Dionyzus

I see a recent topic on underground benchmarks so I thought I'd bring up one with a slight difference.. underwater benchmarks! What I mean is not a deliberately placed one (like in the Lasting Impressions book), but a BM on land that has since been covered by water, such as a reservoir. I've seen some examples of these marks in Texas lakes, such as BL1018. This marker is on top of a bridge abutment, which could probably be located with sonar. Then it's a simple matter of donning the scuba gear to get a close up shot! Of course most Texas lakes are notoriously muddy, but perhaps other parts of the country with clearer water may be better for finding this unique kind of benchmark.

I'm a relative newbie to benchmarking, but based on what I've seen in the Maptech TNPro program, I think the majority of USGS benchmarks are included. It may vary depending on geographic region, but below is a screenshot from a map in south Texas where all BM symbols correspond to NGS entries, which are shown by the red triangles. You can click each red triangle in the software to see the datasheet entry in the NGS database.

Even though it's not in the NGS database, this marker (at 38 31.4 N, 122 28.6 W) is shown on the USGS topographic map for that area: Ken