User:Droll/Elevation

Elevation measurements in the United States

Researching elevations of geologic features in the United States can be confusing for many editors. Here we use the summit of Mount Whitney in the Sierra Nevada as an example. The estimated elevation of the summit of Mount Whitney has changed over the years. The technology of elevation measurement has become more refined and, more importantly, the vertical coordinate system has changed.

The elevation shown on older topographic maps was 14,494 feet (4,418 m). This is the elevation stamped on the United States Geological Survey (USGS) brass benchmark disk on the summit. More modern topographic maps show the elevation in meters as 4,416.9 m (14,491 ft)

Elevations are based on models that attempt to estimate the shape of the earth. This shape is called the geoid. The understanding of this shape has changed over the years and the data used to measure elevation has change as a result. For many years a vertical datum called the Sea Level Datum of 1929 was used. It was based on measurements of mean sea level which, it was thought, corresponded with the shape of the geoid. In 1973 the name of this datum was changed to National Geodetic Vertical Datum of 1929 (NGVD 29). Older estimated elevations are based on NGVD 29.

In 1991 NGVD 29 was abandoned in favor of a new datum called North American Vertical Datum of 1988 (NAVD 88). This new datum is based on an improved understanding of the shape of the earth (the geoid) and other considerations. Using this new datum resulted in a change in the way elevations are measured. This transition has lead to confusion among nonprofessionals. Most websites still give elevations using NGVD 29. For example, PeakBagger.com gives 14,495 feet for the elevation of Whitney, Peakware.com gives 14,494 feet, The National Park Service gives 14,494 feet,

The elevation of Whitney using the new datum, NAVD 88, according to the National Geodetic Survey (NGS) is 14,505 feet (4,421.0 m). It might helpful to understand that the mountain has not measurably grown in the recent past and the survey foot has not changed in the U.S. What did change is where zero is. NGVD 29 defined zero elevation as mean sea level (MSL). NAVD 88 defines zero as the surface of the geoid. Geographers and geodesists have not done a full survey of the United States but have, instead, created a mathematical model which can be used to convert elevations from one datum to the other. The result of this work is an orthometric height conversion tool, called VERTCON. It is available as a tool on the web.

As a working example of conversion consider that the NGS datasheet for Whitney which gives us this data:

                       *CURRENT SURVEY CONTROL
 NAD 83(1992)-  36 34 42.89133(N)    118 17 31.18182(W)     ADJUSTED  
 NAVD 88     -      4421.0    (meters)   14505.     (feet)  VERTCON   

This states that the elevation of 14,505 feet was calculated using VERTCON.

Further down the datasheet it gives us this superseded data:

                         SUPERSEDED SURVEY CONTROL
 NAD 83(1986)-  36 34 42.88215(N)    118 17 31.17367(W) AD(1984.00) 2
 NAD 27      -  36 34 43.11320(N)    118 17 27.85660(W) AD(       ) 2
 NGVD 29 (07/19/86) 4419.1    (m)        14498.     (f) VERT ANG       

This states that the NGVD 29 elevation measured in 1986 was 14498 feet. This is the figure used to calculate the NAVD 88 elevation. Notice that the horizontal data was also converted with a change form the North American Datum of 1927 (NAD 27) to the North American Datum of 1983 (NAD 83).

The National Geodetic Survey is not the only governmental agency publishing NAVD 88 elevations however. The United States Geological Survey (USGS) also publishes NAVD 88 elevations on their GNIS website. These are seldom, if ever, the same as those published by the NGS. The USGS gives 14,452 feet as the elevation for Mount Whitney. This is result of using the National Elevation Dataset (NED). These elevations, given on the Feature Detail pages, are seldom used in Wikipedia. Notice that elevations from the NED are deprived using satellite technology and do not depend on the NGS data set or older elevations from the USGS which were published on topographic maps. The elevations of mountain peaks are usually underestimated, especially if they are precipitous. This is because the satellite data reflect the mean elevation of a measured area and not its highest point.