Conservation Applications of LiDAR Data

Transcription

1 Program: Funding: Conservation Applications of LiDAR Data Environment and Natural Resources Trust Fund Module: Instructor: Exercise: Hydrologic Applications Sean Vaughn, DNR GIS Hydrologist (Funded by the Clean Water Fund) DEM Display Objectives Learn the basics of setting Environment Settings to control raster analysis and processing outputs. Demonstrate methods to display a DEM and associated raster products for the purpose of hydrography identification and delineation. Page 1 of 21

2 ArcMap Setup The set up of ArcMap with the proper toolbars, extensions, and environmental settings is critical for this exercise. In addition to using the functionality of these various ArcMap capabilities we want you to understand how to: locate, load, and setup these toolbars, extensions, and environmental settings. You can retain these instructions for future reference. 1. Open your GIS software. Start by opening ArcMap. 2. Load the Base DEM. Using ArcMap or ArcCatalog (your preference), load a base Digital Elevation Model (DEM) into the ArcMap Table of Contents. Page 2 of 21

3 3. Load ArcMap Extensions. In ArcMap, load the Spatial Analyst and 3D Analyst Extensions. Customize Extensions Note: Depending on the setup of your machine or prior use and instruction, these extensions may already be loaded for you. 4. Setup the Environment Settings at the Application Level. Discussion: Application environment settings are the system wide default setting that are used by every tool. a. Open the Environment Settings one of two ways. 1.) Right Click in the Toolbox, Click on Environments or 2.) From the Geoprocessing dropdown menu, Click Environments. OR Discussion: It s good data management to get in the habit of working with and setting the Environment Settings. This is especially true when working with raster data. As demonstrated in the course lecture it is important to ensure the output raster data you generate match up with your source/input grids. It should be noted that the Environment Settings are not exclusive to raster data. Simply setting your Workspace will ensure all your output data (vector and raster) is sent to a location where you can find them. Consequently, we start you out using the Environment Settings right from the beginning of your training. Page 3 of 21

4 There are four levels of Environment Settings that form a hierarchy: 1. application, 2. tool, 3. model, and 4. model process. In this hierarchy, all levels contain the same environment variables and have the same effect on output results. The levels differ only in how you access and set them. However, Environment Settings are passed down to the next level. These passed down Environment Settings can be overridden at each succeeding level. We feel more comfortable establishing the Environment Settings at the tool level even if previously set at the application level. However, for the following steps in this exercise you will be asked to set the Environment Settings at both the application and tool levels. b. Under the Workspace Tab, set Current Workspace to: Location: %root folder% :\dem_display_data. c. Under the General Settings Tab, set Scratch Workspace Location: %root folder%:\dem_display_scratch. Discussion: If the Current Workspace is set, tools will look for their inputs in this workspace and you can enter inputs for tools using just the name rather than the full pathname. Furthermore, output will be written to the current workspace, you should always check where tool output is being written. Nothing is more frustrating than to run a tool and forget or not know where the output was written. After you've run a tool, you may find that output isn't output to 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 which will include the directory path structure. d. Set Output Coordinate System to (in this example, dem03 ). e. Set the Extent and Snap Raster to the name of your DEM layer. Page 4 of 21

5 Discussion: The Snap Raster setting is what ensures your output raster will snap or mesh with your input raster. Note: The Extent will state As Specified Below the next time you open the Environment Settings. f. Set the Raster Analysis Settings to dem03. Page 5 of 21

6 5. Add the Land Facet Analysis Tool Bar. a. Customize Toolbars Land Facet Analysis 6. Save/Save As your ArcMap project to DEM_display Name: DEM_display Location: %root folder%:\dem_display_project Page 6 of 21

7 Hill Shade 1. Create a Hillshade raster of the base DEM [dem03, in this example] using the Spatial Analyst Tools. a. Select the Hillshade Tool from the Surface Toolbox menu (supporting graphic below). ArcToolbox Spatial Analyst Tools Surface Hillshade b. Select the base DEM (dem03) from the dropdown menu or browse button. c. Accept the defaults for Azimuth and Altitude. d. Ensure the Z factor is 1 because our x,y and z values are all in meters. e. Name the raster: dem03_hlshd (supporting graphic below). Name: dem03_hlshd Location: %root folder%:\dem_display_scratch\dem03_hlshd Note: Why does the value of sometimes show up which is different from the default of 1? This often happens when you have saved an MXD that had previously used a tool that required (or it was by accident) a Z factor conversion between feet and meters. If the x,y units and z units are in the same units of measure, the z factor is 1. This is the ESRI ArcGIS default (not always). If the x,y units and z units are in different units of measure, the z factor must be set to the appropriate factor, or the results will be incorrect. For example, if your z units are feet and your x,y units are meters, you would use a z factor of to convert your z units from feet to meters (1 foot = meter). Page 7 of 21

8 2. Set Display and Symbology for the new dem03_hlshd raster with the settings defined in the grey box below. Display Tab Resample during display using: = Bilinear Interpolation (for continuous data) Contrast: = 0% (default) Brightness: = 0% (default) Transparency = 0% (default) Symbology Tab Show: = Stretched (default) Color Ramp: = gray scale (default) Stretch Type: = Standard Deviations, n: = 2 (default) Statistics: = From Current Display Extent Note: Examples of Display and Symbology Window Tabs in images below. 3. Now that you have created the Hillshade raster (dem03_hlshd), move it under dem03 in the table of contents. 4. Set the Display and Symbology for dem03 you loaded earlier using the following criteria. Display Tab Resample during display using: = Bilinear Interpolation (for continuous data) Contrast: = 0% (default) Brightness: = 0% (default) Transparency = 55% Symbology Tab Show: = Stretched (default) Color Ramp: = {your choice} Stretch Type: = Standard Deviations, n: = 2 (default) Statistics: = From Current Display Extent Page 8 of 21

9 Note: What you are doing is seeing through the DEM with the shaded signatures of the Hillshade influencing the visual perspective of the DEM. Some people prefer to make the Hillshade transparent and place it over the DEM. 5. Take a minute to pan around and zoom in to get familiar with the data, the display settings, and the geographic setting of the exercise area. Notice that it is hard to explicitly define the location of watercourses. Unfortunately you can t spend too much time playing with this amazing data because you have more to do ahead 6. Load an aerial image for the same area you have been working in. It should load into the Table of Contents above the raster dem03. Location: %root folder%:\dem_display\dem_display_image Aerial images: Digital orthophoto quadrangles (DOQs) are aerial photographs corrected to align with the geographic grid. Air photos are available from MnGeo at In the Popular Resources section, click on Air Photos > FSA Photos, select the most recent year, and either download the file for your county or click on WMS service to learn how to connect to the aerial photos without downloading them to your computer. Page 9 of 21

10 7. Turn the DOQ on and off repeatedly to see how the topography looks without the hill shaded DEM. Take a moment to toggle back and forth to see how much topography is visible in the DEM compared to what the high quality DOQ offers. 8. Turn off dem In the Table of Contents Move the DOQ under dem03_hlshd in the Table of Contents and set dem03_hlshd to 60% Transparent. Page 10 of 21

11 Discussion: By placing the DOQ under the hillshade raster (dem03_hlshd) with a high transparency set on the raster, we can give the depth of topography to the image and the signatures of features from the DOQ to the topography of the DEM (raster). Pay particular attention to the water conveyance features which show up very well with this combination. However, do they exhibit signatures suitable for accurate delineation and digitization? As far as hydrography is concerned, we can do better to increase the visualization and interpretation of the hydrography This creates some exciting results but don t get wrapped up in all the splendor, there is more work ahead. 10. Turn off the DOQ. 11. Change the Transparency of the Hillshade raster (dem03_hlshd) back to 0%. 12. Change the Display and Symbology for dem03 a. Change the settings for dem03 to the values below. Display Tab Resample during display using: = Bilinear Interpolation (for continuous data) Contrast: = 70% Brightness: = 30% (negative thirty) Transparency = 40% Symbology Tab Show: = Stretched Color Ramp: = Yellow to Dark Red (5 th color ramp up from the bottom) Invert = Checked (on) Stretch Type: = Standard Deviations, n: = 2 Statistics: = From Current Display Extent Page 11 of 21

12 Note: You can ensure you are using the correct Color Ramp by right mouse clicking on the color ramp and selecting Save to style Fire symbology defined DEMcolor. Discussion: I call this raster symbology Fire. The goal of this color ramp and settings is to put the darkest colors into the lowest features, attempting to highlight the hydrography of the landscape. The hope is to make the bottoms of watercourses black when you zoom in. 13. Save your ArcMap project. Page 12 of 21

13 TPI Raster Creation We will now explore a method of indentifying the hydrography in the DEM with more intensity and definition using a special output raster with simple display and symbology settings. We will do this through a multistep process using a free ArcMap Toolbox called Land Facet Corridor Tools. From that extension we will employ the Land Facet Analysis tool which contains the Topographic Position Index (TPI) functionality. TPI Topographic Position Index TPI is the difference between a cell s elevation value and the average elevation of the neighborhood around that cell. TPI is a simplification of the Landscape Position Index described by Fels and Zobel (1995) and was developed in detail by Weiss (2001). Fels, J.E. and R. Zobel, Landscape position and classified landtype mapping for statewide DRASTIC mapping project. North Carolina State University Technical Report. Different Neighborhood settings (circle, rectangle, annulus) produce different results that may or may not be helpful. Users of TPI are encouraged to experiment to identify which settings are most useful for their task. Positive values mean the cell is higher than its surroundings while Negative values mean it is lower. 1. Make a Topographic Index Raster. Open the Land Facet Analysis Toolbar Topographic Position Index Tools Calculate TPI Raster. Toolbar Menu Appearance as a docked Tool Bar Appearance as a floating Tool Bar a. Create a Topographic Position Index (TPI) raster from dem03 with these settings (supporting graphics below or on next page). Input elevation raster: = dem03 Neighborhood (optional): = Circle Neighborhood Settings: Radius = 3 Units: = Cell TPI Raster: = TPI_cir3 Page 13 of 21

14 Name: TPI_cir3 Location: %root folder%:\dem_display_data\tpi_cir3 2. Check to see if TPI_cir3 loaded into your table of contents (supporting graphics below or on next page). Page 14 of 21

15 Discussion / Review: A TPI raster represents the difference between a cell elevation value and the average elevation of the neighborhood around that cell. 3. Change the Display and Symbology for TPI_cir3 Display Tab Resample during display using: = Bilinear Interpolation (for continuous data) Contrast: = 0% Brightness: = 0% Transparency = 0% Symbology Tab Show: = Stretched Color Ramp: = Gray Scale Stretch Type: = Standard Deviations, n: = 2 Statistics: = From Current Display Extent Page 15 of 21

16 Discussion: We have used a freely available proprietary Creative Commons Tool to generate our TPI for this exercise. You could use the Spatial Analyst Focal Statistics tool and the Raster Calculator to accomplish the same thing. Using Model Builder it would look like the model below. 4. Take a moment to pan around and explore the TPI raster. Discussion: Notice how we can now see the sides of the ditches and other features on the landscape that convey water. Now with the influences of oblique shading removed a technician can digitize hydrography with more certainty resulting in a more accurate placement of the geometry representing these features. 5. Save your ArcMap project. Page 16 of 21

17 1. In your table of contents Turn off dem03. TPI Raster Integration 2. Turn on dem03_hlshd. a. Ensure the Display for dem03_hlshd is set to 0% transparency. 3. Change the Display and Symbology again for TPI_cir3 to: Display Tab Resample during display using: = Bilinear Interpolation (for continuous data) Contrast: = 0% Brightness: = 0% Transparency = 50% Symbology Tab Show: = Stretched Color Ramp: = Gray Scale Stretch Type: = Standard Deviations, n: = 2 Statistics: = From Current Display Extent a. Zoom in on TPI_cir3 Page 17 of 21

18 Discussion: Notice how the TPI raster [TPI_cir3] defines the hydrography in the Hillshade [dem03_hlshd]. The TPI raster tightly defines the water conveyance features while the hillshade introduces elevation. 4. Change the Display and Symbology for TPI_cir3 to the Fire symbol set you set earlier. Note: This is the same color scheme you set up for dem03 earlier. Display Tab Resample during display using: Contrast: = 70% = Bilinear Interpolation (for continuous data) Brightness: = 30% (negative thirty) Transparency = 30% Symbology Tab Show: = Stretched Color Ramp: = Yellow to Dark Red (5 th ramp up from the bottom) Invert = Check (on) Stretch Type: = Standard Deviations, n: = 2 Statistics: = From Current Display Extent Page 18 of 21

19 Discussion: When you are working in areas with minimal elevation changes, as in the examples shown here, the roads and ditch banks pop with this Fire color scheme, making for easy interpretation of the features that convey water on the landscape. Like our earlier TPI illustration, the technician only needs to concentrate on the features in black for digitizing the hydrography, resulting in a more accurate final digitized product. 5. You should now have dem03_hlshd turned on and under the TPI_cir3 raster in the Table of Contents. Discussion / Review: We have now added a slight amount of visual relief to the TPI with the hillshaded DEM which can aid the technician in hydrography identification and the delineation of those features. Incorporating the relief and contours is also beneficial for determining directionality of most water conveyance features. Notice how many water conveyance loops there are on the landscape because of excavation on the landscape. Compare the effectiveness of the TPI with relief to just the DOQ. Not so long ago we were quite satisfied with what the DOQs provided us. 6. Save your ArcMap project. 7. Add the Effects Tool Bar (if it s not already loaded). a. Customize Toolbars Effects Page 19 of 21

20 8. Use the Swipe Layer from the Effects Tool bar to see how the TPI raster lines up with the original DEM and the DOQ. 9. Explore the raster s in your view to see how you could use TPI for results verification on other projects you are involved in. 10. Save your project. END Discussion: TPI Evolution and Availability Andrew Weiss First presented at the 2001 ESRI International User Conference. Weiss, A., Topographic position and landforms analysis. Poster Presentation, ESRI User Conference, San Diego, CA. Jeff Jenness, Wildlife Biologist, GIS Analyst, Jenness Enterprises/CorridorDesign. Originally wrote the code in Avenue for ArcView 3.3. Jenness, J., Topographic Position Index (tpi_jen.avx) Extension for ArcView 3.x. Jenness Enterprises. Available for ArcGIS 10 at: Thomas Dilts, Research Scientist, University of Nevada, Reno. Migrated the TPI functionality to an ArcGIS toolbox. Available for ArcGIS 9.x at: Page 20 of 21

21 Note: The Land Facet Corridor Designer can be downloaded at: Extra Time Project: 1. Create a TPI on dem03 for 1.) A 3 x 3 Rectangle, using cell units 2.) A 1 x 1 rectangle using cell units. a. Does the representation of water conveyance features tighten up or get larger for each? 2. Use the DOQ for your comparison. 3. What do you think the results would be if we made TPI raster s using the 1 meter DEM (dem01)? Document version: June 2013 Page 21 of 21