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My fair city delayed moving off ’29 for a long time because a) the DOT and County hadn’t made the move, and b) because of the volume of legacy projects that were elevated in ’29. A few years back moving to ’88 was discussed and, I’m told, was almost decided on when NGS announced its intention to introduce a new datum “in 10 years”. So the idea was shelved. And then the new datum delayed maybe 5 years. Nevertheless, with the new datum on the horizon nobody who hasn’t already switched to ’88 is going to do it now.
Correct. Many people do not know that. I think I have to deal with most of them.
The point I was trying to make is that while many old local BMs may have MSL values associated with them on an old map, report, or even stamped on them, there may not be any information on how the elevation was derived, nor the setting of the monument. Therefore, I cannot trust the methods by which someone established the elevation as it relates to NGVD29, nor can I trust that the monument is stable enough to maintain its original vertical position over such a long period of time. That is why I require field investigations of these BMs to establish NAVD88 Orthometric Heights.
Of course the first question that gets asked is, “So what’s the conversion factor to NGVD29?” My answer? “If your BM is still in its original vertical position, and we trust that the value we were provided in your report is truly NGVD29, then the difference is A-B=Conversion Factor for this location. The closest NSRS BM with both NGVD29 and NAVD88 says the difference is X. As you can see, that differs from what we have by direct observation by 0.xx.”
@norman-oklahoma
As I understand it, and maybe I don’t, NGS will not be providing a relationship or conversion tool for NGVD29 forward, only from NAVD88 forward.
Totally understood. We have recovered NGVD29 marks in the valley floors of California and tied them into NAVD88 based projects (not as constraints, but repurposed for project control and/or to check vertical stability). Using the newly determined NAVD88 value and applying VERTCON to get a modeled ’29 elevation, we have seen MANY that were 5 or more feet lower than the superseded NGVD29 value on the datasheet. Some have exceeded 10′ lower.
I have also seen them on levees that were used as local control for a nearby stream gauge(s) that were more than 2′ lower than where they had been reported to, and relied upon to be. Makes flood forecasting/modeling a little dicey using stream gauge data.
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