A height and an elevation

The value of the difference may reach several hundreds of milliGalls. 

To illustrate Equation (2.253), consider two points a and b located at different equipotential surfaces and assume that they are located so close to each other that the field between them changes linearly along the vector line. At the same time the separation of these surfaces may vary. First, choose the path ab b, where points b and b are at the same plumb line, perpendicular to both surfaces. Fig. 2.8c. Then, 

As we know, the quantity Ah is called the elevation or an orthometric height of the point b with respect to point b. In particular, if both points are located at the same level surface, this elevation is zero. If we take a different path, for instance, aa b, Equation (2.255) gives a height of the point a with respect to a, which can differ from the first one. 

Suppose that within the interval a—b the equipotential surfaces are parallel to each other, that is, the field does not change along these surfaces. In such a case the 

We assume that with the help of leveling we solved our first problem and found the separation between the geoid and the points of the physical surface of the earth. Our next step is to determine the position of the geoid with respect to the reference ellipsoid. The solution of this fundamental problem was given by Stokes. To begin, 

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