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whitekn3

Finding my first benchmark.

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Back around 1955-1960 my grandfather showed me a brass plate fixed to a concrete pillar that was near a fence corner on the family farm. Ten or 20 years ago I tried to find it again and could not. Someone said they thought a bulldozer clearing brush may have wiped it out.

 

I recently learned that some "benchmarks", and this was a true benchmark with an elevation inscribed on the top, had a marker buried "below plough depth" below the surface and they can be refound. I looked at an overlay of USGS markers on a google map and the marker was not shown at all.

 

Is there any practical way of locating the original marker? Did elevation benchmarks ever have a lower stone marker?

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If there is an underground marker it should be mentioned in the data sheet. Since you probably don't know the designation or pid you'd need to use an app like FindAControl (Free for IOS) that shows the location of all survey marks in your area and allows you to immediately view the data sheets of ones you select.

 

Even if there is an underground mark your chances of finding it are pretty remote. Most vertical controls have "scaled" rather than "adjusted" locations, which means they were probably estimated from aerial photographs. In my experience these are almost useless. They can easily be off by 100 feet, and digging down 18 inches or so over a 100 foot radius circle is obviously not an option.

 

Even if you knew the exact location, most hand-held gps units will only get you within 10 feet or so - still not good enough to find a deeply buried object. Most vertical controls did not have monumented reference marks, but rather included detailed descriptions of distances from nearby objects - telephone poles, highway centerlines, witness posts, etc. IF you can find two of these that still exist you might be able to triangulate on the location using a steel tape. IF the underground mark is made of metal and you can find that old fencepost, your best bet would be to buy or rent a good metal detector. Who knows, you might find some buried treasure while you're at it.

 

In my area, the network of vertical control benchmarks mostly follows along the routes of US highways. The good folks who put them out in the 30s and 40s had no concept of how wide these roads would eventually become, so very few of these marks remain.

 

Good luck, if you do succeed it would be quite a find!

Edited by trmcconn

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Very few elevation marks have an underground backup disk. I've seen some MORC data sheets that had a rivet below the pipe the disk was on, but that was not common practice for either USGS or C&GS/NGS. Only triangulation stations typically had underground disk.

 

If your disk had an elevation stamped it was very likely USGS as C&GS/NGS did not stamp elevations as a regular practice. And only a small fraction of USGS disks were measured to the standards to get in the NGS data base.

 

So I am not optimistic for you but wish you luck.

 

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Edit: the locations of elevation marks were usually SCALED off a topo map, which might be good within a few yards where there were features on the map to compare to, but the the values were truncated (not rounded) to whole seconds of lat-lon. Thus you can figure that even if the disk was perfectly located on the map, you have up to 100 ft north and 75 ft of west of the coordinates to search.

 

The way to find bench marks from the data sheet is to go to the lat-lon coordinates and then put the GPS away. Get out the tape, and measure from the features listed in the to-reach instructions on the data sheet.

Edited by Bill93

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