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Bill93

Is it undisturbed?

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Discrepancies on MH0134 are still nagging me and I haven't logged it to NGS. We were driving by it this afternoon (2 hours from home) and I added a couple things to my observations. My gut tells me it has been moved but I don't have any real evidence.

 

Someday I may try a leveling run from MH0135, the only other undestroyed NGS elevation in that town, about 1800 ft away as the streets run. Today we were too late for that project and my wife wanted to get home. I would plan about 4 setups (avg 225 ft sights, maximum 310 or 320 ft) with a construction level and topo rod (estimating 0.01 ft between 0.05 marks and averaging 3 wires). I would hope to hold 0.05 ft rms over that distance, i.e. requires 0.017 per sighting x sqrt(8 sightings). That is more than adequate to prove or disprove a suspected discrepancy of 1 or 1.5 ft. If I find a discrepancy, we would have to run back to the start to close the loop and check for mistakes.

 

I suppose I could ask the town water and sewer people if they have checked it. It would be more fun to do it myself.

 

Any comments on the log or my hairbrained scheme? Any other amateurs ever try this?

Edited by Bill93

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We made the leveling run on Saturday, and got essentially perfect agreement, 0.005 ft low with hoped-for accuracy of 0.050 ft.

 

So I'll report it as Good, and assume the street was widened asymmetrically and the rail was raised as is common over the decades.

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Fyi

 

Those type posts were precast, 8in sq post tapers 5 ft to 12in at the base.

 

There is one near me that has been washed out by erosion so I seen them first hand.

 

th_SG0213_CRAB_MIGS_1934-1.jpg

Edited by Z15

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Mike, what would you guess my accuracy to be, if I'm not very experienced at it, but didn't make any blunders. Is 0.050 realistic?

 

Methods: Topcon AT-F4 (good construction grade) automatic level and a rod with 0.05 ft marks estimated to 0.01ft for 3-wire readings. All turning points crayon mark on concrete. Individual sight lengths were not all well balanced but overall sum of backsight distances exceeded foresights by only 52 ft in 1684 ft run.

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Mike, what would you guess my accuracy to be, if I'm not very experienced at it, but didn't make any blunders. Is 0.050 realistic?

 

Methods: Topcon AT-F4 (good construction grade) automatic level and a rod with 0.05 ft marks estimated to 0.01ft for 3-wire readings. All turning points crayon mark on concrete. Individual sight lengths were not all well balanced but overall sum of backsight distances exceeded foresights by only 52 ft in 1684 ft run.

 

 

You're close enough to verify the BM was not moved but I don't even think the equipment you used would be good for 3rd order leveling.

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I know it wasn't 3rd order by a long shot. The instrument is probably usable for 3rd order if it had been recently calibrated, but the rod no way, and it takes better techniques we didn't use.

 

Among the techniques are readings to 0.001 ft or 1 mm as appropriate (mine were average of 3 wires to 0.01 resulting in 0.003 ft increments), sights not over 300 ft (I exceeded slightly once), balance on foresight vs backsight distances equal within 30 ft per setup and sums 30 ft for the whole run (I had one set differing by 120 ft-ouch, with total 52 ft off), well defined turning points (not just a sidewalk), and closure of 0.05 ft * sqrt(miles) = 0.028 ft for this run length.

 

I get the feeling that if you follow all the procedures, failing 0.05*sqrt(miles) is pretty rare and a sign that you blew it bad.

 

I'm probably optimistic to think I could hold 0.05 ft total if I kept repeating that run.

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