A published BM is described as being a square cut in the bullnose of a median at the entrance to a subdivision. But is the recovered BM the published BM? It's the same described square cut. Are the cracks in the asphalt indicative of the original median? I believe the recovered BM is 0.023' lower than the published BM. That makes sense to me because the entrance slopes away from the main road.
Do you tie your BMs to street centerlines, sidewalks, driveways, etc., or do you rely exclusively on GNSS positions?
Do you tie your BMs to street centerlines, sidewalks, driveways, etc., or do you rely exclusively on GNSS positions?
Yes.?ÿ
It looks to me like that bit you have painted pink is your benchmark. 0.023' isn't meaningful with a mark like that. The curb doesn't look all that stable. And what do you know about how the elevation was established? By levelling? Total Station? GPS? and relative to what?
Is that picture recent? It looks so warm!!!
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2 hundreths??ÿ ?ÿwas there a tiny pebble in the cutout??ÿ did someone drive over that curb recently??ÿ ?ÿHow sure are you that you're not running .02 feet out??ÿ ?ÿ ?ÿWrite it down and move on.?ÿ IMHO.
How did you come up with the 0.023'??ÿ Also, why would you suspect it is not the recorded BM.?ÿ I understand that it looks like the curb has been moved but why do you suspect the original was on the old curb??ÿ Why would someone mark the new curb?
If it is that critical to be on a specific datum then find another benchmark from the same run and check it.?ÿ If no others can be found then start looking back pedigree.?ÿ For a couple of hundredths it would have to be pretty critical to do much more than fine another bench on the same loop.
If your mission is to recover a benchmark system, then readjust and move on.?ÿ Passive monuments move over time.?ÿ Good benchmarks systems need to be checked/re-adjusted periodically to maintain a good relationship.?ÿ It is also why the NGS rates the probable stability of a monument.?ÿ A chisel mark in a curb would be low stability at best and require more frequent adjustment.?ÿ
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It is .023' low? The other benchmark you are using isn't .023' high? How did you determine which one was wrong?
Looking at this again and seeing something I didn't see at first -?ÿ I now gather that you are concerned that there was once a different curb arrangement with a different chiseled square.?ÿ It is really unlikely that any such alternate chiseled square would agree with your relative elevation to even 0.02'. And what are the chances that one chiseled square would get replaced? And with another chiseled square? (per J. Putnam) Relax. You've got it.?ÿ ?ÿ
Nevertheless, I can't see using such a BM as the basis for geodetic control or anything like that. But good enough for keeping the stuff flowing downhill.?ÿ?ÿ
What would you build you need to worry about your coordinates system tying into less than .02??? Just curious.
I have come across two USGS square cuts that did not appear to be the original cut. Either the elevations, the descriptions or my interpretation of both were deviant enough that I did not use either one. One was one a bridge curb and the other a headwall. Both were too high about the amount of concrete that could have been added to a previous surface. Both used descriptions like ??mid way? and ??down from?. Anybody can cut a square into concrete.
Depending on the equipment, procedures, original work, and distance between BMs, 0.02 could be suspicious...or fantastic. Or somewhere in between.
For a lot of our work, that would be a pretty good check, and we'd hold both BMs fixed in the level network adjustment.
Depending on the equipment, procedures, original work, and distance between BMs, 0.02 could be suspicious...or fantastic. Or somewhere in between.
For a lot of our work, that would be a pretty good check, and we'd hold both BMs fixed in the level network adjustment.
I have a feeling he isn't using a second BM. Just guessing.?ÿ
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Static observation?
OMG! C'mon you guys.
With all the drugs, edibles, legal mushrooms, not to mention Ayahuasca out there these days, I've noticed a 75% increase in the number of people I've caught cutting random chiseled boxes in concrete curbs in SoCal. Must be fun to do. They always have a sharp chisel and a 3 lb. hammer. Gloves, too.
JA, PLS, SoCal
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I did some research this afternoon concerning this BM; it was set in 1999. There's no doubt it could've settled over the years. The BM was set by running third order levels. The original field notes show a distance of 30' +/- to the road centerline; we measured the same distance. This is the original BM. The median-shaped cracks in the asphalt had us scratching our heads as did the discrepancy in elevation. The discrepancy became smaller after we continued our run to an NGS BM and then ran (on paper) backwards to our beginning BM. We plan on running a new loop through our beginning BM because of other elevation discrepancies en route. Some of these discrepancies might be from running between NGVD 1929 and NAVD 1988 BMs.
