Terminology - land navigation

Subject: Terminology - land navigation
From: (chris dahlberg)
Date: Thu, 7 May 1998 16:31:19 +1000 (EST)
Hi all
        I am having trouble following the discussions on mapping and GPS
units. This is a problem that is getting worse as time goes on and is to do
with using initials for brevity. UTM means Universal Transverse Mercator and
is a projection where information from a curved surface is projected on to a
flat surface. UTM is a very effective one that is used around the world for
large and medium scale mapping eg. 1:100 000 or 1:50 000. The earths surface
is not a nice mathmatical shape for surveying calculations and so most
continents adopt a spheroid that is as close as can be made to the actual
shape. Australia uses the Australian National Sphereoid or ANS. At large
scale it is desirable to have a grid for all sorts of calculations but
particularly navigation. A grid is a series of equally spaced parallel lines
set at right angles. An origin is established at the lower left hand corner
of the grid and a position on the grid can be defined by so many metres to
the right or east of the origin and how many metres above or to the north of
the origin. The conventional names for these numbers are eastings and
northings. The name for the grid is the Australian Map Grid or AMG and the
easting and northing coordinates are known as grid coordinates. Latitude and
Longitude are angles measured at the centre of the earth measured north or
south of the equator and east or west of the Greenwich meridian and these
coordinates are known as geographical coordinates. Geographical coordinates
are not usually used for navigating at large scale although they can be
found on the large scale maps.
        And then along came satelite navigation. This has developed into a
Global Positioning System or GPS, is very useful and incredibly inexpensive
for what it can do. It is a mass produced piece of electronic equipment that
is used as a navigation tool. I do not know of any Navy, Airforce,
commercial airline, merchant navy or anyone that is safety conscious that
uses this equipment as their sole navigation tool.
        The ANS is not suitable for use with GPS because ANS is local and
GPS is universal and so a universal or global spheroid was needed. In 1972
one was defined and it was called World Geodetic Spheroid as defined in
1972. This has been abreviated to World Geodetic System 1972 or WGS72. The
projection for large scale mapping, UTM, remains unchanged but the detail is
in a different place compared to the more accurate, for Australia, AMG. The
compromise for navigation purposes is the WGS72 grid. Both grids use the
same UTM projection!!!
        It is unlikely that the AMG will be discarded however it may
disapear from our topographical maps. The difference in the grids is not a
straight "block shift" and so whilst it is 200 metres different in one part
of the continent it will not be 200 metres different in another.
        There are a couple of messages in here and I hope they are accepted
in the spirit that they are given. George's warning is very apt, lives have
been lost by total reliance on electronic gadgetry because it has failed in
the past and I have nightmares about it.
         I for one have yet to eyeball a hand held GPS unit, and do not know
what coordinates are displayed, geographicals, grid or both? I would be
grateful if someone would let me know.
Chris Dahlberg
Chris Dahlberg's SPECIALISED River Tours
Daintree Village
Tropical North Queensland
Telephone and Fax 07 4098 7997
Daily Dawn Birdwatching Tours

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