How To Hydrologically Condition A Digital Dam

Transcription

1 Program: Funding: Conservation Applications of LiDAR Data Environment and Natural Resources Trust Fund Module: Instructor: Hydrologic Applications Sean Vaughn, DNR GIS Hydrologist (Funded by the Clean Water Fund) Exercise: DEM Conditioning Objectives Introduce Digital Dams Apply a TPI grid for hydrography identification within a 3 meter LiDAR derived DEM. Demonstrate the use of the Topo to Raster tool for hydrologically conditioning a DEM for the removal of Digital Dams. o The utilization of the Topo to Raster tool is one method. It works exceptionally well, and it is readily available within the ArcMap Toolbox. There are several other methods for accomplishing the removal of dams including but not limited to: DEM subtraction at digital dam locations. DEM to raster point conversion with specific point removal followed by a point to DEM raster conversion. AGREE application via ArcHydro Break line utilization within the LiDAR point cloud. AMUDEM software ( datasets/anudem ) Page 1 of 15

2 Setup Exercise & Data Location: A link to the data for this exercise is available in the same place where this document is posted. In this document, the root folder is indicated as %root folder%\ = Flash_Drive\_sevaughn_LiDAR\DEM_ CONDITIONING 1. Map a drive to the exercise contents: a. Use the Connect To Folder button to map a drive to the location of the exercise files. In this example, they are on a Flash Drive %rootfolder%. Name: %rootfolder% Location: (%Flash_Drive%)\_sevaughn_LiDAR\DEM_ CONDITIONING 2. Open the ArcMap.mxd project for the exercise named DEM_conditioning.mxd. Name: DEM_conditioning.mxd Location: %root folder%:\ dem_condition_project 3. Check your Geoprocessing/Toolbox Environment settings. Discussion / Review: This ArcMap project has been set up for you. This ArcMap project should have all of the Environmental Settings established. A quick review of the Environment Settings should contain entries familiar to you and suited to this exercise. OR Page 2 of 15

3 a. Workspace should point to the dem_conditioning_data folder. b. Scratch Workspace should point to the dem_conditioning_scratch folder. c. Processing Extent = dem03 (As Specified Below) d. Most importantly, make sure that Snap Raster is set to dem03. Discussion / Review: After you've run a tool, you may find that the output isn't written where you expect perhaps you made a mistake when entering the output name, or you've just forgotten where it was written. If this is the case, open the Results window; it will contain a record of the tool you've run along with its input and output datasets. xc 4. Turn off all layers in the Table of Contents. 5. Turn on dem_03 and dem03_hlshd. Page 3 of 15

4 6. Set Display and Symbology for dem03. Display Tab Resample during display using: = Bilinear Interpolation (for continuous data) Contrast: = 0% Brightness: = 0% Transparency = 55% Symbology Tab Show: = Stretched Color Ramp: = {your choice} Stretch Type: = Standard Deviations, n: = 2 Statistics: = From Current Display Extent 7. Review the DEM. Locate the intersection of the ditch and road and the resulting Digital Dam in the area identified below. 8. Save your ArcMap project DEM_conditioning.mxd.. Page 4 of 15

5 Raster Hydrology Comparison Standard ESRI raster processing for hydrologic applications requires that all cells within the DEM contribute flow. Meaning, all cells contribute water to an adjacent cell. This is accomplished by creating a filled DEM. In such a raster, all cells that contain an elevation value lower than their neighboring 8 cells (sinks) are filled to the elevation at which they will outlet water to an adjacent cell of lesser value. 1. Turn off all layers. 2. To save time the Fill process has been run for you. Turn on the raster named fill1_hlshd to see how the grid was filled to overtake digital dams on the landscape. This large flat area represents a filled (raised) depression on the landscape. We know this is not supposed to be an impoundment. 3. Turn on the reclassed flow accumulation raster named rcls_flwac150. Discussion / Review: Raster flowacc_flow1 is a flow accumulation grid built from the filled DEM. The raster rcls_flwac150 is a reclassification of the raster flowacc_flow1. It represents all grid cells that have a flow accumulation or upstream contrition of 150 grid cells or more. Notice how the flow has shifted locations around the DEM to flow over the digital DAMs at areas away from the actual location of the conveyance feature (ditch) as identified in the previous steps. Page 5 of 15

6 4. Turn off fill1_hlshd and turn on tpi_cir1 raster. a. Place tpi_cir1 under rcls_flwac150 if it isn t already,. b. Zoom into the lower-center part of the raster. Discussion / Review:. Notice how much different the reclassified flow accumulation (rcls_flwac150) is from the hydrography represented in the tpi_cir1 raster.. This demonstrates the effects of Digital Dams on hydrologic modeling using unconditioned DEMs. 5. Now that you have a better idea of what Digital Dams are, turn off the rcls_flwac150 raster. Take a moment to pan around the DEM, notice that every road, driveway and field approach produces a digital dam. Page 6 of 15

7 Topo to Raster Inputs To ensure water will flow within the DEM properly we must hydrologically condition the DEM by breaching digital dams. In this exercise, we won t correct/breach all the digital dams in the exercise dataset. Instead we will focus on some previous identified digital dams (within yellow boxes) for instructional purposes. We will be using the Topo to Raster tool for the processing but first we must prepare some inputs to the tool. 1. The Topo to Raster tool runs on point and contour data. We will create a dataset of points from the base DEM [DEM03] using the Raster to Point tool for our input to the Topo to Raster tool. a. Use the Raster to Point Tool to create a point file out of our base DEM [DEM03] ArcToolbox Conversion Tools From Raster Raster to Point b. Name it dem03_pnt Location: %root folder%: dem_condition_data\dem03_pnt.shp 2. Turn off all layers in the Table of Contents except for tpi_cir1. 3. Turn on the digl_dam_breach_loc dataset. Discussion / Review: This dataset represents 5 yellow polygon rectangles where you are going to create breaches to cut through the digital dams. Make sure you zoom out to locate all five areas. Page 7 of 15

8 4. Load the contours (contours_2ft) from the Ancillary_data Feature Dataset. Location: %rootfolder%:\dem_condition_data\dem_conditioning_geodb.gdb\ancilary_data 5. Create a new polyline shapefile. Location: %root folder%:\dem_condition_data\ a. Name the new dataset dam_breaks. i. You will need to set the coordinate system to NAD 83 UTM Zone 15N. Page 8 of 15

9 b. Create a breach represented as a short arc at each of the 5 locations identified by the digl_dam_breach_loc layer. Discussion To breach the digital dam, these short breach arcs should start slightly before and end on the opposite-downstream side of the digital dam (road etc.). The arcs must point downstream so you need to digitize in the downstream direction. Visually interpret dem03 and the contours by zooming out to full extent to ensure you see the downstream direction of the landscape. This will help ensure that your arcs are digitized in the downstream direction. Zoom into each of the 5 locations (yellow boxes) at a scale of approximately 1:2,000. Digitize the breach arcs as perpendicular to the digital dam as possible while staying inside the watercourse (dark area of TPI). Page 9 of 15

10 Topo to Raster We are now going to use the Topo to Raster Tool to breach five designated digital dams creating a partial hydrologically conditioned DEM for hydrological analysis in a GIS. The Topo to Raster tool is an interpolation process design to create hydrologically conditioned DEMs based on its parent product ANUDEM-algorithms developed by Dr Michael Hutchinson at the Australian National University ( ). The current version of ANUDEM used in ArcGIS is Open the Topo to Raster Tool. Topo To Raster operates from within both the 3D Analyst Tool Set and the Spatial Analyst Tools. a.. Spatial Analyst Tools Interpolation Topo to Raster -OR- 3D Analyst Tools Raster Interpolation Topo to Raster 2. Setup the Topo to Raster tool dialog. a. Load the dem03_pnt point file and select PointElevations from the internal drop down menu. Page 10 of 15

11 b. Set the Field and Type requirements within the internal dropdown menus (see next series of graphics). This functionality is activated by left clicking in the box within the Field and Type columns and making your selection from the drop down lists. Discussion / Review: The GRID_CODE is a field in the point file that carries the elevation value for each cell from the DEM which will then be assigned to each point in the interpolation process executed by the Topo to Raster tool. c. Add 1.) dam_breaks and 2.) Boundary layers to the dialog from the dropdown selection menu. Fill in the feature Type options for the other 2 Feature Layers so the menu looks like this: Page 11 of 15

12 d. Define the output raster to be located in the iteration_1 folder and name the new raster dem03_hydro1. Location: %root folder%:\dem_condition_output\iteration_1 Discussion / Review: The hydrologic conditioning of DEMs is an iterative process. The execution of the correction technique is run several times adding more digital dam breaches each time until the DEM is demonstrating sufficient flow such that the hydrology of the landscape is represented and meeting the objectives of the task at the necessary scale. The outputs of these iterations are best managed by the use of folders with consecutive iteration numbers (iteration_1, iteration_2 etc.). Page 12 of 15

13 e. Fill in the rest of the menu to look like this: 3. Change the Current Workplace in the Environment Settings at the Tool Level to the dem_condition_output\iteration_1 folder (see arrow and graphic above). Location: %root folder%: \ dem_condition_output\iteration_1 Discussion / Review: Notice that for this exercise we have selected all the default values. This keeps the setup simple for this exercise. You may find it to be advantageous to change these setting in your future work with different dataset inputs. Additional information can be found within the ArcMap help. Page 13 of 15

14 4. Hit OK to run the Topo to Raster Tool. This should take 1 to 5 minutes to run depending on the CPU capabilities of the lab machines. 5. The newly corrected DEM should have loaded into your Table of Contents. Change the Symbology to a Stretched data display (if it isn t already). 6. Create a Hillshade of the new hydrologically conditioned DEM named hydro1_hlshd (you created a hillshade in the DEM DISPLAY Exercise which you can use to reference the required steps for this task). Compare this new Hillshade to the original base DEM Hillshade and the Filled DEM hillshde at the locations of your dam breach arcs. Digital Dam Breached Digital Dam (hydrologically conditioned) Page 14 of 15

15 Discussion / Review: You should now be able to see that the digital dams were breached allowing water to flow through the barriers. 7. This exercise is complete. Depending on how the course and day progressed you may have extra time. Please use any remaining time to experiment with the data, make a TPI of the hydrologically conditioned DEM and/or ask any questions you may have. Document version: June 2013 Page 15 of 15