HANDHELD GPS GUIDE
GPS devices make excellent navigation aids, and there are several small handheld units available for use as backups to main units or for the diver who doesn’t wish to be fixed to one boat. Here's DIVE's guide to buying a waterproof handheld GPS.
These useful devices are great navigation aids and have a multitude of features, but there are caveats attached to them. They should always be used in conjunction with paper charts and should not be solely relied on to navigate a passage. Indeed, when you start up a unit, you will notice that the manufacturer usually displays a similar message requiring you to confirm that you understand this point. The reasons for this are twofold. First, they are electronic pieces of equipment and can fail and, perhaps more importantly, most leisure GPS devices navigate in direct lines – unless you specifically enter waypoints to, say, plot a course around a headland. Also, they won’t automatically route you around other hazards such as rocks, shallow water and shipping channels.
What is GPS?
The Global Positioning System is a constellation of between 24 and 32 orbiting satellites, owned and maintained by the US Department of Defense, which beam microwaves to GPS devices – also commonly known as ‘satnav’ (from Global Navigation Satellite System,or GNSS) – here on Earth.
How does it work?
In layman’s terms, all the satellites ‘know’ exactly where they are in relation to a place on Earth. The GPS device simultaneously gathers this information from several satellites, does some number crunching and then can deduce where it is located. The accuracy can be astonishing – to within 5m or so is not uncommon – however, several things can effect this. First, in times of conflict, the US Department of Defense can simply detune the network. A more common problem is atmospheric conditions, or if a direct line of sight to the satellites is obscured by cliffs or parts of a vessel, for instance. A good navigator should always carry charts and keep a keen watch, using the GPS as confirmation rather than trusting it implicitly.
Choosing a GPS unit
Most modern hardboats and RIBs will have fixed GPS units installed, many combined with a chartplotter and sounder, but it is always advisable to have a backup. Moreover, if you regularly charter different vessels, you may want to have all your own waypoints portable so you can always have them to hand, in which case a mobile device is much more practical.
When venturing out to sea, it is highly advisable to keep salt water firmly away from any electronic equipment, so it is essential to buy a waterproof unit. Splashproof devices are fine for the odd shower on land, but salt water will soon corrode them. The specification to look for is IPX7, which means it can withstand immersion in one metre of water for 30 minutes.
The next thing to check is whether you can operate all the buttons when wearing gloves – some even have touchscreens that can be used while wearing the thickest of neoprene gloves. Also, look at the display, not only in the shop but outside in bright sunlight – is the screen still clear?
Finally, the majority of GPS units on the market don’t come with marine software cartography as standard, so you will need to purchase this separately. It usually comes on a tiny memory card, and you simply need to make sure the chart software is compatible with the model you wish to buy. Expect to pay around £150 for it.
Top tips for using a GPS unit
- Make sure the unit has a clear, unobstructed view of the sky – it needs a line of sight of the satellites to get the strongest signal
- Fully familiarise yourself with the GPS unit before setting out to sea. Returning to harbour in fog is not the time to start learning where you are
- Know how to read a chart and use a compass so that transferring information from one to another is straightforward
- These gadgets tend to be power-hungry, so always ensure you have a spare set of batteries
- Label all your waypoints, wrecks or dive sites: it’s amazing how quickly you can forget which ones are which
- Back up all your waypoint data on a PC. If you lose your GPS unit, you don’t want to lose all your marks as well
- If setting out somewhere new, enter all the waypoints before leaving – preferably the night before when you have plenty of time
- When using a GPS unit to calculate distances, don’t forget that it will assume it can travel in a straight line, which may not be the case at sea
- Keep your chart software updated: lights, buoys and channel markers can all change from time to time
- Although the units are waterproof, salt can work its way into the smallest of gaps, so always rinse with lukewarm water after use and leave to dry in a warm place
- Keep it under wraps. As mentioned previously, electronics and sea water don’t mix, and even just one drop of salt water entering a GPS device is enough to render it useless. A simple and cost-effective way to waterproof it, however, is to encase it in a specially designed watertight pouch. Aquapac makes a variety of such bags that are 100 per cent waterproof, flexible and transparent. The 124 model (£20) is a good one to go for and could also be used to waterproof a PDA or mobile phone.
Bearing: the compass direction from your present position to the next waypoint
Course deviation indicator (CDI): displays the sideways distance from the desired course
Course over ground (COG): the current direction you are travelling in
Cross track error (XTE): similar to CDI, this indicates how far off course you are
Distance to go (DTG): how far you have to travel to your next waypoint
Mark: a symbol or icon placed on the plotter display to represent a point of interest such as a wreck
NMEA connector: National Marine Electronics Association electrical standard interface, which provides connectivity between different manufacturers’ equipment
Route: a passage that consists of two or more waypoints
Satellite status display (SSD): a screen that shows technical data about each satellite in view such as identification number and signal strength
Speed over ground (SOG): your actual speed relative to the ground. Note that this may be different to speed over the water as currents and tides affect it
Steering screen: a graphic, highway view of the GPS user’s COG
Time to go (TTG): the time remaining from your current position to the next waypoint, assuming your SOG remains constant
Track (TRK): direction of travel
Trails: path drawn on the display by the GPS unit that enables the user to either backtrack the course or retrace the passage at a later date
Velocity made good (VMG): similar to SOG, but compensates for progress being made towards a waypoint. Thus when travelling off course, the VMG reading will typically be slower than the SOG and a truer indication of the speed being made to the selected waypoint
Waypoint: a specific location that can be stored in the GPS unit’s memory to be recalled and used at a later time for navigation purposes