The consensus from ten or eleven years' worth of Forum contributions on the accuracy of gps devices when measuring distance covered 'on the ground' seems to be that in spite of both increased satellite numbers and new satellite networks, plus improved device performance, the various factors which can affect accuracy are still in place. These include geography (buildings, valleys, forests, water), straight-line versus 'wiggliness' of routes, and even where on the body the device is worn. For height gained and lost (net ascent) and point heights, there still appears to be more than ten percent variation between devices and networks.

(In the case of height of ascent, it is satellite numbers - at least four simultaneous signals needed for measurement in 3D rather than 2D - which affect accuracy: this and very much else is discussed in Greg Milner's remarkable 2016 book 'Pinpoint: how gps is changing our world'.)

I have previously suggested an average of the many devices carried on a hundred-mile event as a first large-scale comparison of body-worn ground-based gps measurements of cumulative distance and height (who else does annual hundred-mile walking events with maybe a hundred gps devices carried on each event ?). So far, there do not seem to have been many formal studies of cumulative gps accuracy. One 2014 cycle-based test* of ten devices over ten laps of a velodrome found an average distance error rate of 3.7%, with only 4 of the 10 devices being within 1% of each other.

However, the accuracy with which gps devices estimate their position on the planet's surface - surely one of humanity's finest achievements, even if originating in warfare technology - has increased significantly over the decade. Again citing Greg Milner's exhaustively comprehensive book, actual rather than claimed pinpointing *may* (repeat may!) now be down from 10 metres to 10 feet, and unless your device is off or the batteries are at below-threshold strength, it is continuously re-adjusting its location (latitude and longitude) with respect to as many as twelve satellites. (I learnt from this book that smartphones also use gps signals to orient themselves, even if not explicitly 'gps enabled'.) Unless you tell it not to (and this may be difficult to do!) your gps device is very good at tracking your continuously changing position on Planet Earth.

My very recent experience of tracking with a new Garmin (eTrex Touch 35) has so far been that it is successful over relatively short distances (10 to 15 miles) but has had problems on longer walks due to battery strength. I am addressing the battery issue by experimenting with its multitude of settings (and getting longer-lasting batteries, but that's another much-discussed issue). Cumulative distance and height totals have been unreliable even when continuously tracked, but the resulting gpx track files are remarkably accurate when uploaded to mapping software, both in showing every small deviation (by intent or error!) from the planned OS 1:25,000 route and in total distance covered.

In other words, the tracked route is very, very accurate even if my Garmin's own estimates of how far and high I went are not, regardless of where in or on my body I was carrying it and whether or not I had looked at it (it has an 'autopause' facility, which I think means - the online manual is awful, the only book on this device so far discovered is in German - that it stops tracking when I stand still.) Apart from a couple of glitches near water (the local canal's towpath, recorded as longer straight lines than the usual precise curves) its tracking capacity is remarkable. So long as remember to switch it on, that is ...

Iain

* http://www.singletracks.com/blog/gps/gps-distance-accuracy-test-smartphone-apps-vs-dedicated-gps

I ran a group for OS ex-employees and GPS enthusiasts. Highstreet handheld GPSs are accurate at the very most to plus-or-minus three metres - their processing capability and internal hardwear is incapable of anything greater. Ignore any claims on the packaging or on-screen display of 1m accuracy, even with 'averaging' - that is impossible. An analagy is the pixel size in a JPG image. Beyond a certain enhancement no greater detail can be discerned.

To achieve accuracy +- 3m, the unit needs to be stationary for several hours!

In everyday use, such as when walking, a rule of thumb is to multiply the 'accuracy' displayed on screen by three, and more is under heavy or damp tree cover (GPSs hate water) or enclosed situation.

To achieve greater accuracy required Equipment costing £1000s or even £10000s of pounds.

Michael, your Etrex 30 has the most up-to-date Garmin hardware and software. You can take accuracy to be in the region of 1%, but read your walk distance from the saved track, not the Trip Computer page. When you ‘save’ the track, the software rationalizes anomalies such as loss of accuracy when under trees etc.

The best way, if you are really interested in finding the distance, is to download the saved track to 1:25K mapping software such as Tracklogs, and then rationalize the route yourself. That will give you the mapped distance which is as close to the walked distance as you are going to get unless you walk with a surveyor’s wheel.

Thanks Raymond. I didn't pay much attention to the height measurements since on the SDW they aren't a major factor, but I did notice that at one point it showed 5m below sea level - which is lower than anywhere in the UK. It got Ditchling Beacon more or less spot on, though - the GPS was showing 249m to the official height of 248, but that was probably because I was holding it about a metre above the ground.

There was a discussion on accuracy on another thread, someone capped it by pointing out that GPS was used for landing planes
When you measure a walk on a map you do not get all the twists and turns which GPS picks up.
Many of my walks end up a mile or so longer by GPs than on the Anquet I use for planning.

I think your GPS mileages will be the correct ones.
However, on height they are hopeless. I did a canal walk yesterday and it gave about 400 feet for 9 locks!,

After going astray on more than one challenge walk, I've finally followed the suggestion made by a few people on here and got myself a GPS (eTrex 30). Its first outing was on the South Downs Way (which I walked in the company of my mother, but that's another story), from which my assessment would be that the waymarking and the Harvey map of the route would have been sufficient most of the time, but the GPS was a useful double-check, especially where there was no waymarking.

I did notice, though, that while the officially quoted length of the SDW is 100 miles, when we reached the end the GPS read 107. We had had a couple of diversions where our overnight accommodation was slightly off the route, and a couple more where we took wrong turns, but I would estimate that those probably added 2-3 miles to the distance - 7 seems surprisingly high. Is such a margin of error usual for a GPS?