Introduction
This procedure outlines steps to merge Digital Elevation Model data into a single file using ArcMap 9.1. The following steps assume the raw data has been prepared to the point of being functional within the ArcMap environment. If there are issues opening the digital elevation data, please see the various procedures available from http://www.brocku.ca/maplibrary/procedures/procedures.htm This procedure uses DEM data from the Ministry of Natural Resources.

Step One: Adding data to a View

• Run ArcMap. From the ArcMap start-up window choose "Create a new map" and "Immediately Add Data".
• Navigate to the location of the files to be merged and add them to the map view.

  The DEM grid files appear in the map window as distinct tiles.

Step Two: Working with Spatial Analyst

• Choose File>Extensions. Scroll down the list and click the box beside Spatial Analyst to load the extension if it is not already loaded.
• Click OK to continue.
• From the Spatial Analyst dropdown menu select Options. The Options dialogue box appears.

• From the General tab, use the browse button to define the working directory where the merge results will be saved.
• From the Extent tab, select "Union of Inputs" from the dropdown list.
• From the Cell Size Tab, select "Minimum of Inputs".
• To accept these options and return to the map window, click OK.

  Step Three: Using the Raster Calculator

• From the Spatial Analyst dropdown menu, select Raster Calculator. The Raster Calculator dialogue box appears.

• In the expression box at the bottom of the window enter the following text

  Merged_30m = Merge(

  Where "Merged_30m" will be the name of the merged DEM and "Merge" is the command for Spatial Analyst to merge the grid files together.

• To finish the expression, double click each DEM layer listed to add them to the expression. Enter a comma between each DEM layer and finish the expression with a closed round bracket. See the image below for an example. IT IS CRITICAL TO HAVE THE PROPER SYNTAX OR THE CALCULATOR WILL NOT WORK!

• Click "Evaluate". If there is a problem with the expression, an error message will appear. Double check the expression for the proper terms and punctuation, then click "Evaluate" again.
• The new raster file will be added to the ArcMap view automatically.

Step Four: Customizing the DEM Properties

• To adjust the colour scheme, double-click the new DEM layer. The Layer Properties dialogue box appears.
• Select the "Symbology" tab.
• From the "Colour Ramp" dropdown, choose a suitable colour scheme. The selection shown below is good for displaying elevation data.

• Click "OK". The resulting DEM should display colours similar to the image below.

To view a DEM in 3D you must use ArcScene. Follow the directions outlined at http://www.brocku.ca/maplibrary/procedures/3d_view/3dview.htm

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