Elevation needed

[Dave Lindell]

Can anyone get me the elevation for the Orange County surveyor's monument at the intersection of La Palma and Walker?

I think it's GPS 3606.

Thanks.

[pls7809]

Unfortunately that is not a vertical control point. Attached is the datasheet.

The nearest OCS bench is at the SE corner of Valley View and La Palma (1/2 mile east from 3606). BM 1H-127-70.

[Anthony Maffia]

And now for something completely different...

According to the datasheet, BM 1H-127-70 was leveled twice. From 1990 to 2005, the NGVD29 elevation dropped by 0.057, but the NAVD88 dropped by 0.092 feet.

How can two datums drop at different rates?

[Dave Lindell]

Thank you all very much.

[bruce hall]

I think we should split the difference.

[7702]

Using the current NGS Vertcon program, 2.0, published in 1994, I get a difference of 0.700 meters between the two datums. The difference on the OC data sheet for the 1990 values is 0.749 meters. For 2005 it is 0.739 meters. Perhaps there was an error in entering the geodetic position if Vertcon was used, or maybe the values were determined by running from different benchmarks that are seeing different subsidence rates.
Maybe the fine folks at OC Geomatics can explain the discrepancy.

[Jim Frame]

"How can two datums drop at different rates?"

It might just be a blunder. The only other explanation that comes to mind is that the realization of one or both datums changed between level runs.

.

[Anthony Maffia]

I immediately thought Verticon also, but the form says leveled, so others in the loop will also have the same variation.

In practice, though, it doesn't matter. Elevations are always relative.

I just found it a curious item, on a slow forum day.