Satellite navigation

Call me old school, but I still view the good old GPS as a backup, not a replacement for good Of course I've been tarnished by the bad old days of systematic errors induced by our American friends in combination with >100 inaccuracies and the helpful information that instead of dangling my toes in the waves on the beach I am, according to the unit, actually 60m underwater. Oh, and any overhead cover more dense than a sparrow wing blocked the signal. That being said, I use satellite navigation extensively in my alternative lives on oceans and on land. There are a wide range of brands and models. www.platatac.comI advise checking out any user reviews. Key elements to consider are weatherproofness, and battery longevity. The good old Garmin eTrex 10 is a relatively inexpensive and reliable model. The wrist models available such as the Fenix 2 pictured are very accurate and light - but expensive. A basic GPS is worth having as a piece of kit. But they are still only backups!

An important technical note (and one that Lyle Brotherton is justifiably militant about), using Sputniks to find your way around is formally referred to as Global Navigational Satellite Systems (GNSS) or Satellite navigation/Satnav. GPS refers specifically to the American military system. Associated with GNSS are additional regional Satellite Based Augmentation Systems (SBAS) which work with the core GNSS to get ridiculously good position estimates.

There are a few important things you need to do when using Satnav, and they all relate to making sure the unit corresponds with your map. In short, Study and Understand the information in the map legend!

1. Understand the map coordinate system and set the Satnav accordingly. These are the gridlines on your map. You will recall from the section on maps and grid references, that it doesn't really matter what the grid squares are called to get a grid reference, as long as they can be read in some metric way ie. divided into tenths. However when translating these grid values to a Satnav value, they have to have the same system. Survey maps are often but not always displayed in UTM. If your map is in UTM, then your Satnav will have to be set to UTM, not Degrees/Minutes/Seconds, Decimal Degrees; Degrees/Decimal Seconds – or whatever your default is.
Let's look a bit more closely at the UTM values the GPS gives you. Say our GPS tells us we at a small restaurant at 31T03902784715387. Gee, that's pretty bloody uninformative... The first two digits are the large longitude segments of the grid. 31 means we are in longitudinal segment 31. The letter 'T' indicates we are in the latitudinal segment T. How important is this to us? Well, for our average hike, not too important. If describing our position to someone several hundred km away who is in a different Number-Letter zone, well yes it is important. The first 7 digits (0390278) describe our Easting position to the nearest meter. If you stand still, watching the UTM position on a satnav, the last digit will usually flicker in value randomly. This is due to measurement error - ignore the meter-level value. We only need to measure to 10m. The second 7 digits (4715387) are the equivalent Northing values to the nearest meter. As above, round to 10m. So really, to read this in a more digestible format think of it as 31 T 039028X 471539X (note the rounding of the meter-level value).
If we are reading the satnav to the scale of 10m (rounding the last digit), we read the last 4 digits of the Eastings and Northings do give us an 8 figure grid reference. In this example, 9028 1539. How do we read this as a 6-figure (ie. to 100m) grid reference? Round the last digit again: 903 154. I can't stress how important it is to know which digits to keep and round. Practise it before you are lost, on the verge of panic, and you have no idea what the string of 14 numbers means.

Not all maps are set to UTM. The British Ordnance and Survey maps I use in some of the examples are - this makes it easy. The Andorra map I use is not - It uses the European coordinate system according to a grid based on the ETRS89 system. On my Garmin this is called the French Grid. The large scale naming and false Eastings/Northings are different to UTM. The US Army FM 2-25.26 manual and Brotherton's book have some of the better more detailed descriptions of this. In short, if your map is not UTM, but your GPS can only be set to UTM grid, you will have to do some arithmetic to read the coordinates from the same start point of the grid. To do this, go to a known point, take a position in UTM and measure the difference in Nothings/Eastings on whatever the map grid is, and apply this difference when converting the two. Don't simply consider a grid value in one system to be the same as another. They differ by two constants - East and North. Pain in the ass? Yes.

2. Set the Satnav grid datum to the map datum. My apologies to Plato (and ultimately Pythagoras), but the Earth is not a perfect sphere. It's kind of a Tellytubby shape. As with Tellytubbies, there is no single place on the Earth to reference all coordinate systems to. Traditionally, a point has just been designated “It” and everything referenced to it. This is the role of those brass surveyor's Bench Marks all over buildings and pavements. Nowadays with modern Satnav, the WGS84 (World Geodetic System 1984) datum is becoming the standard, but some older maps haven't caught up yet. In Europe in particular many if not most modern maps are based on the ETRS89 datum. This differs significantly for WGS84 both in practise and principle - the European datum moves! It was developed to account for European continental drift, and it diverges annually from WGS84 by about 2.5mm a year. The difference caused by setting the wrong datum can be considerable for our purposes. I've seen disparities of 300m. As an aside, don't rely too heavily on satnav for altitude estimates. Remember the Earth is not a true sphere, so the Datum is an approximation. Additionally, there is horizontal error on the estimate, so if you are near a cliff this will translate to vertical error. This is why your GPS, at sea level, may say you are either underwater, or magically hanging above it sometimes. However, some units now include barometric estimators or link to relief maps, so things are improving.

3. For goodness sake, switch the Satnav to metric units. I've worked with my American colleagues who insist on working horizontally in meters, while taking water depth soundings in feet. That's why I have short-cropped hair so I don't tear it out in a fit of apoplexy.

Using the most of GNSS in itself is an art form. I recommend Brotherton's book as a good introduction. If you really get keen, then the surveying literature has even more detailed applications.