Leica Geosystems GPS Basics - Manual (Page 34)

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GPS Basics -1.0.0en Geodetic Aspects
4.6 Map Projections and Plane Coordinates
Most Surveyors measure and record
coordinates in an orthogonal grid
system. This means that points are
defined by Northings, Eastings and
orthometric height (height above sea
level). Map Projections allow surveyors
to represent a 3 dimensional curved
surface on a flat piece of paper.
Such map projections appear as planes
but actually define mathematical steps
for specifying positions on an ellipsoid
in the terms of a plane.
The way in which a map projection
generally works is shown in the dia-
gram. Points on the surface of the
spheroid are projected on to a plane
surface from the origin of the spheroid.
The diagram also highlights the problem
that it is not possible to represent true
lengths or shapes on such a plane. True
lengths are only represented where the
plane cuts the spheroid (points c and g).
0 10 20 30 40 50 60 70 80 90 100 110 E
0
102
03
04
05
06
07
08
09
0
10
0
11
0
N
o
a
b
c
d
e
f
g
h
i
a'
b'
d'
e'
f'
h'
i'
A plane grid based map The basic idea behind map projections