds (-2.0);
Elevation in Middle Watershed
Elevetation in Upper Watershed
280
450
260
400
240
220
350
200
180
300
160
250
140
200
120
100
150
80
100
60
40
50
20
0
0
SUBBA_1268
SUBBA_1233
Aspect in Upper Watershed
2,800
Aspect in Middle Watershed
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)
2,600
2,400
2,200
2,000
1,800
1,600
1,400
1,200
3,000
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
1,000
800
800
600
400
600
400
200
200
0
0
SUBBA_1268
SUBBA_1233
The Upper Battle Creek subwatershed has generally higher elevation than Middle Battle Creek,
and skewed elevation histogram, which means elevation values tend to concentrated on a certain
range of value. It implies relatively uniform elevation (-1.0).
The Middle Battle Creek subwatershed has lower elevation and more evenly distributed
elevation histogram. This wide distribution of heights suggests a more varied terrain (-1.0).
The aspect of the Upper Battle Creek subwatershed is most frequently in the northeastern
direction. The aspect of the Middle Battle Creek is most frequently in the southeastern direction
(-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
- Go to ArcToolbox  Spatial Analyst Tools  Hydrology  Flow Accumulation
- Name your results as “FlowAcc_btl”
Stream Network Map with threshold 1000
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”
Filling Pits
10. Fill pits
- Go to ArcToolbox  Spatial Analyst Tools  Hydrology  Fill
- Name your result as “fill_dem_20ft.”
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.”
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.”
Q2. Compare the stream network and flow direction maps between the original and filled
DEM. What is the major difference between them?
Stream 1000 Not Filled
Stream 1000 Filled
Flow Direction Not Filled
Flow Direction Filled
It is obvious in the stream 1000 network that when filled, a lot of connections are
made between two streams. So it connects all the blue lines (the stream) to where you can
see where it is flowing.
For the Flow Direction, I had to change all the colors to make it uniform. As when I
had done the procedure, the colors did not match to their values (1,2,4,8, etc.), and when I
did, I noticed that most of the area of change was around where the streams were being
connected by the fill process.
The flow direction is very similar for the original and filled DEM, except where pits exist.
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?
The pits appear to be in a position where streams are coming together and in close
proximity to roads. Many of the major pits are in the North and a majority of them are in
the Western portion of the Map in the upper watershed. By filling the pits, it allows the
program to connect the flow paths and show the link between streams. The reason for this
is probably because the system wouldn’t recognize the path for water to travel down and
therefore not register as a link between the stream paths.
Note: Overlay vector files given in this exercises to locate the depression cells.
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.
Q5. What range of drainage area (in hectares) appears to define a stream head in this
area?
87,480 x 400 = 34,992,000
325 hectares
1 square foot = 0.000009290304 hectare
For stream_500, 601 *400 * 0.000009290304 hectare = 2.233 hectares.
For stream_1000, 1346 *400 * 0.000009290304 hectare = 5.002 hectares. (-4.0)
Note: The DEM grid resolution that we used for the analysis is 20ft.
Feel free to ask any question to Yuri Kim (yuri513@email.unc.edu.)
Please use your onyen (odublar?) for naming your report.
(15/25)