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?
In the middle subwatershed, there are many different elevation values occurring across
the watershed, meaning the area is not flat and probably has a lot of hills. The upper
subwatershed, however, has a larger distribution of similar elevations that mostly occur
between 150 and 300. This means this area is most likely pretty flat since all the
elevations are very similar. In the middle subwatershed, most of the terrain faces
southeast, with many also facing south, east, and northeast. In the upper subwatershed,
the terrain mostly faces northeast, followed by southeast, east, and south. This shows that
although their elevations differ, both watersheds face mostly eastward.
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?
The major difference is that when the pits are filled, the stream networks run smoothly
and the flow directions continue along the stream.
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 major pits are found mostly around impervious surfaces like roads and buildings,
especially on the left side of the upper subwatershed. Filling the pits allows the flow
paths to become continuous. With pits filled, there aren’t any gaping holes in the flow
path. This is because filling pits in ArcMap normalizes extreme elevations to allow the
stream to flow through them, instead of being caught in them.
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?
20 ft * 20 ft * 601 cells = 240,400 ft^2 = 2.23 hectares
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.)