Basic Terrain Analysis and Flow Path Mapping

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? Upper Battle Creek Subwatershed Aspect Map Upper Battle Creek Subwatershed DEM Histogram 2,800 VALUE 280 2,600 Flat (-1) North (0-22.5) 260 2,400 Northeast (22.5-67.5) East (67.5-112.5) 240 2,200 Southeast (112.5-157.5) South (157.5-202.5) 220 2,000 Southw est (202.5-247.5) West (247.5-292.5) 200 1,800 Northw est (292.5-337.5) North (337.5-360) 180 1,600 160

1,400 140

1,200 120

1,000 100

800 80

600 60

400 40

200 20

Aspect SUBBA_1268

Middle Battle Creek Subwatershed Aspect Map Middle Battle Creek Subwatershed DEM Histogram 3,000 VALUE 460 2,800 Flat (-1) 440 North (0-22.5) 420 2,600 Northeast (22.5-67.5) 400 East (67.5-112.5) 2,400 Southeast (112.5-157.5) 380 South (157.5-202.5) 360 2,200 Southw est (202.5-247.5) 340 West (247.5-292.5) 2,000 320 Northw est (292.5-337.5) 300 North (337.5-360) 1,800 280 260 1,600 240 1,400 220 200 1,200 180 1,000 160 140 800 120

600 100 80 400 60 40 200 20 0 0

Aspect SUBBA_1233 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?

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.” - Overlay with impervious area

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?

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. Stream 500 is more detailed. Use the map to answer the question. Show the grid number 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. One grid has 20, convert to 400ft^2 (400ft^2)x(601), convert to hectares

Feel free to ask any question to Yuri Kim ([email protected].)

PASTE ALL MAPS IN REPORT FlowDir FlowAcc Stream_1000 Stream_500 Strem Fill Stream without fill