Geoscience Reference
In-Depth Information
measurement: (1) the azimuth is retained by the compass when
one moves away from the sighting; (2) the map does not have
to be orientated to transfer the measurement (step 3, Figure
2.12); (3) the transparent compass-needle housing makes
transfer onto the map easier.
2.4 Global positioning systems
and altimeters
2
Global positioning systems (GPS) use ultra high-frequency radio
wave signals from satellites to trigonometrically derive your
position to within a few metres laterally. A wide range of GPS
systems are available on the market and the reader should refer
to specialist reviews and literature for more information.
Increasingly, mobile phones contain a GPS unit. Global
positioning systems units do not work in deep ravines and on
some coastal sections; they are also not particularly accurate for
altitude. The GPS can be set up for the particular grid system that
you are working with or for a global reference that is based on
latitude and longitude. The global reference World Geodetic
System 1984 (WGS84) is the most commonly used. Instructions
on how to set up your GPS will be in the manufacturer's manual.
After setting it up or modifying any settings, for instance when
you go to a new country, it is a good idea to test it out at a known
location. The unit may take some time to locate the satellites if
the GPS has been moved hundreds of kilometres.
If you use a GPS together with hard copy maps as your main
location device in the fi eld you should ensure that you also
have a ruler with you so that you can accurately plot your
position. A GPS should not be a total substitute for basic
navigation skills. You should also know how to locate yourself
with a map and a compass (Section 2.3.3) so that if the GPS
goes wrong, the batteries fail, or you are in an area where the
satellites are obstructed, you have an alternative means of
location and navigation. Alternatively if you are in an area of
the world where the base maps are poor or only available at a
small scale, a GPS reading will probably be more precise than
compass triangulation and will more easily allow exactly the
same locality to be found again.
Ensure that you have set up
your GPS or altimeter
correctly otherwise you could
spend the entire day being
somewhere other than where
you thought you were!
An altimeter is useful for recording elevation more precisely
than a GPS when mapping or working in steep terrain, for
instance recording a steep stream section. Altimeters use air
pressure to measure elevation and this will vary with the
weather. You need to calibrate your altimeter by taking a reading
when you are at a known elevation at least once a day and
throughout the day if the weather is changing signifi cantly. A
Brunton-type compass-clinometer and Jacob staff (Section 2.5.2)
can also be used to determine elevation above a known datum.
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