That's what FEMA wants me to use for a LOMA in Zone A with out BFE. This is on an island with no benchmark. I used "MEAN HIGH WATER MEASURED AT FISH BAY" The site is 50-70 feet above sea level and the tide only moves less than a foot, so it really doesn't matter. Any ideas?
I'm not sure which island you're located on, but it seems like there are a lot of BM's on St. John:
If you can't easily reach one of the BM's, I have in the past used a series of observations on the waterline (with timestamp activated in the collector) to record water levels. Then I'd use tidecharts to come to a value. Preferably I'd have taken a shot at the time of high or low water.
As you say, you're so high above sea level that an error would have to be catastrophic to put you in the flood zone.
You don't have access to GPS do you?
I attended a couple of webinars from NOAA presented by Gerald Hovis on tidal stuff. (super fascinating) I would contact him and/or Dave Doyle to see what they think. What you are dealing with now may not make a difference. But, if I recall the tidal events recycle every 18.7 years so it might make a difference in years to come.
Larry, is there a public airstrip nearby? In the past, lacking any other "real" BM, I have used the centerline of the runway at the approach threshold and looked up the "recorded" elevation for that runway. Of course, Alaska has a runway in every village, not so much in the lower 48. 😛
-JD-
Thanks guys.
I'll get something out that will work.
none of those USGS disks that are still there have elevations as good as the ocean and no, we have no airstrip, thankfully.
Normally tidal datums in the US refer to "zero" as Mean Lower Low Water (averaging the lower of the two lows over 18.6 years).
Canadians calculate their tides differently, maybe something like Low Mean Water (averaging both lows each day?) often referred to as "chart datum".
Do the Virgin Islands have one or two tide cycles each day?
Check with National Ocean Service.
http://tidesandcurrents.noaa.gov/
They may have tidal BMs nearby. They may even have a running tide gauge nearby you can use to "calibrate" the predicted tide at your location. Chase out the low and take your shot right as it goes slack and before it turns around. Compare the running gauge's measured observation to the predicted low and apply that offset to predicted at your location. Not perfect, and subject to inconsistent conditions at the two sites. BUT you will notice far tighter agreement over sucessive days than using predicted alone.
I guess your normal range is only a foot so close enough right? By comparison, Juneau is more like 18' twice a day and Anchorage is 30'.
Years ago a surveyor here used a "Bench Mark" for a resort project but never checked against the sea, He got in lots of trouble when the whole site needed an extra 2 feet of fill to keep it above the sea.
About 3 years ago I talked to USGS surveyors who were running precise levels from the west end of the island to the center (about 4 miles and 1000 feet up) and then down to the south side. But I dont think the data is published yet. And without GPS I wouldn't get it to this site in a day.
Tim Reed, I think estimating MLL is what I'm after. Thanks.
There is an NOS tidal series at Lameshur Bay. It's only a few miles away.
Benchmarks:
http://www.co-ops.nos.noaa.gov/data_menu.shtml?stn=9751381 Lameshur Bay, St. John, VI&type=Bench Mark Sheets
Measured Tide data retrieval:
http://www.co-ops.nos.noaa.gov/data_menu.shtml?stn=9751381 Lameshur Bay, St. John, VI&type=Tide Data
Larry,
The field crew you ran into was from NGS not USGS. The leveling for all of VI has been completed. A technical article describing the Virgin Island Vertical Datum of 2009 (VIVD09) is in the process of peer review by ACSM, and hopefully by the end of next week there will be a Federal Register Notice published that establishes VIVD09 as the official vertical datum of VI. NGS is in the process of final checks on the leveling data so it can be loaded into the database and made publicly available. If you can give me with some specifics about where this parcel is located I will be happy to search the data before it's loaded.
Fantastic!
Lameshure Bay is only about 2-3 miles fom this site mostly along a beach, but by roads it's over 10 miles. There is no need to run levels now.
I sure would like to get the info when it is out.
The local mean sea level height for the primary bench mark at the tide station at Lamesure Bay, 975 1381 A (pid DL3636) was used to define VIVD09 realization for St. John. I will post a note here when the data is loaded.