Subsidence on Cape Cod

I just ran into a serious issue with FEMA benchmarks set in the 1950's and conversion to NAVD88. I am pretty sure that Thadd will chime in on this as he has been studying this for sometime, around 10 years I'm told.

We are working on a project that requires the "lifting" of various roads above the high tide level, plus 0.5'. I established a bunch of GPS points with long term elevations (in excess of 2 hours) for OPUS computations and then proceeded to run levels off of those points and using them for a rather large topographic survey 6 miles of roads.

We tied into this benchmark from FEMA and found that using the Corps conversion for our area (-0.86') to convert from NGVD29 to NAVD88 was incorrect by 0.40'.

I did check into two other NAVD88 published benchmarks and agreed to the 0.001', which I thought was pretty good.

The problem that has not surfaced is the amount of work that was done previously using the -0.86' conversion!

Beware out there, it's an issue that can only be resolved by appropriate occupation of the benchmarks by GPS and not by simple leveling and converting. Of course this BM is set on a culvert, a chiseled square, that sits in a tidal marsh. I expect some subsidence. Same with the second one I checked yesterday. It is a bolt in a headwall set in the same river, different location but still in the mud subject to tidal flow.

Dtp