Merging DEMs
For the purposes of contour creation, temporary rasters can be generated using the original digital elevation model (DEM) files. When the DEMs are merged, they are combined into a single new DEM that can be used for contour generation without modifying your original DEMs.
Steps:
- Start ArcMap.
 - Load the DEM files you want to use to create contours.
 - On the main menu, click Customize > Toolbars > Production Contouring.
 - 
Click the Merge and Filter DEMs button 
 on the Production Contouring toolbar. 
The Merge and Filter DEMs dialog box appears.

 - 
Click Merge Only in the Operation area. 
Tip:You can filter the merged layer at the same by choosing the Merge and Filter Result option.
 - 
Click the Open button 
 next to the Merging/Filtering Output Raster Workspace field. 
The Choose DEMs Folder dialog box appears.
 - 
Navigate to a current DEM folder. 
Note:The folder you choose must be a preexisting raster workspace.
 - 
Click Open. 
The Merge and Filter DEMs dialog box appears.
 - Type a name for the merged file in the Merged Output Name text box.
 - Click OK.
 
4/19/2012