# How Elevation is Determined

by Curt Sumner, LS (VA and MD)

The definition of elevation as stated in the American Congress on Surveying and Mapping (ACSM) publication Definitions of Surveying and Associated Terms is: The vertical distance from a datum, generally mean sea level, to a point or object on the earth's surface. The terms "elevation" and "altitude" are sometimes used synonymously, but in modern surveying practice the term "elevation" is preferred to indicate heights on the Earth's surface, whereas "altitude" is used to indicate the heights of points in space above the ellipsoid representing the sea level surface of the Earth.

More simply stated, elevation is an expression of the difference in height between points on the earth's surface. The reason a datum such as mean sea level is used is so that there can be a uniform basis on which to evaluate the height differences.

For example, if one point is said to have an elevation of 2000 feet above mean sea level, and another point is said to have an elevation of 1500 feet above mean sea level, it is easy to calculate that the first point is 500 feet above the second point on the Earth's surface.

In order to make this determination, one must physically observe the two points in some manner. In surveying, one would not simply use a series of benchmarks to establish the difference between two points without a checking mechanism. The surveyor may conduct a "closed loop" which would begin at the first point and end at that same point, having been through the second point along the way. The same benchmark points may or may not be used as the surveyor proceeds in each direction, but doing so can be useful in finding any errors that may have occurred along the way.

Another method is known as "trigonometric leveling". This method uses the horizontal distance between two points, and the vertical angle measured between them, to calculate the difference in elevation using a trigonometric formula.

There are no absolutes in surveying measurement. There will always be some amount of error. Controlling that error to be within acceptable standards is the objective.

There are other ways of determining elevation differences between points on the Earth's surface. Two methods that use modern technology are GPS and photogrammetry. To date, GPS elevations have not proven to be as realiable for surveying operations as are the horizontal positions of points as determined using GPS. There are many factors that contribute to this. Even if elevations are determined using photogrammetry, the base points on which they are determined have likely been established by traditional surveying methods.

In any case, a base datum must be used. It is important that the datum used is clearly defined so as to lessen the incidence of "mixing datums" which leads to inaccuracies in the use of the elevations.

For local use, one can establish an arbitrary datum by assigning an elevation to some point, and then determining the relative elevation of other points with respect to that point. This method is only useful when it is not necessary to know the points' elevations with respect to some datum such as mean sea level. It is useful only when the differences in the elevations of points is to be used within a specified area.

To re-state, elevations are simply an expression of the vertical relationship between points on the Earth's surface. What is important to understand about elevations is that one must know on what datum they are based in order to use them properly.

Curt Sumner is Executive Director, ACSM/NSPS

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