The Cape cod commission just published a cool GIS map of sea level rise and what roads will be KOed based on foot by foot analysis up to 6 feet. The Metadata states that NAVD heights were translated to MHW heights... most of the tidal stations lack published NAVD heights, so I am unsure how that worked.
The punchline was the road analysis. If a road had on section under the MHW plus n feet then every road after was nixed. There is one section that is low and when sea level rises by 3 or 4 feet, then the whole outer cape is not accessible from the mainland... I have a feeling they would remedy the issue pretty quickly when it happens...
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I think the answer to the question or rather the question to the answer is '1 foot accuracy relative to what?'. Perhaps they only care that the relative accuracy of the pointsyou are surveying are accurate to the foot. If they don't know or seem to have someone above them requiring a specific datum, then go with NAVD88 and be sure to document your work.
I, for one, would never refer to mean sea level as a relevant datum. While the NGS definition of the geoid is something along the lines of: an equipotential surface of the earth's gravity field that represents global mean sea level in a least squares sense, it and datums like NGVD29 and NAVD88 are not referenced to mean sea level.
While many surveyors along the coasts are familiar with NOAA COOPS products, others are not. Below is an extract from one of their data sheets:
Tidal datums at CORPUS CHRISTI, GULF OF MEXICO based on:
LENGTH OF SERIES: 16 YEARS
TIME PERIOD: January 1986 - December 2001
TIDAL EPOCH: 1983-2001
CONTROL TIDE STATION: 8779770 PORT ISABEL, LAGUNA MADRE
Elevations of tidal datums referred to Mean Lower Low Water (MLLW), in METERS:
HIGHEST OBSERVED WATER LEVEL (09/13/2008) = 1.709
MEAN HIGHER HIGH WATER MHHW = 0.497
MEAN HIGH WATER MHW = 0.468
MEAN SEA LEVEL MSL = 0.282
MEAN TIDE LEVEL MTL = 0.268
North American Vertical Datum NAVD88 = 0.136
MEAN LOW WATER MLW = 0.068
MEAN LOWER LOW WATER MLLW = 0.000
LOWEST OBSERVED WATER LEVEL (02/28/1984) = -0.836
Note the divergence between MSL and the national vertical datums. If you sample sites in different regions you will see the differences show above to vary. A fundamental flaw in NGVD29 was that it constrained heights to tide gauges that each had its own MSL value but were assumed to each be linked to global mean sea level.
The US NGS site has a lot of details about US national vertical datums as well as presentations and papers on the related issues. They are well worth reading.
Williwaw, post: 364232, member: 7066 wrote:
What to do .... :bored:
The first question I ask, "What are you doing with this?" Until I knew that, I would have zero expert advice on how to proceed.
Victor.Plymouth, post: 364276, member: 11292 wrote: I wonder if you can help me understand the situation?
Living in the small island of Great Britain, our national mapping service provides 90 CORS so that you're never more than 75km (47 miles) from one. We're advised by the RICS (a professional institution) to observe for 1 hour (or 2, 4, 6) when the CORS is within 20km (or 30, 50, 100). As we have an algorithm to adjust to horizontal or vertical coordinates, we can achieve a value to a datum based upon a version of MSL to within +-5cm.
Does the NGS provide something similar? Is it country-wide?
Thank you
Yes, OPUS. But, without actual benchmarks in the area to verify, you are subject to the accuracy of the current model. This is likely very good in the UK, but here, it can be less accurate.
