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mcsew2k

How do I find benchmarks on old USGS Topo map?

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I have been looking for several benchmarks on a 1915 USGS topo map. For example, on the Searles Lake, CA map it indicates BM 2707 with an "X" near a dirt road. My question is, how accurate is the elevation? Can I look along the 2700 elevation line on a modern topo map and expect to find the benchmark nearby? Any other tips on how to find it?

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Without more specific information about the marks location my best guess is that the info below published by the U.S. Geological Survey in Bulletin 766-U is the mark you're looking for:

 

image.png.25cf714c6d9e1f99b538fc1b73fdbd09.png

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Wow, Dave came up with info I wouldn't have known to look for.  But he didn't address the general question of old vs newer elevations.

 

The vertical datum in use has changed over the century and I don't have a feel for how much in a given area.  First there were regional datums. I don't recall when the first national adjustment occurred.  The NGVD29 national adjustment was used for the modern topo maps. 

Nowadays most work is done with NAVD88, but continuing work may stay on NGVD29.  Corps of Engineers projects and city sewer systems for example may not have converted. Next there will be the 2022 datum that relies on GPS instead of old disks.

 

I'd suggest you try to find a mark on the old topo that is also either on the newer topo, or else has an NGS data sheet with a superseded NGVD29 elevation in order to estimate the datum shift for the area between maps.

 

The shift is likely to be small enough to ignore for map reading, but I can't promise that.

Edited by Bill93

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Hi all,

 

Just showed this one to a friend after picking him up at the Devils Creek (Fort Peck Reservoir, Montana) boat ramp at the end of a three day kayak trip.  Seems like I have noticed several it that region with a plus two to three foot difference.   kayakbird

SQ0371* NAD 83(1986) POSITION- 47 34 52.4    (N) 107 38 16.5    (W)   HD_HELD2  
 SQ0371* NAVD 88 ORTHO HEIGHT -   882.147 (meters)     2894.18  (feet) ADJUSTED  
SQ0371_SETTING: 66 = SET IN ROCK OUTCROP
 SQ0371_STAMPING: 2891.782 L-38-1931

 

 

 

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