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Rambli Ahmad, a postgraduate student at the Universiti Malaysia Sarawak, taking a GPS reading during the People and Plants Certificate Training Course in Applied Ethnobotany (CTC).
Gary J. Martin
 

Finding geographical positions
with a GPS Receiver

Required Tools:

  • GPS receiver
  • Notebook for recording information about localities
  • Topographical map (optional)
  • Globe of the earth (optional for teaching the technique)

Description: GPS stands for the Global Positioning System, a network of satellites created in the 1970s and 1980s by the Department of Defense of the United States of America. The system, which cost over 13 billion US dollars to develop, consists of 24 satellites that revolve around the earth at an altitude of 20,000 kilometers, following six distinct orbits. Although the GPS was originally designed solely for military purposes, many civilians now access it to find their precise latitude and longitude on the face of the earth.

A GPS receiver is a hand-held device that allows you to determine your position within a small margin of error, usually 100 meters or less. It is used extensively for calculating speed or navigating - finding how to go from one place to another - by people who travel in boats, planes and other vehicles, or who are walking cross-country. The receiver works by triangulation: it measures the travel time of signals transmitted by each of at least three satellites, computes its distance from them and then calculates the current location and speed of the person holding the receiver.

As with many technological devices, GPS receivers were very expensive to begin with, but have become less costly over time. At present, you can find personal models that range in price from US$300 to US$800. Sophisticated research models cost thousands of dollars.


Background concepts:

z Strictly speaking, the Global Positioning System is more than a network of 24 satellites. It also includes the GPS ground-control stations and the people who use receivers to determine their position or to guide navigation.

z When the earth is represented as a sphere, the lines that run around the globe from the poles to the equator mark latitude. They divide the earth into northern and southern hemispheres. At right angles to the latitudes, there are longitudinal lines that divide the world into eastern and western hemispheres. Together, latitude and longitude are referred to as coordinates, and are measured in degrees, minutes and seconds. Each latitude reading is accompanied by N or S (for Northern or Southern hemisphere) and each longitude reading is accompanied by E or W (for Eastern or Western Hemisphere). Latitude ranges between 0 and 90 degrees, whereas longitude ranges between 0 and 180 degrees.

Useful facts:

The U.S. Department of Defense limits the accuracy of GPS receivers employed by unauthorized users to 100 m horizontally (longitudinal and latitudinal position) and 156 meters vertically (i.e. elevation). This policy, referred to as selective availability, was to be suspended in late 1996, but has apparently continued to this day.

Degrees of latitude are equally spaced, but the slight flattening at the poles causes the length of a degree of latitude to vary from 110.57 km at the equator to 111.70 km at the poles. At the equator, meridians of longitude 1 degree apart are separated by a distance of 111.32 km ; at the poles, meridians converge. Each degree of latitude and longitude is divided into 60 minutes, and each minute divided into 60 seconds, there by allowing the assignment of a precise numerical location to any place on earth. [Encarta ‘96]

This means that in the tropics, a second measures approximately 31 m and a minute equals about 1.850 km.

The circumference of the earth at the equator is over 40,000 km

Alternative methods:

Another way of finding your latitude and longitude is to consult a topographic map, but this is usually less precise and requires more skill than using a GPS receiver.

Applications:

GPS receivers are commonly used by botanists to record the precise position of collecting localities (that is, where they have collected a certain plant or animal). The receivers are also used to describe specific localities named by local people (toponyms), which are usually of special importance to them. Colleagues involved in community mapping projects use GPS receivers to map key topographical features. These data are often incorporated in Geographical Information Systems (GIS), which integrate topographical, biological and others types of observations in a single computerized representation of the landscape.

Related exercises (planned):

Documenting toponyms, Making plant collections, Measuring elevation with an altimeter, Community mapping

How to do it:

Although you will have to consult the user’s guide of your specific GPS receiver for details, the general procedure is simple. Find a relatively open spot near where you would like to take your position. Turn the GPS receiver on, and wait for a reading. The receiver must be in contact with at least three, and preferably four, satellites. You may have to wait for several minutes, so you may wish to turn the receiver on while you are explaining to others how it works.

Once you have a reading, record it in a notebook with other relevant information such as the local name of the place, type of vegetation, topography, slope and exposure (see Documenting a toponym). If you are teaching this technique in a workshop or course, you can ask participants to find their location on the face of the earth and on a topographic map and world globe. Ask them to observe the landscape around them to see if it corresponds to the features on the topographic map at the position indicated by the receiver.

Practice saving the position coordinates in the memory and giving the location a name. Then walk to another location and go through the procedure again. When you have two or more locations stored in the memory, you will be able to calculate the distance from one place to another, and design a path that passes through the points. If you record many points, you can map out the trail that you took.

Tips:

l GPS receivers use a lot of energy, so be sure to bring along plenty of extra batteries (usually AA alkaline batteries are used).

l Although a GPS receiver will give you an elevation reading when in contact with at least four satellites, this measurement is rarely as accurate as that given by an altimeter

l GPS receivers rarely work well under dense or even moderate forest cover. One solution is to purchase an antenna that can be carried up into the tree canopy, but most people simply walk to the nearest clearing, such as a cultivated field, forest gap or river.

Helpful Illustrations:

The February 1996 issue of Scientific American has excellent illustrations of how the Global Positioning Satellite system functions (see cover and inside pages). The Trimble guide book (pages 1-2, 1-3) is also a useful source of illustrations. Microsoft Encarta ’96 has nice illustrations of the globe with lines of latitude and longitude.

Example: none

References:

F GPS - the Ninth Utility, an 80 page booklet produced by Trimble

F Herring, T.A. (1996) The Global Positioning System. Scientific American, February, pp. 44-50

Useful addresses:

Trimble (supplier of GPS receivers)
9020 - II Capitol of Texas Highway North Phone +1.512.3438980
Suite 400 Fax +1.512.3459509
Austin, TX 78759
USA

Prepared by: GJ Martin, based on experiences in various training workshops.


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