Bikemagic/Garmin Guide to GPS

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A few minutes spent setting up will avoid confusion later

If you’re acquainted with the basics of grid systems and how GPS works, you’ll appreciate that before your GPS receiver can tell you anything useful you’ll need to spend a little time on some basic setting up. Different receivers hide these settings away in different places – look for Setup, Preferences, System or similar and you’ll not be far off.

Time

As we’ve learnt earlier in this series, GPS relies on knowing the time to figure out where you are. You don’t actually need to set the clock on a GPS receiver – the first time you turn it on there’ll be a protracted initialisation procedure while it figures out which satellites it can see, what time they all say it is and gradually narrows down its location and sets its own clock. What you will need to do is set the correct time zone. In most cases you’re presented with a list of countries or regions. Assuming you’re in the UK, most receivers will automatically switch themselves between GMT and BST as appropriate.

Grid systems and datum sets

By default, GPS receivers tend to use longitude and latitude (see Part 1 for a fuller explanation). Longitude and latitude works well for navigating oceans, but for more local use you need the information relating to the grid system used for mapping wherever you are. In the UK this means an Ordnance Survey grid reference.

Somewhere in the menus of your GPS receiver you’ll find settings for Position Format (sometimes called Coord System) and Map Datum. If you’re using a Garmin receiver in the UK, you need “British Grid” as your position format and “Ord Surv GB” as the map datum. Other brands may have “OSGB” for the grid and “GBR36″ for the datum. Once these are set, the receiver will give you familiar Ordnance Survey grid references, allowing you to quickly pinpoint your location on a map.

Magnetic or grid North

As well as a choice of grids, you’ll also have a choice of Norths. If you choose Grid North, your GPS will align its north with that of your maps. If you choose Magnetic North, it’ll agree with your compass. The difference between the two Norths varies over time and with location, so you can set that too. Given that a compass is a vital piece of backup equipment, it makes sense to have your GPS set to Magnetic North too, although using Grid is less of a brainache if you’re referring to a paper map.

Measurement units

You’ll be presented with a range of options for measurement units, although most riders will be choosing between kilometres and miles for distance and metres and feet for altitude. Which you choose is really up to you – many people find the Imperial units more intuitive, but going metric makes sense given that the grid squares and contours on the map are in km and m. Measurement units can be set to your personal preference of either Kilometres for distance and Metres for altitude or Miles and Feet if you prefer.

Other settings

Once you’ve got the basics set up you’ll have a whole range of options to personalise the settings to suit how you want to use it. One setting that’s worth a look is the backlight – lighting the screen up is a big drain on the battery, and turning the brightness and shut-off times down can extend battery life significantly.

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Bikemagic/Garmin Guide to GPS

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Garmin’s Edge 705 is a bike-specific GPS receiver

What is GPS?

The Global Positioning System allows a suitably equipped receiver to pinpoint its position anywhere on the planet’s surface. While originally designed for (and still operated by) the US military, over the last 12 years it has had a major impact in navigational systems for everything from walkers to pilots. Bike-specific GPS receivers are a relatively recent development.

How GPS works

A constellation of satellites orbit the Earth at a height of around 12,000 miles and send out a stream of messages by radio. Those messages contain the time the message was sent and precise orbital information about the satellite in question (how high it is, what angle it’s orbiting at, how fast it’s going and so on). The receiver measures how long the signals take to travel from the satellite and thus computes the difference. Once it knows how far it is from four satellites, it can use trilateration to determine its position.

(If you’re wondering why it needs four rather than just three, the reason is that the satellite signals travel at the speed of light, which is a very big number. Even a very small error in the receiver’s clock would, therefore, result in a big error in the location. With four satellites, the receiver can do its sums not just for x, y and z coordinates (with z being altitude) but also correct its own clock.)

The satellite network is arranged to cover the whole planet. The original design called for 24 satellites, although there are actually 31 at the moment. Because of the arrangement of the satellite orbits, different numbers of satellites will be in view from a particular point on the surface of the globe at different times, although at least six should usually be within range. Most receivers will be able to tell you how many satellites they can see and how strong the signals are, as well as letting you know the accuracy of your position.

Limitations of GPS

All GPS itself does is give you a position. What you can do with that information depends on the capabilities of your GPS receiver and any other software you’re using. Most receivers will log your position at time intervals to give you a track of where you’ve been, and allow you to mark important locations or landmarks. Some have built-in mapping capability, but pretty much all will connect to a PC running mapping software.

GPS is not a replacement for the traditional navigation skills of map and compass use. For a start, unless you’re actually moving it doesn’t know which way you’re facing. It’s an additional resource that may be used either on its own or in combination with these traditional skills. One of the big attractions for MTBing is the ability to see where you’ve been on a PC after the ride, with accompanying speed information. The ability to upload tracks to the GPS receiver to follow is also an increasingly-popular feature.

It is important to remember that your personal safety is your own responsibility at all times, and whilst GPS is a useful tool it should not be relied on as the sole means of navigation in the outdoors.

Does GPS work everywhere?

Because the system uses radio waves, the ability to receive a signal is partially dependent upon the surrounding environment, although increases in the sensitivity of the receivers are improving this situation. Dense woodland, tall buildings, and canyons in particular can make reception difficult or reflect the signal giving a false reading.

The recent SiRF III chipsets have gone a long way to reducing this problem and a GPS receiver based around this chipset should be suitable in almost any outdoor location – and even indoors within reasonable distance of a window or line of sight. Despite the advances in chipset sensitivity, ionospheric distortion, caused by high solar activity, can cause dramatic malfunctions by slowing the progress of the radio signals through the ionosphere.

This is one of the reasons why GPS isn’t a total replacement for map and compass skills.

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Pinpoint your location with GPS

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The OS grid divides the country into named squares 100km on a side…

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…with numbered gridlines within each square. The first three digits are the horizontal coordinate, the final three are the vertical.

At its most basic level, a GPS receiver tells you where you are. To make any use of that information, though, you need to know a little bit about how maps work. Here in the UK, we’re blessed with Ordnance Survey maps – highly detailed, legendarily accurate and covering the whole country at a range of scales. To pinpoint your location on the map, we use a grid reference.

Grids are a slightly arbitrary but very useful map feature. Because the planet is (roughly) spherical, on a global scale longitude and latitude is used, with lines a fixed number of degrees apart. Lines of longitude run north-south and tell you how many degrees east or west of the Greenwich meridian you are. Lines of latitude run east-west and tell you how many degrees north or south of the equator you are.

Because lines of longitude converge at the poles, the further from the equator you are the smaller the actual distance on the ground between any two lines a given number of degrees apart becomes. At the equator, one minute of arc (a sixtieth of a degree) is equivalent to one nautical mile. At the poles, it’s zero. Lines of latitude are all parallel to the equator, so the relationship between degrees and miles stays constant no matter where you are.

Clearly this is all very complicated, with calculating distances on the ground requiring a firm grasp of spherical trigonometry. Hence the use of a grid over smaller areas like, for example, Great Britain. Sticking a bunch of parallel lines over the map lets you essentially ignore all that latitude and longitude stuff and work with a simple coordinate system.

The vertical lines on a map are aligned to what’s called Grid North, parallel to a particular line (or “meridian” of longitude. True North is the direction along the actual lines of longitude, converging at the pole, so the further away from the meridian to which the grid is aligned you are, the more Grid North varies from True North. Again, you don’t need to worry about this for most purposes. You also don’t need to worry about Magnetic North, which is the way a compass points and a very small amount different from either of the other norths.

But to return to the grid, it’s arranged on a number of scales. The lines on an OS map are 1km apart. Each one has a two-digit number. Obviously there aren’t enough two-digit numbers to cover the 1,100km length of Britain, so the grid is divided into 100x100km squares and each of those given a two-letter code. The grid lines within each square are then numbered 00 to 99 both north-south and east-west.

GPSes designed primarily for navigation that are OSGB-aware will give you a number like ST125413. With any luck you’ll know roughly where you are so you may be able to ignore the letters. The rest of the grid reference is made up of the actual coordinates, with three digits for the “eastings” (reading crossways) and three for the “northings” (reading upwards). The first two digits in each case relate to the numbers printed on the grid lines, with the third telling you how far between that line and the next you are. So “125″ is halfway between 12 and 13, while “413″ is a third of the way between 41 and 42. And there you are.

The same operation in reverse gives you a grid reference that you can punch in to your GPS so that it can tell you in which direction (and how far away) somewhere you’re looking for lies, but there’ll be more on that later in the series.

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