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GPS Vehicle Navigation in Australia

This was my first book, published in two editions. It is now out of print and I have no plans to release a third version because nowadays technology is so simple to use there’s little need for a book. Back when I wrote this we had to connect serial cables to PDAs, use external antennas for GPS receivers, scan and geo-reference our own maps and much more. Today, you just download an app and tap an icon, it’s all done for you.

However, the explanations in the book of UTM, lat/long, types and north and other navigation information are as current as they were when the book was first released, as they have been for a few hundred years, and will be into the future!

Here’s some snippets from the book:

Elevation and Altitude
Altitude is how high you are above the ground. For vehicle navigation,
this is often 0 metres. Elevation is how high you are above a
given point, which is usually sea level. If your GPS receiver has a 3D fix, which means it can see four satellites, then it also becomes an altimeter so it can give you your elevation. Due to the way the position is computed, the elevation reading is less accurate than the location reading by a factor of about one or two. If your GPS receiver has a position error of 5m, expect the elevation to be out by 7 to 15m.

The Mapping Problem
The earth is not flat, yet maps are flat. Therefore, maps are inherently inaccurate because a flat map cannot perfectly represent a spherical object, or part thereof. You can accept this, or you can try yourself. Go and wrap a ball in paper without wrinkling the paper. If you succeed, everyone will be impressed and you will need to rewrite everything humans know about

To make matters worse the earth is not a perfect sphere (a sphere is a perfect ball). It is a spheroid, which means it is nearly a sphere. In fact it is an oblate spheroid, or ellipsoid which means it is somewhat fatter around the equator than a perfect sphere would be. A rugby ball/AFL ball/egg is an extreme example.

The reason for this small deformity is because the earth rotates around the poles and centrifugal force has pushed the equator out. The equator is about 42km longer than the equivalent around the poles.

Maps are often referred to by scale, for example 1:50,000. This means that one unit of measurement, say a centimetre, is equivalent to 50,000 centimetres in the real world. Often the thousands are abbreviated to k, for example 1:50k instead of 1:50,000. Common scales include 1:25k,
1:50k, 1:100k, 1:250k and 1:1 million.

Cartographers speak of large and small scale maps. Large scale are not what you think; they are the maps of scales such as 1:50k and 1:25k. Small scale are the 1:250k, 1:500k and similar scales. The scales in between are medium.

All these definitions are open to interpretation, but if
you want a map that shows more detail you need a larger scale map. For a ‘zoom out’ you want a smaller scale map. Remember it by ‘large scale maps have large details’.

Australian Map Datums

All Australian maps from 1999 onwards should use the GDA94 (Geodetic Datum of Australia) datum which is equivalent to WGS84 (well, the difference is a few centimetres), so the newest maps should not be a problem. Select WGS84 if you have a GDA94 map. The associated grid for
GDA94 is MGA94 which is essentially UTM. However, most maps are not brand new and even ‘new’ maps may well be based on old data, perhaps ten or twenty years old. Check the original date on the map, not the reprinted date.

Here’s some GPS FAQs

Where can a GPS receiver work?

The ‘G’ in GPS means Global, so anywhere in the world the receiver can see satellites. A clearish view of the sky is required to read the satellites. GPS receivers do not work indoors and not very well in cities. Caves are bad too.

What are GPS coordinates?

They do not exist. There is no coordinate system specific to GPS. Instead, GPS uses any of the existing coordinate systems, for example UTM or lat/long. What people mean by ‘GPS coordinates’ are coordinates that are entered into a GPS receiver.

Can GPS be switched off?

Yes it can, as it is controlled by the US Military. However, they are unlikely to do so. Nevertheless, you should always have a backup navigation system in the form of a map, compass and the knowledge to use them.

How accurate is GPS?

Given a view of four satellites, the manufacturer usually states the receiver will be accurate to +/- 15m. In practice, accuracy is often closer to +/- 5m.

How much does it cost to use GPS? Are there any ongoing costs?

It costs nothing to use GPS and there are no subscription costs. You just buy the receiver.

A GPS receiver is an electronic map. I can throw all my maps away!

No. A GPS receiver essentially tells you your location in terms of map references such as lat/long. You can create a moving-map system by hooking it to a computer, but you should always have conventional maps as a backup.

If I drive one track and return on it, why does the track not overlap exactly?

GPS receivers usually have position errors of 5-10 metres, often a bit more. Also, remember that you drive on one side of the road only! This is especially apparent on dual carriageways. Another reason is if you go around a sharp corner the receiver may ‘cut’ the corner as it marks a trackpoint just before and just after the change of direction.

Is a GPS receiver all I need to navigate?

No, you need a map and the knowledge to use it. A GPS receiver is simply a navigation aid. However, in extremis, you could use just GPS.

Can a GPS receiver interfere with…? (or, does a GPS receiver transmit?)

A GPS receiver only receives, it is not like a mobile phone. As a GPS receiver does not transmit, so it will not interfere with anything more than other passive electronic devices. Most such

devices do radiate some small electromagnetic fields as a by-product of their normal operation, but this is negligible compared to a proper transmitter.

Is GPS hard to learn?

You mean ‘is navigation hard to learn’. Expert navigators will understand GPS immediately, as they understand datums, coordinates and so on. Navigation is not hard, but it does take some learning and practice.

Do I need a computer?

No, you do not, but buying at least a data cable to link the GPS receiver to the computer will make it much more useful.

My GPS receiver is inaccurate! It shows me the wrong coordinates!

Are you sure the datum is set correctly on the receiver? Does it have a 3D fix?

I set this waypoint and it is way out!

Did you ensure the receiver had a good 3D fix with a low EPE? Are the datums correct?

Aren’t GPS receivers only accurate to 100m or something? Isn’t it called SA?

The US military used to introduce an intentional error called selective availability or SA into GPS. This was indeed an error of +/- 100m. However, on 1 May 2000 President Clinton had this error removed. Tthe United States Government has stated that it has no intent to ever use SA again. There has been no change in this policy.

You may also find this blog post of interest.