Marhofn 196.11 - May 2009

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Hill surveying part 2: practice

John Barnard

In my article in last year's Marhofn I reflected on what we might do during 2008. Until then we had only used level and staff, to measure height differences of up to 30m, concentrating our efforts on Nuttalls and Deweys. We really wanted to be able to measure SubMarilyns and marginal Marilyns, but the 150m of ascent is really too much for level and staff, as it would take about two full days to complete each survey. I know I am dedicated, but I don't think I could hold my staff vertical for two days! So having on a number of occasions dismissed the idea of purchasing survey-grade GPS equipment, we went and did just that. What triggered this change of mind and how did we get 'permission' from our wives to buy this equipment, instead of spending the money on more mundane things such as new kitchen work surfaces? The 'life changing' experiences were our surveys of Mynydd Graig Goch and Craig Fach in August 2008. These surveys required measurement of the absolute heights of these hills, and differential GPS is the only way to do this. Fortunately, James Whitworth from Leica GeoSystems offered his services, along with SmartRover 1200 GPS equipment. Having seen this type of equipment in operation, we knew we had to have it. I am not sure exactly how we got the green light for the purchase from our wives, but I suspect it was something to do with seeing their husbands on television, following the enormous publicity that these surveys attracted. Maybe we were not so mad after all.

Unfortunately, the expenditure does not stop once you own the equipment. Differential GPS requires corrections to the collected data in order to get the height measurement accuracy down to a few centimetres. This correction data can be received during the survey itself through Leica's Internet-based SmartNet service. However, the SmartNet subscription cost of £100 per week or £2350 per year is prohibitive. So we have had to take the other approach, which is to download Rinex data from the Ordnance Survey website. This requires hiring a 'dongle' (an electronic key) to process the downloaded data. We plan to hire this dongle on a weekly basis about five times a year. Although the cheapest option, this still amounts to a few hundred pounds per year.

Having bought the equipment in October 2008, the next step was to learn how to use it. Actually, once the machine is set up for the type of survey work we want to carry out, then its operation is quite simple. You set it up where you want to measure a height, leave it for a while to take the readings you want, typically for 30 minutes to two hours, and go off and do something else while this is happening (we still keep it in sight just in case...). The trickier bit is to become conversant with all the features; to maximise accuracy, monitor performance to make sure you are getting sensible measurements, and learn what to do if strange messages appear on the display.

I think this will be a continuing learning experience, but we are now well able to take reliable and accurate measurements. So now we have all the equipment needed to do any kind of survey work (apart from measuring heights in a forest), what have we done with it so far? Our main findings to date are:

What will 2009 bring? Still plenty in the 'in-tray' to get stuck into. Now that we have the GPS equipment we plan to use it to maximum effect. We will put some serious effort into surveying the SubMarilyns and marginal Marilyns listed with 147m to 153m of re-ascent. However, we still have to finish our project on the marginal 2000-foot hills of Wales before we start on those hills in England. We also hope to increase our surveying activities in Scotland, so I guess it is a question of just watching this space. If you have any suggestions do contact us.

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