Is the sea rising or the land sinking?

How does one measure that over time?   With today’s technology, a mathematical ellipse (OK- Clarke’s spheroid of 1888)  in space is used to measure land and water.  At Battery Park at the south tip of Manhattan, the high precision GPS antenna records visible satellite position vectors every second.  The tide station on the pier records water levels every six minutes.  These items are posted on line in near real-time.

Land and Water measured together

The tide data looks like this:

 NOAA tide Station 8518750  The Battery, Lower Manhattan, New York City, NY

Date/      Time           Predic.   Observd.  Diff    Barom.  Air temp  Water temp
(Local Time)               (ft.)    (ft.)    (ft.)    (mb.)    (oF)      (oF)       

10/30/2011 09:36:00  EDT   4.75      4.45     -0.30    1020.0   38.3     56.8
10/30/2011 09:42:00  EDT   4.85      4.54     -0.31    1020.0   38.7     56.8
10/30/2011 09:48:00  EDT   4.94      4.69     -0.25    1020.0   39.2     56.8
10/30/2011 09:54:00  EDT   5.02      4.81     -0.21    1020.2   39.6     56.7
10/30/2011 10:00:00  EDT   5.10      4.90     -0.20    1020.3   39.6     56.7
10/30/2011 10:06:00  EDT   5.17      4.99     -0.18    1020.4   40.0     56.6

(Water level data is referenced to Mean Lower Low Water daily over 19 years)

The daily deviations of the land based GPS antenna look like this:

Daily station deviation from published position

In 1811 the Commissioner’s of the City of New York laid out the street grid system, and established an extensive series of elevation monuments at corner intersections, based on high tide of the era at zero.   EarthImage was asked to calibrate that system in relation to the modern determination of tidal level and land elevations.   Our conclusion was that the City Elevation Zero of 1811, now known as Manhattan Borough Datum, is 4.03 ft over modern mean lower low tide (the average daily low from 1978 to 2001.)  As the daily high today is 4.73 ft higher than that, one can speculate that tides have risen 0.70 feet (8.4 inches) since 1811.

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