Merge NRCan DEM (CDED) Data
Introduction
This procedure outlines steps to merge Natural Resources
Canada Digital Elevation Model data files into a single file using ArcView
3.2. First you must copy the avenue script available at http://support.esri.com/search/KbDocument.asp?dbid=19740
or save the text to a file called "mergegrids.ave".Save
the script in a directory where you can find it later in this procedure.
This procedure assumes you have worked through the initial steps of importing the raw DEM files from NRCan. I would also recommend that you work through the conversion process to transform the data from Decimal Seconds to Decimal Degrees. This procedure can be accessed at http://www.brocku.ca/maplibrary/procedures/DEMshift.htm
Run ArcView 3.2 (you can perform this operation using ArcGIS ArcMap 8.2 however it will be easier to import 1 grid file that is merged as opposed to many individual files). When you are prompted to create a new view, click YES. When you are prompted to add data to the view, click NO.
You must have Spatial Analyst loaded to add GRID data to a view. Choose File>Extensions. Scroll down the list and click the box beside Spatial Analyst to load the extension. Click OK to continue.
Adding data
Click the Add Data button 
. In the Add Theme dialogue box change the "Data Source Types" to
Grid Data Source. Navigate to the DEM you wish to add, select
it and click OK. You can select multiple files at once by holding the shift
key down and clicking other files.
The grid files appear in the view. Turn each grid on by checking the box in the legend. The image below illustrates 2 grid files turned on. Notice the coordinates in the top right corner of the display. They should reflect either latitude/longitude decimal degrees or UTM coordinates.
Loading the Script
From the project window (see image below), click on the scripts option
at the bottom, then click New.
View of the "project" window
A blank script window appears. From the Script dropdown menu choose "Load text file...". In the "Load Script" dialogue box, navigate to the directory where you saved the script earlier, select the file and click OK. If you didn't create a file called "mergegrids.ave" as I suggested in the beginning you will need to copy, then paste the script text from http://support.esri.com/search/KbDocument.asp?dbid=19740
Blank Script window
Load Script dialogue box
The blank script window is now filled with the text of the "mergegrids.ave"
script file. Click the compile button 
to compile the script file.
Running the script
A WINDOW IS ACTIVE WHEN THE TITLE BAR IS DARK BLUE. THIS IS VERY IMPORTANT TO REMEMER FOR THE NEXT STEPS!
Complete the following steps in precise order:
.
You will be prompted with the "Create Grid Mosaic" dialogue box.
Select one or more (hold shift key) DEMs to merge with the first DEM and click
OK. 
The merged grid is added to your View. Turn the new layer on to see the merged file.
Saving a permanent copy of the merged file
Go to View>Properties.
From the Map Units dropdown list select "Decimal Degrees".
Beside Distance Units, leave the units as "Unknown". Click OK.
Select Theme>Save data set. Navigate to your directory, give the file an appropriate name and click OK.
In order to use the data efficiently in ArcView 8.2 you should
assign a projection to the file using ArcTools. Follow the procedure outlined
at http://www.brocku.ca/maplibrary/procedures/projecting.htm
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This page is: http://jeeves.brocku.ca/maplibrary/procedures/DEMmerge.htm |