The purpose of this assignment is to use open data in Erdas and ArcMap to create a combined mosaic. In this case the terrestrial data has a resolution of 30 arc seconds with no bathymetric topo while the global data has 1 arc minute accuracy across oceans and land. Combining these two data sets will result in one dem with terrestrial dem data that is of higher resolution than the bathymetric dem. For more information on arc seconds I recommend referring to http://www.esri.com/news/arcuser/0400/wdside.html. To download the global dem at 1 arc minute resolution go to http://www.ngdc.noaa.gov/mgg/global/global.html. I used the ETOPO1 Bedrock: Grid Registered Geotiff.
Next go to http://www.ngdc.noaa.gov/mgg/topo/gltiles.html and choose a continent. You will create a dem of that continent so for ease I recommend not using an area with islands. Download the tiles necessary for your area (i.e for Africa F, K, G).
Assign Location to GLOBE Data in ERDAS
The GLOBE Data needs to be assigned an origin location. To do this import it into ERDAS using the Import Data option in the manage data ribbon. Then use metadata change map model to adjust the origin. More information is available in this tutorial.
Mosaic the GLOBE Data
The GLOBE data is going to get clipped to the shape of your continent. There are several ways to do this which will be discussed, but the first step is to download the world landfile polygons. http://openstreetmapdata.com/data/land-polygons
This data can be downloaded to whatever detail level you are interested in. For Africa I downloaded the largest polygons available. These sources were all separate polygons, but were that not the case you would utilize the explode tool which is accessed by setting your multipolygon file to be edited, then opening the advanced editing options. The file I downloaded actually required that the polygon including Africa be separated from the rest of Europe and Asia. In order to split the polygon the layer was set to be edited and the split polygon tool from the basic editor was used. The unwanted polygons were deleted and the feature was extracted as a new shapefile.
As mentioned, there are two ways to clip the globe data to a DEM. The first method occurs in ERDAS during the mosaicking step. The second occurs in ArcMap AFTER the mosaicking step.
The next step was to Mosaic the GLOBE Data in ERDAS. The process of mosaicking is discussed in this tutorial. I had best results using default mosaic settings. If you are going to mosaic in ERDAS follow the “Erdas Clip” Guide below before running the mosaic. Otherwise run as shown in the tutorial and prepare to load ArcMap for the “ArcMap Clip” option.
Next you can change a setting in Mosaic pro to in Edit –> Output Options and add a clip boundary. This should clip to an AOI. To create an AOI with your shape file you must open the shape file in ERDAS, select and copy the vector in the shapefile, then paste it into your new AOI layer (new –> 2D Map –> AOI).
The method I used was the “Extract by Mask” tool in ArcGIS Spatial Analyst. This creates a subset of your Africa Data.
Next we clip our etopo data to a square around the target continent. This can be done in a similar fashion in Arc or completed in the next step which mosaics the two different topographies. If you are already using ArcMap, I would recommend clipping it in this environment by creating a new shapefile with a square polygon that denotes the area of interest. Use the Extract by Mask once again but on the etopo layer. Extract any layers created in ArcMap to *.img files by right clicking on the file and selecting Data — > Export Data and select the location and *.img file.
Mosaic the DEMs
Before the DEMs can be mosaicked in ERDAS (This step can also be done in ArcMap) the etopo data must be projected to geographic wgs84 lat long. Do this in metadata –> edit –> change projection. You may also need to ensure that the new GLOBE mosaic has the same and correct projection. Recompute the statistics on both again as shown in the tutorial above. Load them into mosaic pro ensuring that the africa GLOBE is the top layer.
The data being loaded in should look like this
Run the mosaick in ERDAS and use your created rectangle as an AOI (could also just create this in ERDAS to avoid exporting it from ARCGIS).
Alternatively use the mosaic tool under datamanagement–>raster–>raster dataset –> mosaic. Make sure that africa (in this case) is set to be used as your first layer.
The next step is to create a painted relief terrain image using the terrain ribbon. Additionally the DEM was loaded as a pseudocolour image. The colour ranges were set separately for land and water by querying the necessary records and applying a colour ramp using the table –> colour tool.