Have any of the NYC surveyors out there heard of this benchmark? I received a map from a client of a survey done in '12, it cited;
"Elevations based upon Borough of Manhattan Datum, reputed to be 2.750' above MSL A Sandy Hook, Ref BM 429A, Elv.=16.90"
I found no such benchmark on NOAA, NGS, FEMA, USGS or even a Google search.
It seems to conflict with RTN in NAVD88 translated -1.652 (Manhattan Borough) by .13'.
I don't know how this elevation was transferred to the site, doubtful by GPS.
Any help in this benches location would be appreciated.
Manhattan Datum predates NGVD 1929 and obviously NAVD 88. I'm attaching a FEMA table which explains how to do the conversion once you get NAVD 88 from your GPS. You can get Manhattan BMs from the Borough President's Topo Office if you can ever find them open.
techls, post: 366119, member: 6448 wrote: Manhattan Datum predates NGVD 1929 and obviously NAVD 88. I'm attaching a FEMA table which explains how to do the conversion once you get NAVD 88 from your GPS. You can get Manhattan BMs from the Borough President's Topo Office if you can ever find them open.
I have that chart taped to my field book & hanging on my office wall, use it all the time!!!!
What I am really interested in is finding said BM, I would like to verify it or at least if it was properly transferred to my project. As I said, that was done in '12, designs were based off of it. We get called in to do some work, client (construction management company) asks for a BM from us in Manhattan. Says he knows of no existing BM's or topo on site & we will need to bring in our vertical which we do. That was 3 months ago, suddenly they find the previous topo & send it to me today asking why elevations are different.
Like I said the Manhattan Borough Presidents office has a Topographic Bureau which maintains a record of bench marks. Many of them still survive on school and church steps,etc. Problem is finding the Topographic Engineer, if he still exists. Try the Department of Design and Construction at 1 Centre Street, they have extensive records regarding horizontal and vertical control.
techls, post: 366127, member: 6448 wrote: Like I said the Manhattan Borough Presidents office has a Topographic Bureau which maintains a record of bench marks. Many of them still survive on school and church steps,etc. Problem is finding the Topographic Engineer, if he still exists. Try the Department of Design and Construction at 1 Centre Street, they have extensive records regarding horizontal and vertical control.
Thank you!
I've attached a copy of the Manhattan benchmark listing, all elevations are in Manhattan Highway Datum.
drakej6, post: 366191, member: 6751 wrote: I've attached a copy of the Manhattan benchmark listing, all elevations are in Manhattan Highway Datum.
Wow, interesting stuff, much appreciated.