Digital elevation data is now readily available from multiple sources. There are several tools that can help to visualise elevation data, Cesium being one. Recently I have been researching how to visualise terrain data in cesium and I could not see a  complete example that I could replicate on my own. This post will highlight the steps I took to visualise elevation data in cesium.

1.  Download elevation data. You can download the data from https://dwtkns.com/srtm30m/ or using the SRTM Downloader plugin in QGIS. In this example, I am using a GeoTIFF from https://github.com/kartoza/docker-mapproxy/blob/master/data/E020N40.tif
   
2. Modify your downloaded images to create a seamless layer. You can use gdalbuildvrt or merge the individual tiles to a single image using gdal_merge.py.
3. Spin up an instance of GeoServer. In this case, we will use the kartoza/geoserver docker image. To render the terrain data, we also need to install a GeoServer BIL extension. We run the following command to spin up GeoServer with the correct extension.
   

   docker run -d -p 8080:8080 --name geoserver -v `pwd`/data:/data -e COMMUNITY_EXTENSIONS=dds-plugin kartoza/geoserver:2.17.2

4. Open your GeoServer instance and publish your DEM. 
   NB: Remember to define the BIL format settings for your raster layer when publishing the layer.
   
5. Navigate to layer preview and test if you can preview your raster layer.
6. Proceed to download the cesium terrain workshop material https://github.com/CesiumGS/cesium-workshop
   NB: The cesium workshop shows a lot of information which is not necessary for our example. We need to modify index.html and Source/App.js to be minimalistic for our example.
7. Delete some lines from the index.html by running:
   

   sed -i.bak '24,54d;' cesium-workshop/index.html

8.  Add a minimalistic App.js to replace the current one in the folder cesium-workshop/Source.
   

   (function () {
       "use strict";
       var west = 7.1;
       var south = -11.2;
       var east = 72.9;
       var north = 41.2;
   
       var rectangle = Cesium.Rectangle.fromDegrees(west, south, east, north);
   
       Cesium.Camera.DEFAULT_VIEW_FACTOR = 0;
   
       Cesium.Camera.DEFAULT_VIEW_RECTANGLE = rectangle;
       var viewer = new Cesium.Viewer('cesiumContainer', {
           scene3DOnly: true,
           selectionIndicator: false,
           baseLayerPicker: false
   
       });
   
       // Remove default base layer
       viewer.imageryLayers.remove(viewer.imageryLayers.get(0));
       viewer.imageryLayers.addImageryProvider(new Cesium.IonImageryProvider({assetId: 3954}));
       var terrainProvider = new Cesium.GeoserverTerrainProvider({
           url: "http://localhost:8080/geoserver/wms",
           layerName: "kartoza:E020N40",
           //styleName: "tradecraft:graytocolor",
           waterMask: true
       });
       viewer.terrainProvider = terrainProvider;
   }());

9. Add a declaration for using Cesium-GeoserverTerrainProvider.
   

   sed -i '/App.js/i <script src="Source/GeoserverTerrainProvider.js"></script>' cesium-workshop/index.html

10. Use a web server to render your HTML file. For my use case, I am using Nginx in docker.
    

    version: '3.4'
    services:
    
      ngnix:
        image: nginx
        volumes:
          - ./cesium-workshop:/web
          - ./sites-enabled:/etc/nginx/conf.d:ro
        logging:
          driver: json-file
          options:
            max-size: 200m
            max-file: '10'
        ports:
          - 8091:80

    Where my nginx conf located in sites-enabled looks like
    

    server {
    
        listen      80;
        # the domain name it will serve for
        server_name localhost;
    
    
        # max upload size, adjust to taste
        client_max_body_size 15M;
    
        location / {
            add_header "Access-Control-Allow-Origin" "*";
            add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS, PUT, DELETE';
            add_header 'Access-Control-Allow-Headers' 'X-Requested-With,Accept,Content-Type, Origin';
            root /web/;
            index index.html index.htm;
        }
    
        error_page 500 502 503 504 /50x.html;
        location = /50x.html {
            root /usr/share/nginx/html;
        }
    
    }

11. Navigate to http://localhost:8091
12. Rotate your map and you should be able to visualise the terrain as depicted below.
    
13. In the production environment remember to replace the cesium default ion access token with your own
    

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