Frequently Asked Questions

The book has a FAQ section. Here are some additional FAQs noted whilst cruising discussion forums. Further detail can be found in the book.

What GPS receiver should I buy??
Is my GPS receiver an accurate speedometer?
Is my GPS receiver an accurate altimeter?
Why does my receiver record these amazing speeds and/or suddenly jump from place to place?

If you have a FAQ you'd like to see an answer to here, contact me.

1. What GPS receiver should I buy?

This question is very similar to "what computer should I buy?" and equally frustrating!
The answer, of course, is "it depends". Everyone has different needs and wants, so your choice of receiver naturally varies.
What you need to do to answer this is work out what you want to use the receiver for, and the constraints you have, for example budget. Knowing what a receiver can do will take some research; obviously it can show you where you are in terms of coordinates, but there are a lot on the market and each has special capabilities.
At the very least decide what you are likely to do with it. Driving, hiking, flying? Is battery life important? What about maps? Do you want, or need, turn-by-turn directions? (autorouting). Do you want it to be waterproof? Is size and weight important? Will it only ever be connected to a laptop computer?
Once you've had a bit of a think and done some research, by all means ask a fairly specific question such as "I need a GPS receiver that I'll use in the car, and occasionally while walking so it needs to be fairly robust. I've got $500 to spend, including accessories; what do you recommend?".

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2. Is my GPS receiver an accurate speedometer?

Yes it is. It's probably more accurate than the speedo of your vehicle, which tends to over-read so the car manufacturer can't be blamed if you get caught speeding. The GPSR works out its speed by Doppler shift, so although it doesn't know precisely whre it is (if you call +/- a few metres imprecise) it does know exactly how fast it's moving.

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4. Is my GPS receiver an accurate altimeter?

The short answer
Not really, and you need to know what you're measuring your "altitude" against.
Longer answer
First let's define altitude and elevation. The two terms have slightly different meanings, and their exact definition does vary depending on your reference source. However, altitude is generally used to refer to height above the earth's surface. Elevation is generally used to refer to height above a given datum. As an example, a village on a mountain may have an elevation of 1500 metres above sea level, the most common datum used for measuring elevation. An aircraft flying by might be at an altitude of only a couple of hundred metres, if the pilot was using a local airfield and measuring altitude that way, or maybe 1700m if he was using something close to mean sea level, or something different again.
Now we have definitions out of the way, we can examine how GPS receivers measure elevation (or altitude!)
GPS receivers have an in-built, idealised, model of the earth's surface and they generally measure their elevation from that. This model is called a geoid and may be, or may be not, close to mean sea level which is probably where what maps and other elevation definitions use. So if you want to use your receiver to check your elevation against a map, ensure you baseline your receiver's elevation display against the map to see what the difference is. This won't be consistent over a very large area, say 100km+, as the distance between the geoid and mean sea level does vary. Mean sea level itself does vary from place to place too.
The other point to consider is that GPS is designed for horizontal, not vertical accuracy. The vertical error is likely to be 1.5 time that of the horizontal EPE.
There are some GPS receivers have built-in barometric altimeters, which function like those fitted to aircraft. The altitude/elevation that these units display will vary according to the pressure setting, and the reading may or may not agree with any conventional GPS receiver.

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3. Why does my receiver record these amazing speeds and/or suddenly jump from place to place?

It's called multipath error. The receiver expects the signal to go directly from the satellite to the receiver. But sometimes the signal bounces off other objects before it gets to the receiver. The same signal is then received, twice or more, via different paths (hence "multipath"). This is a source of some confusion to the receiver and results in strange behaviour, for example fantastic recorded speeds, or wildly fluctuating position. Driving through Melbourne's CBD or other places with tall buildings is a good way to get a multipath error, although you may get these errors almost anywhere.

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