The Dutch dictionary says navigation is
the 'knowledge that is needed to guide a vessel to a target'.
To navigate has to do with determining position and course and
avoidance of difficulties underway. Navigation seems to be something
for ships only - in the water as well As in the air. Navigation is
also important for the ships of the desert. The guides of caravans
through the Sahara desert maybe are the eldest navigators in the
world.
Navigation is all about:
To navigate several aids and appliances exist. The Global Positioning System and the use of a GPS- Navigator (GPS in short) combines techniques of using maps and a compass in a smart way. With the use of only one device position, course and destination can be determined. Thanks to electronics speed, distance and track are kept in memory. With the use of a personal computer routes can be made and loaded into the device for use later on.
Use of GPS also has disadvantages. A GPS-Navigator is a vulnerable piece of electronics, that uses lots of electricity. Four AA alkaline batteries of a good quality take 4 to 5 hours to wear out completely. Furthermore a GPS needs movement to provide useful information. On a fixed spot a GPS-Navigator only displays your actual position, accurate (atomic) time and the time of sunrise and sunset. A GPS-Navigator never can replace a compass!
Those who want to navigate well should be able to navigate with minimal device: map and compass. With these not only the basic principles of navigation can be learned. They also make you least vulnerable to weather conditions (clouds that hinder receiving satellite signals) and empty batteries. Best is to use a GPS-Navigator in combination with map and compass. The combined use of GPS and compass may save a lot of your battery power: the course the GPS tells you to follow to your next waypoint can be set out and followed with a simple compass! A good map is needed to prepare a trip and determine waypoints.
The Global Positioning System (GPS) holds 24 satellites that circle around our globe. Each of these satellites travels a predetermined course and sends a unique signal. A GPS-Navigator can receive these signals and make something useful of it.
A GPS-Receiver or Navigator processes the signals of the satellites with a built-in almanac. This almanac contains descriptions of the orbit the satellites travel trough space. together with the unique signal of a satellite and the time it takes that signal to reach the GPS-Receiver the exact location on earth can be determined. To measure time each of the satellites has its own and very accurate atomic watch.
Places on earth with equal time intervals between satellite and position are on a circle. To determine one unique position on earth a minimum of three satellites are needed. The Navigator is at the spot where the three iso-chrono circles meet. To provide information about the altitude a fourth signal is needed.
The GPS is maintained by the American
Ministry of Defence. For safety reasons the given signal is somewhat
distorted. This is called selective
availability (SA). Because of this
the position the Navigator provides is not accurate. Sometimes SA is
set off. During the Gulf war allied forces had to use GPS in de
desert not to get lost. Because the military didn't have enough
Navigators themselves SA was set off to be able to use commercially
available devices.
Besides the on-purpose inaccuracy measuring distances always has some
inaccuracy. On Garmin Navigators this is called EPE - Estimated Position Error. Nevertheless a handheld GPS-Receiver is fairly
accurate. Under ideal conditions and with SA position can be
determined with 15 metres accuracy.
Greater accuracy you get with DGPS - Differential Global Positioning System. The satellite signal is compared to a radio signal from a fixed beacon. The position of this beacon is known precisely. With the aid of DGPS a position can be determined with 5 cm accuracy. This is unnecessary for outdoor-sport use but useful in building roads or dredging shipping lanes. Because of the DGPS the need for selective availability diminishes. Sometime this might trigger abolishment of SA.
In fact using the GPS is an old form of
navigation with new devices. The use of a jacobsladder (a long skewer
with two crossbars) and later on the use of a sextant the height of a
star or planet to the horizon was determined. The combination of
height, date and time differs for each position on earth and was
described in almanacs. 'Looking at the sun' wasn't without any risk.
Many a sailor lost an eye making a position and stared too long into
the sun. Sextants for this reason have different dark
lenses.
The Decca navigational system is based on radio beacons along the
shores and is therefore no satellite navigation.
Just like any other electronic toy handheld GPS-Navigators become cheaper and cheaper. In 1992 the Garmin GPS38 cost 550 euro. Today the much better GPS12 receiver costs only 180 euro. Newer devices of same size and weight are able to do more and do it faster. Serial 8-channel receivers like the GPS38 are followed up by 12-channel parallel receivers like the Garmin GPS12XL. This Navigator can keep track of 12 different satellites at the same time. This means that you get a more accurate position in less time. Receivers also became more powerful and seem to work under trees as well. Recently GPS handheld Navigators come with road maps already programmed.
I think the lower end of the market is still a good choice for recreational use. Basic functions of these Navigators work well and battery power is not spilled on things like aging road maps (that accidentally does not depict the road your ride). Coosing different icons for each waypoint is standard nowadays. The use of turned over displays as on the Garmin GPSII will always be unknown to me. The external antenna is unsuitable for use on a motorbike. Used in the woods the ability of more expensive receivers to make a fix under the canopy of leaves is not always working. Sometimes it can be useful to put more waypoints into your receiver but pen and paper could help the same way. For recreational use the speed of determining position is of less importance. Using the GPS continuous this also is of minor importance. Most time to determine position is used at a 'cold start' - after the device was shut off for more than 2 hours or travelling 800 km from the last used position, something that happens not every day. A warm start also makes a swift position on less expensive devices.
When buying a GPS-Receiver be sure it can handle a common system of coordinates like UTM. Inform yourself what systems are used in your part of the world or where you want to travel and the system used on your favourite maps. This assures you can easily determine waypoints from an ordonnance survey map and put them into the Navigator.
The describings below are based on my own Garmin GPS38. Other receivers often have similar fundions. All Garmin Navigators should have more or less the same pages.
The Garmin GPS38 has different pages that have the following characteristics (from left to right)::
satellite-status page | receiving of the signal and the strength of it (bars) |
position-page | position in different measurements (degrees-minutes-seconds/thousands, UTM etc.) |
plotter-page | 'map' with waypoints, changable scale (0,5 tot 600km) |
compass-page | compass or highway display to point out direction |
menu-page | management of waypoints and routes, make new routes, calculate distances, sunrise and sunset, system controls |
Following funcions of the Navigator are used most:
The moment the Navigator is turned on the staus-page appears (1st page). The bars depict which satellites are received and the intensity of the signal. Hollow bars mean data is being received. The numbers and the circles point oit where the different satellites are. As soon as a position is determined the position-page (2nd page) comes into view. On this page position, altitude, time, speed and direction are given. There also is a trip-odometer that can be reset to zero. When the receiving of sattelites is hindered for a short time the tripmeter holds counting and picks up as soon als the signal is received again. In differnt terrain you will miss about 10% of the real distance you travelled. In teh upper part of the display there is a compass scale that only works when moving around. Movement also immediately alters the position.
Pointing out ones position several formats can be used. Next to degrees, minutes and thousands the Garmin GPS38 and give position in UTM. This format often is used on ordonnance survey maps. A position taken with a Navigator can be found on the map in return.
A GPS-Navigator is ment to guide you to a certain destination or a serie of destinations. These destinations are called waypoints and are easy to program. To program waypoints into a GPS you can be done:
To get coordinates from an ordonnance survey map you should use a goniometric ruler that can measure angles. Especially designed to do this these rules can be bought at outdoor equipment shops and cost only a few bucks.The angled scales on the ruler here have 50 metres accuracy on a map of scale 1:50.000 and 25 metres on 1:25.000! A coordinate in UTM has a zone- indication and two sets of 7 digits that give the distance to the Zero-meridian and the Equator. To program a waypoint in UTM you do the following (the right column gives the values for the position of my home):
take the characters of the UTM zone (they are with the legenda on the map) | 31 U |
take 4 digits of the gridline left to the waypoint, if there are only three put a 0 in front of them | 0646 |
take the distance with the ruler between the gridline to the left an the waypoint, always 3 digits, max. 999m | 810 |
take 4 digits of the gridline below the waypoint, put eventually A 0 as fourth digit in front | 5770 |
take the distance between the gridline below and the waypoint | 785 |
The coordinates of the waypoint can be compiled from the zone, the
distance from the zero-meridian and the distance from the equator. In
the case described above this is: 31U 0646810
5770785. These valuse can be put into the GPS Naviagtor.
It is important to use the correct map-datum with the GPS. This is a
sort of code for the coordinates you use. The system of coordintes
changes over time and maps are adapted to this. Often the map-datum
is not very easy to find. To get some idea watch for dates that are
printed on the maps you have. A map-datum should be before the date
of reconnaicanse or the date of print of a map. By making a waypoint
of a known location on a map and compare this to the position the GPS
gives the map-datum can be guessed. A false map-datum can result in a
position error of abour 100 metres. On maps with bigger scale the
mapdate is less relevant.
Programming waypoints in degrees, minutes and thousands is done in a same manner. Nevertheless there are a few differnces. With this system there are no zones and the coordinates have the order 'deviation from the equator' - 'deviation from the zero-meridian'. The characters N or S or W or E are given. If a deviation from a gridline should be taken from the left or the right side of the line depends on the fact if the waypoint is east or west from the zero-meridian. For the southern hemisphere this is the same. To calculate a position in degrees, minutes and thousands goes as follows (the right column gives the values for the Pyramid of Austerlitz, Michelin map of the Netherlands 211, 1:200.000):
take the value of the line below the waypoint, N for northern hemisphere, S for south | N 52°00' | |
devide the distance in milimetres between the line below and above on the map by the grid-distance, measure distances at the sides of the map | 10'/93mm=0,1075
| 5,376' |
take the value of the line left from the waypoint, E for east from the zero-meridian, W for west | E 5°20' | |
devide the number of milimetres between the left and right gridline on the map by the grid-distance, measure distances at the sides of the map | 10'/57mm=0,1754 | 0,878' |
add the degrees, minutes and thousands, don't forget that the base gridline you used can give minutes as well! | N 52°05,376' |
With the Garmin GPS38 it is possible to make a new waypoint based on the actual position or an earier programmed waypoint. By giving a reference waypoint (or from the actual position on the plotterpage), bearing and distance the new waypoint is calculated. To make a compass bearing in the terrain a regular compass should be used.
The bearing can also be taken from an ordonnance survey map. The
bearing or course lies between the known point and the new waypoint
(see the drawing). To measure the bearing put the compass unfolded on
the map. By turning the lines of the compass card in line with the
card grid the bearing can be read. On the earlier described
goniometric ruler there often is such a compass card.
Outdoor equipment stores often have illustrating and explaining
leaflets for free - like the one of Recta used here.
If several waypoints are put into the GPS Navigator routes can be compiled. In a list distances and bearings are given between waypoints. For the active route (the one you actually use) time of arrival for the waypoints in between are given. When following a route the GPS automatically guides you to the next waypoint. When a waypoint is within 1 minute distance a message is shown and when display light is on you are automatically able to read it in the dark. When approaching a waypoint (this can be set between 0.25 to 5km) the next waypoint is displayed.
Programmed waypoints are seldom 100% accurate. Fortunately small differences of the coordinate have only little effect outdoors - unless you meet a ravine or river in between. The examples below are illustrative.
On the Equator:
- 1 minute equals 1 nautical mile=1852 metre
- 1 second equals 30,87 metre
- 1 thousandth equals 1,85 metre
At 52° north or south the these distances are less great. In the middle of the Netherlands or on the Falkland Islands you will get the following numbers:
- 1 minute is 1131,5 metre
- 1 second is 18,86 metre
- 1 thousandth is 1,13 metre
Values in between lay for example in Tarifa in the South of Spain at 36° north, or Melbourne, Australia south:
- 1 minute equals 1490 metre
- 1 second equals 24,83 metre
- 1 thousandth equals 1,49 metre
Differences of a coordinate that are within a quarter of a minute come within the regular position error of a GPS device and the selective availability.
The newsgroup sci.geo.satellite-nav gives access to discussions on using GPS. When you look to the signatures of persons that post messages interesting sites can be found. Through this site a regular updated FAQ can be found. Other sites on the Global Positioning System are linked below.
Information on this page is not for commercial purposes.