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New to Benchmarking...


AKStafford

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Posted

I've been Geocaching for awhile. We were out this summer with my parents and I happened to snatch a picture of a benchmark.

 

I've read the FAQ's and some on the info links and it appears this marker is not in the Geocaching data base. IMG_0660.JPG

 

This is the Data Sheet on it: http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=TT1488

 

It's location is approximately N 61° 30.890 W 144° 26.287.

 

Any thoughts?

Posted

That matches the description for TT1488, which for some reasons is not in the geocaching.com database.

 

What's interesting is how clean and fresh the concrete looks for a 1923 benchmark in Chitina, Alaska!

Posted

It was set in the sidewalk of a recently built (at least it looked fairly new) little rest stop type area.

I'm guessing maybe in construction process it go clean up a little?

 

If a well-meaning contractor 're-set' the mark in a new sidewalk, then it's destroyed.

Posted

 

If a well-meaning contractor 're-set' the mark in a new sidewalk, then it's destroyed.

 

Please forgive a total newbie question but if the mark is re-set in exactly the same position how is it then destroyed?

Posted

If a surveyor took care in referencing it out and replacing it, then it could still be usable, having changed much much less than its positional uncertainty, given at about 7 cm (2 3/4 in) in each horizontal axis. Vertically, it could be reset with some confidence to 0.002 ft, which is better than GPS observations of height. But the surveyor should have submitted information about his reset.

 

If a contractor took it out and put it back where they remembered it being, then it has moved an unknown amount, large or small, and is useless.

 

But notice that even though this disk has been around since 1923, it has no superseded NAD27 nor NGVD29 data. Its data is all from GPS observation. I'm not smart enough to deduce whether that was 2007 or before. So it is quite possible any reset occurred BEFORE the current data was measured.

 

If there was no valid geodetic data in 2000, then it would not have shown up in the snapshot of the data base that geocaching captured.

Posted

 

If a well-meaning contractor 're-set' the mark in a new sidewalk, then it's destroyed.

 

Please forgive a total newbie question but if the mark is re-set in exactly the same position how is it then destroyed?

 

So you see, 'close' is good enough for hand-grenades and horseshoes (and thermo-nuclear devices), but not good enough for surveying and benchmarks. :anicute:

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