Adjustment Computations, 6th Edition

CHAPTER 24COMBINING GNSS AND TERRESTRIAL OBSERVATIONS

24.1 INTRODUCTION

Ellipsoids define the mathematical shape of the Earth, or a portion thereof. Ellipsoids are commonly defined by the length of their semimajor axis, a, and the flattening factor, f. Commonly used ellipsoids are the Geodetic Reference System of 1980 (GRS 80) and the World Geodetic System of 1984 (WGS 84). Table 24.1 lists the length of the semimajor axis and flattening factor for these ellipsoids.

TABLE 24.1 Defining Ellipsoidal Parameters

Ellipsoid	a (m)	1/f
GRS 80	6,378,137.0	298.257222101
WGS 84	6,378,137.0	298.257223563

A network of points determined with respect to each other on an ellipsoid is a reference frame, which is also called a datum. These reference frames define the geodetic coordinates of the points, and thus the origin and orientation of the datum. Most reference frames are regional in nature since the network of points only covers a portion of the Earth. These are known as local reference frames. For example, the North American Datum of 1983 (NAD 83) is a local coordinate system consisting of a network of points in Canada, the United States, Mexico, and some Caribbean islands. However, the International Terrestrial Reference Frame (ITRF) is an example of a global datum defined by a multitude of points located on all major land masses of the Earth. Similarly, the coordinates ...