Saturday, 9 July 2011

On GPS Part I: General Principles

RIPLEY: Can't be. That's inside the room!
HUDSON: It's readin' right, man.  Look!
HICKS: Well, you're not reading it right!
                                              Aliens (1986)

The community of outdoor bloggers, particularly in the UK, Europe and in the US, have posted at length on the use of GPS systems within mobile telecommunications devices as well as handheld GPS-specific units and, in particular, the use of those systems with mapping software provided by a host of companies. Each of these GPS systems, whether they be dedicated GPS or as part of a mobile telecommunications device, and each of these mapping software systems have advantages and disadvantages and the primary conclusion I have come to is that there is no best option for either the device or the mapping system. I myself use Anquet, more because at the time it was just available for the Mac, was highly rated and seemed to cover exactly what I wanted - at the time, I was not interested in a GPS device on which I could place OS mapping software - I simply wanted a GPS device from which I could discern longitude and latitude and a map which I could manipulate in terms of size, area covered and print out.

This year I will undertake the
Classic Haute Route - a high altitude, alpine traverse from Chamonix to Zermatt crossing numerous high-level cols, glaciers and climbing at least 3 peaks above 3,000m. During the Tour du Mont Blanc last year, Eli had a GPS system (a weighty monster of a thing which I am convinced could, if necessary, launch ICBMs from Trident submarines based anywhere in the world). He used that GPS system, loaded as it was with mapping of the Mont Blanc massif, to chart his route and to work out various intriguing bits of data such as distance covered, speed and ascent/descent for the day. I rather liked this and he emailed me the maps, complete with the route covered, and data extracted, for each day. I used it to construct the posts I wrote for the TMB and from time to time, I reminisce by looking at the maps and deconstructing our route. It's a geeky kind of fun and I am unapologetic about it. So, during and after the CHR, I would like to be able to do the same. I am not particularly fussed, even now, about securing a device which permits me to examine mapping on the device - that seems unnecessary and, for battery reasons, a smaller device is more suited for longer trips in areas where a recharge is impossible. I'll come back to that.

Next year I am planning to spend a week in the Sarek National Park in Swedish Lapland. Although this trip is a year away, I am planning some of it now - a vague, notional groundwork process rather than anything more detailed or specific - intelligence gathering if you like. Sarek is, the Swedish Environmental Protection Agency seem desperate to point out, a dangerous place not for the inexperienced. There is a lengthy treatise from them seemingly intent on dissuading people from coming. There are no trails in Sarek and therefore navigation becomes all the more important. As Steven Horner observed in a very good post of his, not everyone is happy using GPS - a fact which I personally find difficult to understand, except in the context of those who do not have the time or inclination to learn a new skill. I begin with this premise - GPS does not replace maps, even when there is mapping software available on the unit being used. I would always take a map of the area alongside any GPS unit with a good compass. However, to not take advantage of perhaps the most comprehensive and effective navigation aid since contour lines seems rather pointless to me. In Sarek, where navigation will be critical, I wanted a unit that had two features: replaceable batteries, and lightweight. I'll go through the Garmin Foretrex 401 in due course but what I want to do now is to highlight basic areas of a GPS unit and the use of such a unit before applying that, in a second post, to the Foretrex 401. Most of the posts within the outdoor blogging community seem to focus on the principle that everyone knows how to use GPS and what it offers - this post is designed to assist those learning this new skill for the first time as I begin to refresh my memory having not used GPS regularly for years.

GPS uses one derivation of a Geographic Coordinate System - longitude, latitude and, sometimes, elevation. On a map, latitude lines, also known as 'parallels' since they are parallel and an equal distance apart, run horizontally. Each degree of latitude is approximately 69 miles (111 km). There is, however, a disparity due to the fact that the earth is an imperfect sphere (it is, in fact, a biaxial ellipsoid, i.e. slightly egg-shaped). Zero degrees is the equator, 90° north is the North Pole and 90° south is the South Pole.

The vertical longitude lines are also known as 'meridians'. They converge at the poles and are widest at the equator (also about 69 miles or 111 km apart) - therefore they are not parallel. Zero degrees longitude is located at Greenwich, London (in the United Kingdom) and is designated 0°. The degrees continue 180° east and 180° west where they meet and form the International Date Line in the Pacific Ocean. If it is of interest to you, Greenwich, the site of the British Royal Greenwich Observatory, was established as the site of the prime meridian by an international conference in 1884.

For greater accuracy, Ptolemy (around 150AD) divided longitude and latitude degrees into minutes (') and seconds ("). There are 60 minutes in each degree. Each minute is divided into 60 seconds. Seconds can be further divided into tenths, hundredths, or even thousandths.

Of course, life is never simple. Besides using the degree-minute-second (d-m-s) notation, a decimal notation (and even a combination of both) is used across the world. Decimal notation still uses degrees, but converts the minutes and seconds to a decimal number.
 This system largely came into effect as the d-m-s system proved cumbersome, particularly for computerised devices, so the decimal system is used, for example, by many Geographic Information Systems (GIS). However, it's easy to convert from a decimal back to the sexagesimal system:

The whole units of degrees remain the same (Becs des Bosson in Vissoie sits at 46.16806° lattitude, so start with 46°).
Multiply the decimal by 60 (0.16806 * 60 = 10.0836).
The whole number becomes the minutes (10').
Take the remaining decimal and multiply by 60 (0.0836 * 60 = 5.016).
The resulting number becomes the seconds (5'). Thus Bec des Bossons sits at 46
°10'5" which is correct.

Also bear in mind that longitude and latitude can be based on several different geodetic systems or datums, the most common being WGS 84 (World Geodetic System 1984), a global datum used by all GPS equipment and NAD83 (North American Datum 1983). Other datums are significant, chosen as they were by regional national cartographical organisations as the best method for representing their particular region, and these are the datums used on printed maps for those regions. Using the latitude and longitude found on a map may not give the same reference as on a GPS unit unless you change the datum on the unit.

On the Swisstopo Vissoie (1307) 1:25,000 map, 2006 Edition, the Datum is WGS 84 - the information is to be found at the bottom of the map. The same cannot be said of the Fjällkarten Sareks National Park (BD10) 1:100,000 map, February 2009 Edition, which uses SWEREF 99 TM. However, this is considered to be almost identical to WGS 84 and the map itself suggests it is consistent with WGS 84 within 1 metre. Either way, once you know what datum is used, you can set your GPS Unit to that datum. My unit, the Garmin Foretrex 401, and indeed Garmin Basecamp, does not have SWEREF 99 TM but the map anticipates this and informs us that the datum used is identical to the European ETRS 89 which both do list.

There are other Geographic Coordinate Systems - OS maps used the Terrestrial Reference Frame OSGB36 (i.e. a grid system based on land navigation and physical observation rather than satellite navigation), but since GPS does not generally involve itself in these systems, I am not going to go into them in any detail. There is however a PDF document which OS publish which is outstandingly comprehensive and interesting read for map geeks like me.

What can a GPS Unit do?
Basic principles for a GPS Unit are simple. Comprised of 24 to 32 satellites in space, orbiting the earth, the GPS (Global Positioning System) is a GNSS (Global Navigation Satellite System). A very comprehensive but really quite interesting article in Wikipedia explains the history and the current composition of the GPS GNSS. Essentially, GPS will locate the position of the unit anywhere in the world and relay that information through whatever interface the unit is using but most regularly, and simply, through longitude and latitude co-ordinates. Often elevation is also recorded. Many units will also make reference to local, national grid systems (such as OSGB36) but not all.

That is not the only function of a GPS unit of course but every ancillary function stems from this basic premise. Speed (and ascent and descent) requires the plotting of two, or more, points and then a mathematical calculation based on that information referenced to time travelled as well. Positioning on a map is even more simple as that mapping software is overlaid with longitude and latitude co-ordinates. It may even be overlaid with altitude/elevation information but the GPS Unit can provide that information without mapping software. 

Most Units will also include 'waypoint' functions and the ability to input 'pre-prepared routes'. I'll come back to this but I want to say this now - GPS is not a substitute for a map and a compass. In my view, you need those skills to fully benefit from GPS and in case it fails. You cannot, and must not, rely completely on GPS for navigation.

Waypoints and Routes
A waypoint is essentially a point or location, loaded onto the GPS Unit by the user, which is intended to be either the final destination or point through which the user will travel in order to reach his final destination. Clearly, since travelling in a straight line outdoors is impossible, any route will necessarily involved many twists and turns, each of which could be inputted into a GPS unit as a waypoint. Waypoints essentially correspond to 'legs' in traditional navigation theory. The number of waypoints (or legs), and consequently the level of detail required and the frequency with which the user would have to have reference to the unit is entirely personal choice. Those confident with their navigation skills might only have a handful of waypoints - others may have perhaps more than 20 or 30. A waypoint is a point to aim for which the user knows is on his route and is not too far from his current location making navigation easier. The GPS assists navigation in addition to a manual compass in that it constantly adjusts according to the user current location, so if forced off course by the terrain the GPS will constantly adjust the bearing you need to take to reach your target. 'Pre-loaded routes' are essentially a series of waypoints which can be loaded onto the unit and are either manufactured by the user prior to the hike or manufactured by a host of other organisations either free of charge or for a cost. A good GPS unit will also be able to give and estimated time of arrival at a waypoint, and/or final destination, as well as current ground speed, assisting navigation.

In conclusion, GPS is in my view an essential tool in modern navigation and to ignore it is unnecessarily restrictive. Whilst being able to navigate by map alone (with reference to the shape or relief of the landscape) and with compass in the inevitable hill fog that ensues in the UK, is utterly fundamental to time spent outdoors, to avoid learning to use GPS makes little sense to me. I accept that reliance on technology should only be learned in the context of a comprehensive skillset should that technology fail, but if it's there - use it.

I'll deal with my newly purchased unit, the Garmin Foretrex 401 in another follow-up post, which will detail how GPS can be used to secure all manner of information and data.


  1. How intresting you are looking at Serek.  You don't don't do Twitter.  If you did you would have read about my intention to go to Serek next year a couple weeks back.  Have the maps and a plan.  Getting to the start is a detail I am working on.

    Navigation is going to be: map, compass, GPS and Viewranger 1: 25 000 maps with GPS.   GPS is fine.  I tend to forget mine a lot on trips but did pack it on the Challenge.  See it as a navigation aid. Better to stay located on the map but sure is handy to have a GPS to help if you get it wrong.  

    Plenty of rails marked on the map of Serek.  Also google earth images show trails as well.  Lot more bridges than claimed as well as hill phones etc.  Its remote and wild looking which appeals to me.    But it aint the Brooks Range in terms of remote.  

  2. I didn't know you were going there, but knowing you as I do, I can see why it appeals to you - largely for the same reasons it appeals to me. I am looking at June next year so perhaps, if we're there the same time, we can meet up at one of the major towns at the conclusion and compare notes...!

    I am surprised you have a 1:25,000 map as the only one I could find was 1:100,000 - but I guess that is the advantage of maps on GPS units. Personally, I am happy with the Fjällkarten - it shows me what I need and the navigation will be reasonably straightforward given the Rapa Valley will be our main route. We'll start from Kvikkjokk and head up to Laitaure then head north west along the valley. We'll avoid glaciated terrain so we don't have to take ice-axes and crampons but otherwise, the terrain looks superb.

    There do seem to be a few trails that I can see, so it's not as remote as it would like to claim but I am not being complacent. I only have 5-6 days at most and the Northern Lights are intriguing both myself and Mrs M, so the time of year may be dictated by that. Great to hear you're going there and perhaps I should get tweeting...

  3. Smashing post Maz. Lots of good honest facts and advice. I take a Satmap Active 10 with me but i don't depend on it. Like all gizmo's battery life is an issue for long trips. And also it has circuits and connections that can fail. You can also get them waterlogged like i did a few weeks ago.
    They are great for giving you mileage, height climbed etc and of course your location when you are misplaced or in the merk.
    I  agree with you that not using technology when it is available is just like being blinkered. Use them but not in place of the old map and compass.
    Serek sounds very interesting. I enjoy the planning stage. I went to the Jotunheim for a 17 day backpack a few years ago and although it's not as wild as Serek the planning is the same and a most enjoyable place to go.

  4. August was my plan and getting high at times hopefully.   Big issues are if I go solo and getting there.  I wonder if a satellite phone was worth hiring for the week to give peace and mind to loved ones.   

  5. I did wonder if you were thinking of going solo which is the way your comment sounded. I would be quite reticent to do so, given what I have read, but I also want the company anyway. Getting high sounds good and the Rapa Valley has a few 1600m+ parts as well - just depends on avoiding glaciers and being careful above the snow line as I do not want to carry ice-axes and crampons. Satellite phones are very expensive - must be a cheaper way to keep in touch...?

  6. Jotunheimen and the Handangervidda were our other options, as well as the Laugavegur trail in Iceland. Eventually, Sarek just hit the spot - wild, challenging and beautiful. I think it might have been a Backpacker Magazine article that swung it for me. That said, I have always loved Jotunheimen - never been but we were headed that way a few years back but chose Glaskogen instead. The Besseggen Ridge looks amazing. 17 days is a real trek too.

  7. Solo is often my only option as I don't have many friends up for remote walks,  Sat phone would only be hired.  Snow line is an issue there.  But trails are there to follow and big valleys seem easy navigation corridors.    

  8. Hi Maz,
    You have plenty of time to do the Jotunheim and the Hardangavidda. I have done the Laugavegur trail in most parts. It was done in conjunction with a round of the south east glaciers. I also went up Hekla which is now on the verge of erupting.
    I have a photograph of myself on the Bessegan ridge in my younger days, when i was extremely fit and carried rucksacks without worrying about the weight, it is one of my favorite places routes. I will never forget it as it was one of the fastest "walks" i have done to Olavsbu in a heavy thunder and lightening storm.  I will send you a link when i find it.
    I havn't been to Glaskogen.

  9. Besseggan Ridge


  10. I used a GPSr (a ~10 year old hand-me-down Garmin Geko from my son) for the first time on the hill on Friday. I've been using this and 2 x new Garmin Extrex for running geocaching sessions at work and I'm enjoying getting to grips with the technology. Anyway, on Friday I used the Geko to check my position a couple of times when wandering on compass bearings right through the centre of the map in your first shot. I'd climbed Cairn Lochan when the cloud dropped quickly. I wanted to walk over to the edge of the Lairig Ghru to take photos down to the Falls of Dee. I felt a lot more confident walking on compass bearings, knowing I could check my position at intervals.

  11. Sheila, I think a lot of people agree with you and more and more are using GPS as a supplement to their navigation arsenal. Particularly in dense, fast-descending hill fog, it really helps. You should be able to navigate without it but good navigators used to think you should be able to navigate without a compass by contour/relief alone (maybe they still do) but the compass has become such an important part of our kit that few would leave home without one. GPS is different as it requires an extraneous element to make it effective (satellites) whereas the compass does not (I doubt that the magnetic poles will disappear anytime soon). Thanks for the comment!

  12. Maz, looks like an interesting post.  I will read soon as poss. Need to finish off some work this evening first- back soon !

  13. Nice informative post Maz.  Wish I was articulate enough to put all of what you just put together!  My two cents worth about GPS is it gives me peace of mind if walking in thick scrub with limited identifiable features around me.  Just being able to confirm distance travelled can also be handy as I find I can misjudge the amount of ground covered if the terrain is tough.  It's not about giving up on map or compass, but a device to make a walk a little easier when a decision has to be made.  It does it for me :)

  14. I agree completely. It's an additional piece of kit that is really very useful. Thanks Greg. Distance covered in tough terrain (or fog) is a good point - one of the make navigational tools.


  15. Maz, finally got around to reading your post. As always highly informative. I entirely agree that one should never totally rely on them and I like many people take a map and compass to go hand in hand with the GPS unit ( and as a backup to a GPS if things go wrong).  Just as importantly is the ability of a GPS to act as a key piece of safety equipment. Several years ago when coming off Scafell in a early winter blizzard, I became disorientated where it was difficult to work out the ground from the sky, but with the waypoints on my Garmin, I found my way off the mountain safely.  This would have been a rather more frightening affair without it.

  16. Beware that SWEREF 99 TM is not equivalent to WGS 84. If you read coordinates from a GPS unit set in WGS 84, you will not be able to identify the point on swedish maps with SWEREF 99 TM grid. The reason is that you willl most likely be in zone 34 while SWEREF 99 is based on zone 33. Not exactly the same thing.

    As to swedish mountain maps (Fjallkartan), they are available in 1:100 000 only.

    Enjoy Sarek.