Benchmark Stability

[+gpsblake]

It seems obvious what it should mean but I'm curious about the different grades they give out. Some BM seem to have a B or a C but I really don't know what this means.

 

And I am just curious about BM's like in earthquake zones or faults.... Since the landmass moves a few inches each year, does that make the BM useless after a while out there???

 

Cheers

over 300 BM found and growing.

[Z15]

from NGS Bluebook (specs)

 

There is more info to read here

 

CODE DEFINITION

A Monuments of the most reliable nature, expected to hold

their elevations very well.

B Monuments which generally hold their elevations fairly

well.

C Monuments which may be affected by surface ground

movements.

D Monuments of questionable or unknown vertical stability.

Edited December 9, 2006 by Z15

[+gpsblake]

Thanks.

 

I can only get dial-up here so I can't download a 13 meg file easily.

 

But I would think with all the ground movements in California and other earthquake areas, that a benchmark for altitude purposes would be useless in a few years or after a major quake.

[2oldfarts (the rockhounders)]

Thanks.

 

I can only get dial-up here so I can't download a 13 meg file easily.

 

But I would think with all the ground movements in California and other earthquake areas, that a benchmark for altitude purposes would be useless in a few years or after a major quake.

 

We had wondered the same thing and got our answer from a surveyor in California and can be found in this link.

 

Shirley~

[+gpsblake]

Off topic but I would like to know how they were able to determine that altitude and lat/long exactly before the days of gravimeter's and GPS units. Even over 100 years ago, I wonder how they were able to tell EXACTLY where the state line's were at. Really neat, wouldn't think of these things if not for BM hunting.

[Papa-Bear-NYC]

Off topic but I would like to know how they were able to determine that altitude and lat/long exactly before the days of gravimeter's and GPS units. Even over 100 years ago, I wonder how they were able to tell EXACTLY where the state line's were at. Really neat, wouldn't think of these things if not for BM hunting.

To simplify a complex situation:

Latitude was measured by measuring the angle of the north star (actually the center of the small circle the north star travels) above the horizon.

 

Longitude was measured by transporting very accurate chronometers (clocks) from one place (say Greenwiuch England) to another and measuring the time of noon (sun passes zenith) or times of transits of Venus, etc.

 

True north was measured by observations of the north star as above.

 

Longitude was the hardest. With the invention of the telegraph it was improved since time signals could be transmitted.

 

Google "Longitude problem"

 

Altitude was measured by "leveling" - basically sighting from one site to the next with an instrument to measure the altitude difference. So start at the tide guage (sea level) at Sandy Hook, NJ and sight to the next point, then the next, the next, etc. till you cross the country. Very exacting and very tedious. Impressive to say the least.

 

Google "Differential Leveling"

Edited December 11, 2006 by Papa-Bear-NYC

[+jwahl]

While I have limited expertise in this, it is my understanding that in cases where there are changes in elevations or position of a large area such as subsidence and earthquakes. It usually requires new work to be done and the work re-levelled or re-adjusted based on new measurements at some point in time when it is recognized as becoming a problem. This has happened several times in southern California, and I believe in the New Orleans area. Elevations that may be in error in terns of an absolute value may usually still maintain relative consistency as well as to other features on the ground so that engineering still fits them. The big exception is things that deal with water, it still has to run downhill no matter what the benchmarks say.

 

So from a practical point of view if a large area subsides it doesn't aedversely affect use of the benchmark monuments and is sometimes corrected from time to time.

 

Horizontal movement is similar but different. If the pacific plate moves 1 foot relative to the rest of the continent, again all the boundaries, buildings, highways, fences and other survey monuments move with them. So you wouldn't want to have everyone's property lines and improvements fixed except relative to the ground. Local movements, landslides etc., are a different story.

 

- jlw

[Papa-Bear-NYC]

There was an interesting example of this after Hurricane Katrina hit the New Orleans area.

 

The gorvernment knew certain BMs had subsided, but the contractors needed to start work. There were legal requirements for certaing buildings that they had to be x feet above sea level. Now some of the marks were actually below sea level. Contractors and property owners of course didn't want to hear that they could not legally rebuild.

 

So I remember a newspaper report of a meeting where some NOAA official was meeting with contractors and saying "BM such and such is down about a foot and a half, BM so and so is down about 15 inches", etc. etc.

 

We think of these things as neat things to find as a hobby, but in the real world, they can be very useful and important. And with legal implications to boot.

[+Harry Dolphin]

If I remember aright, Lasting Impressions has a section on using the benchmarks in Yellowstone to measure the rise and subsidence of the land.

[Relic_Hunter_Finder]

Statement: I enjoyed this forum, for the reason that I have learned a lot; thanks.