chapter 5 digital elevation model and 3d … · qgis tutorial 1 indian institute of information...

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Indian Institute of Information Technology Management – Kerala (IIITM-K)

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.

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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.

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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)

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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.

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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.

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Click on identify tool see different option can do with 3D terrain

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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

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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).

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chapter 5 digital elevation model and 3d … · qgis tutorial 1 indian institute of information...

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