(8/iii 2000: this is a sketch of procedures, for eventual augmentation with screen dumps and crosslinks to other pages)Load Spatial Analyst and Spatial Tools extensions; set working directory as desired, and specify UTM, meters, 1927, zone 17
http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/7_min_dem/states/VA.html has an index of DEMs for VA (Getting them into the usable format requires moving them to Liberty, uncompressing, moving back, putting them through the import process as below...)
Add a DEM as a Theme, and make it Selected (NB: http://home.wlu.edu/~blackmerh/maps/sdtsconv.html shows how to extract and convert DEMs in SDTS format). Change 'No Data' from black to transparent.
From the Transformation menu, choose 'Merge Grid...' and navigate to the storage location of the adjacent grid you wish to add to the Selected
A new Theme is created with the merged grids. The procedure can be repeated (using the latest new Theme as the Selected) to tile in other DEMs, and geoTIFFs can then be brought in as new Themes, and used as to provide the basemap to create yet another new Theme containing the desired features (like sinkhole ponds, etc.) on a single layer.
One of the possible flies in the ointment: _are_ all DEMs in the same units, or are some in feet instead of meters... (3.2808333 feet/meter) [used Field Calculator to add a column to the table with values in feet... but I'm not sure how efficacious it turns out to be]