Leica Geosystems GPS System 500 - Technical Reference Manual - Manual (Page 273)

273 11. Applications Technical Reference Manual-4.0.0en
11. Applications
Applications contains a number of
miscellaneous functions that are not
necessarily related.
From within this menu item you may
determine coordinate systems, carry
out point management functions,
access an on-board calculator, define
wake-up sessions and access any of
the standard and/or optional
application programs (assuming the
security code has been entered).
For further description of the optional
Application programs please refer to
the appropriate manuals.
11.1 Determining a Coordinate System
GPS gives coordinates relative to a
global datum known as WGS84. This
coordinate datum is however
relatively new in terms of the length of
time that people have been giving
specific points coordinates. The
WGS84 datum is therefore not used
as the datum for coordinates in the
vast majority of countries around the
world.
As surveying developed through the
centuries, individual countries
adopted datums that best suited their
individual requirements.
Therefore when surveying with GPS,
coordinates are first obtained relative
to the WGS84 datum. These
coordinates then have to be
transformed into the local coordinate
system.
There are several methods by which
this can be done. One is the Helmert
approach where the coordinates are
transformed from the WGS84 Datum
to the local ellipsoidal datum and then
a map projection is applied to obtain
grid coordinates. Other methods
involve transforming the coordinates
directly from WGS84 into a local grid.
System 500 receivers contain three
different methods: the Helmert
method, the 1-Step method and the
2-Step method. The method used
depends on the results required and
the quality and extent of known
points.
Which method to use?
This question largely depends on
local conditions and information.
If you wish to keep the GPS
measurements totally homogenous
and the information about the local
map projection is available, the
Classical 3D approach would be the
most suitable.