Gauss and UTM Conformal Projections and the Plane Rectangular Coordinate System - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 295

6.5 Elements of the Geodetic Control Network Reduced
to the Gauss Plane
6.5.1 Reduction of the Geodetic Control Network
on the Ellipsoid to the Gauss Plane
According to the method for “reducing terrestrial observation elements to the
ellipsoid” introduced in Chap. 5 , we can reduce the elements of field observations
(the horizontal direction, zenith distance, terrestrial distance, and astronomical
azimuth, etc.) to the ellipsoid and then solve geodetic problems. Hence, the
geodetic control network constituted by points on the Earth's surface can be
reduced to the control network formed by geodetic points on the ellipsoid. How-
ever, the practical attempt to solve geodetic problems on the surface of the Earth
ellipsoid has been proved remarkably complex. In addition, the control network on
the ellipsoid cannot provide direct control for topographic mapping. In order to
meet the needs of controlling topographic mapping and simplifying computations
of the control network, we need to employ the method of Gauss projection to reduce
the geodetic network constituted by geodesics on the ellipsoid to the geodetic
network connected by straight lines on the plane. The computations entailed by
the process of reduction are hereby presented specifically.
First, a few concepts are introduced below.
True North and Grid North
The so-called true north of a point refers to the direction of the northern end of the
true meridian (geodetic meridian) which passes through the point, namely the
direction towards the Earth's geographic North Pole. Grid north is the direction
of the northern end of a straight line which passes through a point in the Gauss plane
parallel to the vertical axis (northern direction of the north-south grid lines).
True Bearing and Grid Bearing (Azimuth)
True bearing is the angle between the true meridian and the geodesic, i.e., the
geodetic azimuth. Grid bearing is the angle between the direction of the true north
and that of a straight line on a plane. It is measured as positive clockwise from the
grid north.