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Converting Tide Elevations to the NAVD88 Datum

Stephen M. Dickson, Maine Geological Survey
February 25, 2008

1. Look up the mean tidal range and spring tidal range in a book of tide tables or on the web (URL below). In Portland the mean range is 9.12 feet and the spring range is 10.53 feet.

2. Determine the difference between the Mean Tide Level (MTL) and NAVD88. Note that NAVD is higher than MTL. Here are the best examples I can find on the web:
• 0.35 feet at Portland using 19 years of data in the new 1983-2001 Epoch,
• 0.31 feet at Bar Harbor using 17 years of data in the new 1983-2001 Epoch,
• 0.32 feet at Eastport using 19 years of data in the new 1983-2001 Epoch.
More benchmark details. Not all benchmark locations have good values relating NAVD to the tides. The best sites are those with tide gauges in place for the last 20 years or longer (in the list above).

3. Divide the range by 2 and then subtract the vertical difference between NAVD and the MTL to get the height of Mean High Water (MHW) or Spring High Water (SHW).

Example 1: The spring range of tide in Portland is 10.53 feet and the height of the MTL is 0.35 feet below NAVD88. So the height of SHW in NAVD is:

SHW = (10.53 / 2) - 0.35 = 5.26 - 0.35 = 4.91 feet NAVD88.

Example 2: Stonington, Deer Isle: MHW = 9.7, SHW = 11.2 and Bar Harbor is the reference station. So, using the mean range and spring range from Table 2, MHW and SHW can be calculated:

MHW = (9.7/2) - 0.31 = 4.85 - 0.31 = 4.54 feet NAVD88 and

SHW = (11.2/2) - 0.31 = 5.60 - 0.31 = 5.29 feet NAVD88.

Notes:

The Highest Annual Tide cannot be calculated by this method due to asymmetry in the high and low levels about the MTL. Current year HAT values along the coast can be obtained from the Maine Department of Environmental Protection's Shoreland Zoning staff or by accessing their web site.

Due to the accuracy of some values, it is best to round values to the nearest tenth of a foot after making a calculation.

Last updated on November 19, 2010