Monday, November 17, 2008

How high is my field site?

A couple of useful links for figuring out your field site elevations relative to the tidal datum and nearby National Geodetic Survey benchmarks:

NOAA Tides and Currents

Under the products menu in the top left, you can choose Water Levels. Then you can use the search box at the top of this page to find your local tidal station. If I enter Monterey CA, it takes me to the Monterey Tidal Station page:
Monterey Station ID #9413450

If you scroll down to the bottom of the page, you'll find a linke for "Datums" on the left, which takes you to the datum information page for this tide gauge:
Monterey Station Datums
This lists the relative heights of several things, such as Mean Lower Low Water (i.e. the zero tide level, = 1.031m above the North American Vertical Datum at Monterey), and the height of the tide station relative to the North American Vertical Datum (NAVD88, 0.988m at Monterey). These two bits of information are potentially useful, since it tells you the difference between the NAVD88 and zero tide level at this locality. Now if you know the NAVD88 height for another NGS benchmark (say at your nearby field site), you can relate the height of that benchmark to the zero tide level at your site. Over relatively short distances (a few km) in areas without complex bathymetry that messes up the tidal range, you can feel relatively comfortable extrapolating the NAVD88-MLLW difference (NAVD88 is 0.043m below MLLW at the Monterey tide gauge) to your local benchmark. In a place like Monterey, where the tidal range is more or less the same around the peninsula and Monterey Bay, this approximation should work. If your field site was at the end of a long inlet with a tidal amplitude that was substantially damped relative to the tidal range at the nearby tide gauge, this approximation would fail.

If you know the difference between NAVD88 and MLLW, the next step might be to figure out the elevation of your local NGS benchmark. One handy tool for determining this is the NGS's Survey Control Map:
NGS Map.

That link opens up a map that you can use to zoom in on your field site. When you get zoomed in to your area of interest, click on the "Display Stations" button on the right side of the map. This will overlay the various benchmarks in the area and pop up two more windows, one of which will contain a list of all the benchmarks in view.
In the popup window that lists all the benchmarks, you might find it useful to uncheck the box that says "Always append new". Then go back to the main map and choose the thing that looks like a triangle from the tool menu on the left. You can now click on individual benchmarks on the map, and they will pop up in the info window.

If you then click to highlight the benchmark, and hit the "GetDatasheets" button, you'll get a text datasheet with all the station info for that benchmark. This includes the latitude/longitude and usually an elevation relative to the North American Vertical Datum (88).

In this case, you can note that the NAVD88 elevation for this benchmarkis 6.84 m. Since we determined earlier that the difference between NAVD88 and MLLW in Monterey is 0.043m (the NAVD88 zero height is slightly lower than MLLW), the elevation of our benchmark above MLLW at this site is ~6.80 m. In other words, if we descended 6.8m down the shore from the benchmark, we'd be at Mean Lower Low Water. This can allow you to take measurements relative to this fixed benchmark on land over the years without having to do the usual song-and-dance of running down to the current water level, measuring a height, and then trying to guess at what the current tide level was at that time based on tide tables. The benchmark provides a stable reference point that you and others can come back to year after year to make new measurements.

You might want to save the "Pid" number for you benchmark for later reference. In this case it was GU3971. You can use this to quickly retrieve the datasheet for the site in the future using this NGS webpage:
NGS DatasheetPID form

Some other potentially informative/useful sites:

Info and definitions on tidal datums:
Tidal Datums info at Tides and Currents, NOAA

A software tool for converting between vertical datums and among various geoids:
VDatum user's guide

Lastly, when looking up the Mean Lower Low Water datum, don't overlook the Tidal Epoch that MLLW is based on. The epoch is explained at the Tidal Datums page listed above. In short, the zero tide level (Mean Lower Low Water) is re-adjusted every 19 years or so. This is done to account for long-term changes in sea level due to a number of factors including sea level rise or local geological processes (isostatic rebound from glaciation for example). We are currently using the 1983-2001 National Tidal Datum Epoch, which went into effect around 2003. Therefore it's not going to change for quite a while, but if you're working with older studies that were referenced to MLLW, the absolute position on the shore may have shifted between then and now. The change in the height of MLLW is specific to a given tidal station, so it is useful to not only note the epoch you're referencing (NTDE 1983-2001) but also the tidal station that your MLLW level is based on, i.e. (NTDE 1983-2001, Monterey, CA).