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Wintertime

Benchmarks show subsidence in California

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The Center for Investigative Reporting just did a report about ground-level subsidence in California's Central Valley, which I first heard about from KQED Radio in San Francisco. The stories are based on benchmark height information gathered by USGS scientists. Here are links to both the original report and KQED's story:

 

CIR story

 

KQED story

 

I'm giving you both links because the KQED page has a larger version of a recent photo taken between Los Baños and Chowchilla of a USGS hydrologist showing how far a certain area dropped between 1988 and 2013. The benchmark she's near is H1235 RESET, PID HS1427:

 

HS1427

 

The other photo, taken in 1977, mentions BM5661 (or possibly S661). I can't find its datasheet anywhere in the NGS database, either under active or destroyed marks. But yikes, look at the subsidence in 50 years!

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Maybe S 661 (with space) GU0103

Thanks, Bill, that could well be the one; it isn't far from Mendota, which is mentioned in the CIR article.

 

May I ask how you found it? I had tried "661," which only turned up one mark in Ventura County.

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I tried several variations on the designation. I figured BM was not part of the designation itself. The rest looked like either a 4-digit number or a letter and 3 digits, so I tried each. The standard format for a series of marks has a space between letter and number. The NGS search seems to have a wild card at the end of the characters you enter, but not at the beginning. You have to get the start correct.

Edited by Bill93
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I tried several variations on the designation. I figured BM was not part of the designation itself. The rest looked like either a 4-digit number or a letter and 3 digits, so I tried each. The standard format for a series of marks has a space between letter and number. The NGS search seems to have a wild card at the end of the characters you enter, but not at the beginning. You have to get the start correct.

Aha, that must have been my problem on the NGS site.

 

And when I tried "661" on Gc.com, I got caught on this limitation: "If you use keywords of 3 letters or less, the query will look for an exact match." Sigh...

 

I must not have tried "S 661" on the correct site. Thanks for finding it!

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I went to a talk at USGS the other night in which the topic of subsidence came up. The presenter (USGS ecologist Tom Suchanek) showed the photo of Joseph Poland that was in the CIR article I linked to in my original posting.

 

I got to wondering whether there's any record of the elevation changes in the NGS database. Here again is the datasheet for GU0103, S 661, which we believe to be the benchmark Mr. Poland was referring to:

 

GU0103

 

Mr. Poland didn't file a found report in 1977, but there was one in 1975. An NGS team reported it not found in 1988.

 

I've looked at the datasheet, but I can't tell whether the "SUPERSEDED SURVEY CONTROL" height was superseded because the ground around the benchmark sank and the new height is from a later date, or because the height from a given year was recalculated to a new geoid. I admit to never having gotten around to learning about geoids.

 

There are quite a few other survey marks in the vicinity, but the closest ones were monumented decades after this one so wouldn't show as much evidence of subsidence even if that information is on the datasheets. Still, they might offer some clues to those who know how to read them properly.

 

Is there anything in the datasheet for GU0103 that demonstrates the land subsidence where it was?

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This is an old pic

 

LandSubsidence-1024x793.jpg

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Yes, that's the photo that was in the CIR article I cited and was part of Mr. Suchanek's talk the other night.

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Posted (edited)

I can't tell whether the "SUPERSEDED SURVEY CONTROL" height was superseded because the ground around the benchmark sank and the new height is from a later date, or because the height from a given year was recalculated to a new geoid.

 

The way I read the current NGS data sheet, NGS probably has only one set of measurements for this mark.

 

The superseded elevation is in NGVD29, the adjustment done shortly after 1929. The old measurement data for many bench marks plus that for some new marks were fitted in an adjustment with different assumptions/measurements for gravity around 1988 to get the NAVD88 datum elevations. This disk apparently didn't have enough data to suit them or some other problem so it was not actually fitted in the adjustment. Its updated elevation was computed using a generalized VERTCON model of lower accuracy to convert from NGVD29 to NAVD88.

 

Also note the newer elevation estimate is higher than the old one, due to differences in the underlying model, so certainly those numbers do not reflect subsidence.

Edited by Bill93
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Okay, thanks, Bill. I was hoping that the "VERT ORDER - FIRST" might mean that they had been keeping detailed elevation records on this one, but maybe not.

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If I interpret that picture correctly, the ground has subsided about 50 feet since 1925, is that right?

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