Tutorial 1: Downloading elevation data

Transcription

1 Tutorial 1: Downloading elevation data Objectives In this exercise you will learn how to acquire elevation data from the website OpenTopography.org, project the dataset into a UTM coordinate system, and generate a hillshade of the topography colorized by elevation. Figure 0. Hillshade of State College, PA and surrounding area, colorized by elevation. Note that this is the only time in the semester where it is acceptable not to have a scale bar, north arrow, and legend!! Background Elevation data, traditionally in the form of a topographic contour map (such as those produced by the US Geological Survey), typically form the base upon which geologic mapping is performed. In recent decades, these data have been increasingly available in the form of digital elevation models, or DEM, a raster (grid) dataset that can be visualized and analyzed using a geographical information system (GIS). Here you will learn the basics of downloading publically available elevation data, and preparing a DEM for analysis in the software package ArcGIS, which we will be using for the remainder of the semester. 1.0 Types and sources of elevation data There are 3 common types of elevation data that you may encounter: The US Geological Survey for much of the 20 th century used stereo-photographs taken from aircraft to generate contour maps for much of the United States, which you may be familiar with (USGS 7.5 quadrangles). These contour maps have since been digitized and are available through various USGS websites as the National Elevation Dataset, or NED (10 meter grid spacing). In 2000, NASA flew the Shuttle Radar Topography Mission (SRTM) and collected topography for much of Earth s land surface at a grid spacing of 30 meters. Because this is a global dataset, and it takes less computing power to work with the resulting data, we will use the SRTM dataset for the remainder of the course. Finally, in the last 10 years there has been an explosion of very high-resolution (1-3 meter grid) topography data collected from airborne laser swath mapping using light detection and ranging (lidar) technology. The existing coverage is limited, but grows increasingly every year.

2 1.1 Acquiring publically-available elevation data from OpenTopography.org OpenTopography.org is the web portal to a topographic data distribution center run from the San Diego Supercomputing Center at UC San Diego in California. Their main purpose is to serve as a one-stop-shop for research-grade lidar data in the US, but they also host the best interface for downloading the 30 m SRTM dataset worldwide. Figure 1. Landing page for OpenTopography.org. Navigate to and click on Data\Raster (Figure 1) to see the available raster data sources (Figure 2). Figure 2. List of global raster datasets hosted on OpenTopography.org. Select the Shuttle Radar Topography Mission (SRTM GL1) Global 30m dataset to bring up a world map showing the data coverage. Once on the map page, zoom into Pennsylvania, click the blue Select A Region button and draw a box surrounding State College (Figure 3).

3 Figure 3. Selecting the spatial extent over which to download elevation data. Once you have selected a region, you can choose which format you would like your data to be in. Select GeoTiff if it is not already selected, and deselect all other checkboxes (Figure 4). Figure 4. Download output options Next, enter a brief job title and description, and enter your address to have the link sent to you. (Figure 5).

4 Figure 5. Job description entry form on OpenTopography.org. Once you submit your job, you can either watch the screen until your file is ready for download, or wait for an with a link to your download (Figure 6). Figure 6. Download progress screen on OpenTopography.org. Once you download your file, unpack it (using 7zip or similar software) to a folder on your computer where you will be working from (My Documents\Geosc497c\lab1\ or similar) Opening ArcMap and customizing toolbars and windows Open up ArcMap and get yourself acquainted with the layout. You will need to set up the extensions, toolbars, and catalog the first time we start up. Because we will be working with gridded raster data, you will need to activate both the 3D Analyst and Spatial Analyst extensions. Pull down the Customize menu, select Extensions, make sure the boxes for 3D Analyst and Spatial Analyst are checked (Fig. 7), and click Close. Figure 7. Adding extensions in ArcMap

5 Next, right click anywhere on the toolbar, and make sure the following toolbars are loaded: 3D Analyst Draw Editor Layout Standard Tools You can move these around wherever they are most helpful to you (recommended!), and you can customize which icons appear in each toolbar if you wish (not recommended!). Next, make sure you have the following windows open, which are selected by clicking the following icons on the Standard toolbar (Fig. 8). Figure 8. Icons on the Standard toolbar The Table of Contents window shows you which datasets are currently loaded, the Catalog window shows you the file system of your machine and enables you to move around or load datasets, and the ArcToolbox window contains the library of tools that we will use throughout the course to analyze these datasets. At this stage, your screen should look something like Fig. 9 below: Figure 9. Window and toolbar layout.

6 1.3 Creating a file geodatabase and adding data to the Table of Contents You will need to create a folder in My Documents to organize the lab datasets for the semester perhaps call it Geosc497c. For this week, make a folder called Tutorial_1 or something similar, and move/extract the GeoTiff you downloaded from OpenTopography here if you haven t already. For ArcMap to recognize this folder in the Catalog window, make a Folder Connection by clicking the folder connection icon and pointing it to the Geosc497c folder. Next, create a file geodatabase named state_college.gdb by right-clicking in the folder and choosing \\new\file geodatabase. A file geodatabase is the standard database structure used by ArcGIS for storing raster and feature class data, and is similar to the folders on your computer. Once you have created state_college.gdb, right click on the file and select Make Default Geodatabase. This sets the location for storing temporary files generated during your mapping session, and helps alleviate issues with write access privileges that often occur when using public computers. Your Catalog should look something like Figure 10. Figure 10. Catalog view showing Folder Connections and the File Geodatabase for this lab. To load a dataset into your current project, simply drag and drop the file (e.g., output_srtm.tif ) into the main window (Figure 11). Alternatively, you can click on the Add Data icon on the Standard toolbar and add datasets individually. ArcGIS also enables you to load pre-compiled web-based datasets, including satellite imagery and USGS topographic maps. Try loading the layer USA Topo Maps by clicking on the small down arrow next to the Add Data icon and selecting Add Basemap. You can turn layers on or off by clicking the check box next to them.

7 Figure 11. ArcMap window after loading output_srtm.tif into the current Data Frame. You should now see the dataset output_srtm.tif in the table of contents under the Data Frame Layers. A Data Frame is a map element that defines the geographic context and display properties for one or more layers or datasets in ArcMap. Right click on the Data Frame Layers and select Properties (Fig. 12). Here you can rename the Data Frame, change the coordinate system and display units, and control how the Data Frame is displayed in Layout View (we will go into this more in later labs). Figure 12. Data Frame Properties dialog box.

8 Now double-click the layer output_srtm.tif to bring up the Layer Properties window, and select the Source tab (Figure 13). Here you can see information about the dataset including the resolution, number of rows and columns, extent, and spatial reference (i.e., geographic or projected coordinate system). Note that the dataset we downloaded is in a geographic coordinate system (lat-long) with units of degrees. Figure 13. Source tab in the Layer Properties window showing information about spatial reference Projecting raster datasets Often times, data acquired from the web will be in a different spatial reference than we would like to work in. In this case, the file output_srtm.tif is in a geographic (lat-long) coordinate system, and we would like to project it into a Cartesian (x-y) coordinate system, so that our horizontal units are the same as the elevation units (meters in this case). To do this, navigate to and double click on the tool \\Data Management Tools\Projections and transformations\raster\project Raster in ArcToolbox (Figure 14). Figure 14. Project Raster tool in ArcToolbox.

9 For the input raster, choose output_srtm.tif, and for the output raster dataset, navigate to the file geodatabase state_college.gdb in your work folder, and give your dataset the name sce30_dem with no extension. This saves it in the ESRI grid format native to ArcMap. For the output coordinate system, navigate to \\Projected Coordinate Systems\UTM\WGS 1984\Northern Hemisphere\WGS_1984_UTM_Zone_18N (Figure 15). Depending on longitude, the particular UTM zone will change in central PA, we are on the border of zones 17 and 18, so either one will work (Figure 16). Next, change the resampling technique to Cubic this is a crucial step, and failure to do this will result in striping on the final dataset. For the output cell size, round up to choose both X and Y values of 30 (units are in meters). Click OK and wait for the processing to finish (dataset will automatically be loaded into your table of contents). Figure 15. Project Raster dialog box with appropriate values chosen.

10 Figure 16. UTM zones for the continental US (Wikipedia). You will note that although the new dataset has a different projection, nothing appears to have changed in the data frame. This is because ArcMap will project datasets on-the-fly into the coordinate system of the active data frame. This can be helpful in some instances, but a nuisance in our case, where it is crucial that all datasets and the data frame are in the same coordinate system and projection. To change this, first remove the data set output_srtm.tif by right clicking on it in the table of contents and choosing remove. Next, double click on the data frame ( Layers if you have not changed the name) to bring up the Data Frame Properties window and go to the Coordinate System tab (Figure 17). Here, rather than navigating through the projection folders, as you did above, go directly to the folder \\Layers\WGS_1984_UTM_Zone_18N (Figure 17). You will also need to change the map and display units in the General tab to meters (Figure 18). Figure 17. Changing the coordinate system of the Data Frame.

11 Figure 18. Changing the map and display units of the Data Frame Creating a polygon feature class and clipping the extent of a raster Your elevation dataset sce30_dem should now look distorted, as expected from the projection. In this section you will learn how to clip the extent of the raster dataset so that it is a nice tidy rectangle again. First, you will need to create a feature class, by navigating to and opening the tool \\Data Management Tools\Feature Class\Create Feature Class (Figure 19). Figure 19. Create Feature Class tool in ArcToolbox Here, choose the file geodatabase state_college.gdb as your feature class location, and choose a feature class name of clipping_extent. Set the geometry type as POLYGON, and set the coordinate system to WGS_1984_UTM_ZONE_18N, as for the other datasets (Figure 20).

12 Figure 20. Create Feature Class dialog box. Next, go to the Editor Toolbar and select Start Editing under the Editor menu (Figure 21). Figure 21. Editor Toolbar showing location of Editor menu, Edit tool, and Create Features button. Click the Create Features button to bring up the Create Features window. Then, click on the layer clipping_extent in the Create Features window, and choose the feature type Rectangle in the Construction Tools box. You can now draw a rectangle anywhere in the data frame, but it will be arbitrarily aligned. To lock your rectangle into x-y alignment, press the Tab key on your keyboard (this toggles the alignment back and forth). Experiment a bit. You can delete any mistakes by using the Edit tool to select the offending feature and pressing Delete on your keyboard. Once you are satisfied with your extent (e.g., Figure 22), select Save Edits and then Stop Editing from the Editor menu.

13 Figure 22. Rectangular feature class created that will be used to clip the raster dataset. To clip your raster dataset, navigate to and open the tool \\Spatial Analyst Tools\Extraction\Extract by Mask in the ArcToolbox (Figure 23). If you get an error here, it is likely that you forgot to activate the Spatial Analyst license in Section 1.2 above (Figure 7). Figure 23. Extract by Mask tool in ArcToolbox. Here, select the input raster to be sce30_dem, the feature mask data to be clipping_extent, and name the output raster sce30_dem_clip (Figure 24). Once the tool is finished processing, remove the layers sce30_dem and clipping_extent from your data frame. Figure 24. Extract by Mask tool dialog box.

14 1.6 Generating a hillshade colorized by elevation. To generate a hillshade of the elevation data for clearer visualization, navigate to and open the tool \\Spatial Analyst Tools\Surface\Hillshade in ArcToolbox. For the input raster, use sce30_dem_clip, and name the output raster sce30_hillshade_clip in the file geodatabase state_college.gdb. Leave all other values at their default (Figure 25). Figure 25. Hillshade tool dialog box. Once you are finished you should see a new raster dataset in your dataframe that looks something like Figure 26 below. Figure 26. Hillshade of State College, PA. It will be more informative to combine the hillshade with the elevation, so drag the dataset sce30_dem_clip to the top of the table of contents, and double click on it to open the Layer Properties dialog box. Navigate to the Symbology tab and select a colored scale bar (greenbrown-white is best for elevation data) (Figure 27). Be sure to change the Stretch to Minimum- Maximum. You can also control the transparency of each layer in the Display tab of the Layer Properties window (Figure 28). Experiment a bit here! You should be able to reproduce the image shown in Figure 0 at the start of this exercise.

15 Figure 27. Symbology tab in Layer Properties dialog box. Figure 28. Changing transparency in Display tab of Layer Properties dialog box.