Has anyone yet measured the full length of the Offa's Dyke path with a GPS to get an accurate distance measure?
Having just completed it,the official distances seemed very conservative with the time I was walking daily. I track my walking times daily,and after eliminating all stops,adding diversions,I find the following when comparing it to the PW and C2C:
Offas Dyke 177miles 75hr24min Avge 2.35mph Av Day 10hr46min
Pennine Way 268miles 99hr38min Avge 2.69mph Av Day 9hr58min
Coast2Coast 195miles 75hr35min Avge 2.58mph Av Day 9hr27min

It is primarily the central sections I have an issue with:
a.Kingswood-The Quarry,after Bakers Hill - offically 25miles,
yet I take 11hours dead despite 5 hours being dead flat 3mph walking down the river Severn and Montgomery Canal.
b.Dolley Green-Kingswood - officially 26miles yet I take 11hour 35mins,most of it easily graded,apart from maybe 30minutes extra time cost on the Switchbacks ascents.
c.The Quarry,Bakers Hill-Gweryd Lodge -offically 24miles,yet I take 10hrs50min despite plenty of level walking.
d.Hay on Wye-Dolley Green officially 23miles yet 9hrs40 min.

Several walkers I met coming south were also of the same opinion, and it also ties in with comments of accommodation owners,where people booked in at certain times would arrive much later than expected,despite most of the path being easily graded.
Comments please?
Thanks
Alan

There has been a longish thread on the C to C section about the problems of obtaining a definitive distance on any walk.

If you plot the route on a GPS then this is still a rough estimate and possibly no better than using a piece of string on a map. It is, at its best, still a straight line distance rather than a walking distance.

If you use a gps during the walk then the distance recorded can be quite a substantial amount beyond that quoted. My SATMAP updates itself every second, so the recorded route takes in every minor deviation around a bog, to find an easier river crossing etc etc. It also records every minor ascent including climbing over a stile, taking a short cut over a bump etc etc.

A final problem is that maps are a representation of a 3D world on a 2D surface, and, as such are a compromise. If a base line is taken up the centre of England an a map is drawn at a scale of 1:25000 from this baseline, by the time the extreme coasts are reached at each side of the country then the error between the map position and the land position is quite significant (due to several factors but mainly the curvature of the earth). To counteract this (I was once told by the OS) maps are drawn at a consistently incorrect scale in order to equal out and reduce the significance of this error. So ... getting inaccurate distance is a major problem.

My favoured method is to count the number of grid lines that you cross and half it. You get quite an accurate estimate of the distance this way.

When I do my walks, I use the old fashioned method of measuring the miles on the map. It may not be 100% accurate but it does me fine.

My measurements for Offas Dyke were 171 miles, working out at:

the start to monmouth - 18.8 miles
monmouth to pandy - 15.6 miles
pandy to hay - 17.2 miles
hay to kington - 14.4 miles
kington to knighton - 12.6 miles
knighton to little brompton - 15.4 miles
brompton to llanymynech - 21.2 miles
llanymynech to llangollen - 18.6 miles
llangollen to clywd gate - 14.2 miles
clywd gate to bodfari - 11.4 miles
bofari to prestatyn - 11 miles

As i say - this may or may not be right!

That is why there are map datums that must be used in conjunction with position co-ordinates... (ie O.S with uk grid or WGS 84 with lat/long etc) that allow for curvature to reduce these errors.....Lat/long position may vary depending on the datum ....If your gps is set to the correct datum for the co-ordinate format your map is using, you shouldn't be far off at all

To work out a distance between two points the datum plays almost no role as the distances are between points anchored in the same relative measuring system. Datums are needed to identify location, not distance. I temper this comment with the proviso that the datum must be appropriate for the general location.

More important is the frequency of measuring the position. A high frequency will record every little step around an obstacle and so deliver a longer distance compared to a less frequent measurement which will tend to cut corners and smooth out those minor variations of course.

What Mark F posts about the relative frequency of taking measurements is a critical factor. The SATMAP updates every second so, if you have to navigate a ladder stile it updates your ascents by the height of the stile. If you have to negotiate a boggy area your distance log is updated by the total distance that you travel to get around it rather than the straight line distance across. These distances, although small in themselves, can aggregate to make a significant distance to the actual length compared to a plotted route length or the distance measured on a map.

Good point..I was thinking position error not distance between points

Well, as nobody has come up with any GPS measured distance for OD,I've gone back to the 20th century solution and used my Map Measuring wheel on each of the O.S.Maps (courtesy local library,each section measured twice as a check),and came up with the following for Days 3 -7,which I thought the Trailblazer guide understated:
Hay-on-Wye - Gumma Farm,Dolley Green
Guidebook 23 miles,Map wheel 24.5 miles
Gumma Farm,Dolley Green - Kingswood
Guidebook 26 miles,Map wheel 28 miles
Kingswood - Carreg-y-big
Guidebook 25 miles,Map wheel 26.7 miles
Carreg-y-big - Gweryd Lodge
Guidebook 24 miles,Map wheel 26.2 miles
Gweryd Lodge - Prestatyn Beach
Guidebook 26.5 miles,Map wheel 28.8 miles
As I was quite happy with the first two day's mileage,this now adds up to a walked mileage of 187.4 miles.After deducting diversions to campsites,this gives a total OD Path mileage of 186 miles.You always walk a little further on the ground,so I believe it is still conservative,but I'm more comfortable with 186 as a total mileage,rather than the guidebook quoted 177 miles.
Alan F

Off topic Alan, but you did OD in 7 days, how many days did you do the PW in. Do you camp or B&B?
Dave.