Please see the update below (additional to Strider and the website) from Phil Bardswell who is leading the group walk this Saturday 30 June. Please contact Phil direct philandjools@btinternet.com if you have any questions. Do not reply to this email as it is from an unmonitored email account used for outgoing messages only.

Martin Callow
Group Secretary
Dorset Group
Long Distance Walkers Association
dorset.ldwa.secretary@gmail.com
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We will be catching the 9:06 am train from Dorchester West station to Thornford. This will take 25 minutes. The Group is buying your ticket, so could you report to me at the station so I know how many tickets to buy.

The walk finishes in Fordington, but you won’t find a parking space there. Good options are to park in the streets to the south-west of Dorchester West station, or in the Fairfield long term car park, which you pass if you come into Dorchester from the south, at a cost of £4 per day.

We will visit the Thornford Benchmark and then walk back to the Dorchester Benchmark, a distance of a little over 20 miles. It’s likely to be hot and the going is slow to start with, so I don’t expect to be back before 6 pm, and it could easily be later than that. We will be stopping at Cerne Abbas after 11 miles for a late lunch / early tea. There are three pubs, a café, and a shop there so you should be able to find something to suit. Remember suncream, water and sunhat.

For those who are wondering what this is all about here’s a brief introduction:

You will all have seen and visited trig points. These were used by the Ordnance Survey to produce the two dimensional maps that we all use. However, the maps also include contours and spot heights, and these height measurements were produced by a second network of surveying using benchmarks.

All heights in the UK are measured relative to the mean sea level at Newlyn in Cornwall. Before the advent of GPS, to measure the height of another point in the country you had to laboriously measure the difference in height over a survey line between Newlyn and the place in question. This process is called geodetic levelling.

To survey a line, the surveyor puts a staff on a benchmark, knocks a bolt in the ground about 100 metres away, and puts another staff on the bolt. Then he puts his level (basically a telescope with a levelling bubble on top) exactly halfway between the staffs and measures the difference in height. The levelling survey proceeds in this way across the country.
To make things more efficient and to reduce the possibility of large errors creeping in, the geodetic levelling surveyors established a network of very accurately measured and very stable fundamental benchmarks. There are about 200 of these in the UK, and three in Dorset: at Dorchester, Thornford, and Wimborne. Unless you’re a connoisseur, you’ve probably never seen one as they are inconspicuously sited. Along the line between Thornford and Dorchester, they used a series of flush brackets (you’ll have seen these on the side of most trig points) and bolts (hard to find, but we will see one) as intermediate reference points. In the 1920’s survey they went up the railway line, and in the 1950’s they went up the road (the route we’ll be walking more or less), so you will be travelling along both survey lines in a day.

To find out more, you’ll have to come along on the day!