Horizontal geodetic datum consists of a starting point and an ellipsoid on which to
longitude of the initial point. The azimuth of a line from the initial point gives a direction with which
the computations are made. A change in any one of these five quantities will change the datum; hence,
the coordinates of all points on the datum would also change.
(2) The vertical datum, to which the elevations of the points are referenced, is usually the MSL
surface. However, it may be any arbitrary level surface defined by an assumed elevation for some point.
b. Geodetic-datum orientation may be established using a single astronomical point and one
astronomical azimuth, an astronomical-geodetic method, or a gravimetric method.
(1) Single Astronomical Point and One Astronomical Azimuth. After a reference ellipsoid is
determined or established, the other three parameters or factors must be defined. The simplest means is
to select a first-order triangulation station located at the center of the surveyed area. At this point, the
astronomic coordinates are observed and the astronomic azimuth from this point to another control
station in the net is determined. In this method, it is assumed that the normal (perpendicular) to the
ellipsoid coincides with the plumb line or the normal (perpendicular) to the geoid (Figure 1-16).
Therefore, the observed astronomical coordinates and azimuth are adopted, without any corrections, as
coordinates and azimuth on the ellipsoid. The two components are defined as zero--the deflection of the
vertical and the undulation of the geoid.
(a) Reference Figure 1-16 and note that a datum oriented by a single astronomical point
produces large geoid separations. Also, it is not earth-centered, and the rotational axis of the ellipsoid
does not coincide with that of the earth. The inconvenience of such an orientation is that the positions
derived from different datums are not directly comparable in any geodetic computation.
(b) This type of orientation could be used locally for positions in the net, although large
errors would probably be introduced as the survey is expanded because the entire net would be shifted
relative to the axis of the earth.