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Not directly comparable
- Elevation values from maps using different vertical datums (e.g., NGVD 29, NAVD 88, MLLW, and MHHW) are not directly comparable because they are based on different frameworks and starting (or zero) points.
www.mass.gov › info-details › north-american-vertical-datum-of-1988-navd-88
People also ask
Are GPS elevations based on a USGS map?
What is a vertical datum?
Is a vertical datum just a reference surface?
What are the different types of heights in a vertical datum?
Oct 29, 2023 · A vertical datum is a reference surface of zero elevation to which heights are referred to. Datums are used to measure height and depth from mean sea level.
Elevation values based on different vertical datums cannot be used together directly since they are based on a different vertical reference point (See graphic).
A vertical datum is a surface of zero elevation to which heights of various points are referenced. Traditionally, vertical datums have used classical survey methods to measure height differences (i.e. geodetic leveling) to best fit the surface of the earth.
Oct 3, 2014 · The most likely answer has to do with different vertical datums used, as mkennedy suggests. You should probably trust the most recent published data regarding absolute elevations. When comparing elevations, at least use values from the same era.
USGS topographic maps published after 2009 are georeferenced to the North American Datum of 1983 (NAD83; which is virtually equivalent to WGS84 at topographic map scales), but uses the North American Vertical Datum of 1988 (NAVD88) for vertical control of elevation.
Elevations provided by your GPS receiver might disagree with elevations on a USGS map, but they could both be correct if they reference different vertical and/or horizontal datums. The default horizontal datum setting on most GPS receivers is the World Geodetic System of 1984 (WGS84) and the elevations are based on the NAD83 ellipsoid.
Nov 4, 2020 · The vertical datum used by surveyors across the country is mean sea level, obtained through a tide gauge. Of course, measuring the sea is a lot trickier than measuring the floor in your house to check a person’s height.
