CHAPTER 5 DIGITAL ELEVATION MODEL AND 3D VISUALIZATION A digital elevation model (DEM) is a digital model or 3D representation of a terrain's surface. A DEM can be represented as a raster (a grid of squares, also known as a height map when representing elevation) or as a vector-based triangular irregular network (TIN). The TIN DEM dataset is also referred to as a primary (measured) DEM, whereas the Raster DEM is referred to as a secondary (computed) DEM. The DEM could be acquired through techniques such as photogrammetry, lidar, land surveying, etc. DEMs are commonly built using data collected using remote sensing techniques, but they may also be built from land surveying. DEMs are used often in geographic information systems, and are the most common basis for digitally produced relief maps. Exercise: 5.1 Visualize the Digital Elevation Data in a 3D environment using QGIS Step: 1. Installing the plugin Open the QGIS plugin dialog (Plugins > Manage and install plugins.), and then install Qgis2threejs plugin. 1
Step: 2. Obtaining elevation data 30 meter ASTER GDEM elevation data can be downloaded freely from the distribution site. http://gdem.ersdac.jspacesystems.or.jp/ (User registration and login are required). For our exercise purpose, DEM data given in C: // Training/ DAY 5/Data. Step: 3. Loading DEM data Drag & drop the downloaded zip file to QGIS window (or load the file using Add Raster Layer dialog). You don t have to unzip it. 2
Step: 4. Coordinate Reference System setting Horizontal unit of ASTER GDEM data is degree, whereas vertical unit is meter. For appropriate visualization, you need to transform the DEM data to a projected CRS. QGIS can perform the CRS transformation on the fly. So, let s enable the On The Fly CRS transformation and change the map CRS to a projected CRS. Click the CRS status icon in the bottom-right corner of the window to open the project properties dialog. Activate the Enable on the fly CRS transformation checkbox and then select a suitable CRS for the DEM extent. Here we will use the WGS84/UTM Zone 43N projection (EPSG:32643) 3
Step: 5. Layer styling Open the raster properties dialog for the DEM layer and colorize the DEM layer richly. An example (Singleband pseudocolor render type and inverted Spectral color map): Step: 6. Exporting Zoom to a part of the DEM layer extent as the map canvas is filled by the colorized DEM. Click the qgis2threejs plugin icon in the web toolbar. 4
Step: 5. Output HTML file path edit box Select output HTML file path. Usually, a js file with the same file title that contains whole data of geometries and images is output into the same directory, and some JavaScript library files are copied to under the directory. Leave this empty to output into temporary directory. Step: 8. Click Run to start 3D visualization Exporting starts when you press the Run button. When the exporting has been done, the exported page will be opened in web browser. 5
Click on identify tool see different option can do with 3D terrain 6
Exercise 5.2 Draping openlayers over DEM 1. Overlay DEM on top of Google hybrid layer. 2. Zoom to a part of the openlayers extent as the map canvas is filled by the colorized DEM. 3. Click the qgis2threejs plugin icon in the web toolbar 4. Click on World option and change the vertical exaggeration to 3 5. Select DEM option in the Qgis2threejs dialogue box 6. Check the display type as Layer image and click on Select layers 5. Now the box will show the different layers in the map canvas 8. Select Google hybrid from layers list and give OK 9. Specify output file and click Run button. 10. You will be directed to a Web browser 7
Identifying Location and Elevation of the area When you click on an object, layer name that the feature (object) belongs to and the clicked coordinates (in order of x, y, z) are shown. Save Image Press Shift + S to show save image dialog, then enter image size and click the OK button. In addition, with some web browsers, you need to click a link to save image. The image file format is PNG. To change label color and/or adjust label size, edit Qgis2threejs.css (print-label class).. 8