Wednesday, 26 January 2011

Club Run - 25/01/2011

GPS Track (see below)

 Another Tuesday night, another great club run from Church Stretton. Tonight we headed out up Ragleth Hill (Helen says this is the route to use descending in the Skyline and Long Mynd Hike). We made the mandatory return journey along the ridge to the post, then dropped down to Ragdon Road and did a bit of tarmac bashing down to the layby at the top of Sandford Avenue. From here we hiked up steeply to the Gaerstone and continued along the ridge to the highest top of Hope Bowdler Hill. We descended to Cwms under Caer Caradoc and then down through the very muddy field onto Cwms Lane and back into Church Stretton.

I experimented with a new GPS solution on the phone - this one is called Trails. It's a bit clunkier than RunKeeper but allows export of a GPX file and also uses much better mapping than Runkeeper (the OpenCycleMapTopo which has 10m countour intervals when zoomed in and shows a lot of the paths too). I had a problem with the recording in that it starts straight away, before a decent z co-ordinate is acquired, so the first two points had zero altitude and thus the climbing was over-estimated by about 600' (the altitude of the car park in Church Stretton).

One of the advantages of the GPX format is that it's very easy to look at the data and edit these points out. I did this and then imported the data into a web based app, This enables you to do track and profile plots with a good degree of control over the output. The track plot is at the top of this post, and here's a profile based on the GPS altitude readings:

You can see that there's a lot of noise arising from the errors in GPS readings, particularly near the start where we were close to buildings and under trees. The levels themselves seem to be within 30 feet or so of spot heights on the OS map generally, although one or two points have errors up to 70'.

A better way of doing profiles is to use only the x,y track data. GPSvisualiser can process the GPX file to replace the GPS measured altitudes with ones calculated using a DTM (digital terrain model). This gives a smoother profile which probably tends to slightly underestimate the amount of climbing:

Note that the profile line is coloured to indicate the speed. Red bits are where we stopped to regroup! The process of producing these is really quick and I like the degree of control you get which is missing in RunKeeper.

Another cool trick is that GPS Visualizer can produce .kml files to import into Google Earth from the .gpx output of the iPhone. Here's a sample, looking down into Church Stretton along the last mile or so of the track:

For the record today's stats were 6.9 miles and some ascent (variously about 1,850' according to the GPS, 1,820' according to the OS 1:25k map, and interestingly only 1,420' from the profile derived from the DTM). Call it 1,820' then.

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