Converting Tide Elevations to the NGVD29 Datum
Stephen M. Dickson, Maine Geological Survey
February 25, 2008
- Look up the mean tidal range and spring tidal range in a book of tide tables or on the web. In Portland the mean range is 9.2 feet and the spring range is 10.53 feet.
- Determine the difference between the Mean Tide Level (MTL) and NGVD29. Note that NGVD is lower than MTL. Here are the best examples I can find in my archived files and on the web:
Current benchmark details. Not all benchmark locations have good values relating NGVD to the tides. The best sites are those with tide gauges in place for the last 20 years or longer (in the list above).
- 0.39 feet at Portland using 19 years of data in the new 1983-2001 Epoch,
- 0.39 feet at Bar Harbor using 16 years of data in the old 1960-1978 Epoch,
- 0.29 feet at Eastport using Corpscon 6.0 software and interpolation.
- Divide the range by 2 and then add the vertical difference between NGVD 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 0.39 feet above NGVD29. So the height of SHW in NGVD is:
SHW = (10.53 / 2) + 0.39 = 5.26 + 0.39 = 5.65 feet NGVD29.
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) + 0.39 = 4.85 + 0.39 = 5.24 feet NGVD29 and
SHW = (11.2/2) + 0.39 = 5.60 + 0.39 = 5.99 feet NGVD29.
Tide predictions for the field.
Historical tide data.
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