]> SQUAREWHEELS.org.uk – GPS Survey & Gradient Profile for Richard’s Cambridge & Oxford cycle route


GPS Survey & Gradient Profile
for Richard’s Cambridge & Oxford cycle route

This page was assembled on 16 June 2006, and last modified on 25 October 2015.

The GPS track data of this route are available for download in the following formats:

It may be necessary to rename the file you save, to end with .gpx or .kml as appropriate.

GPS survey & Gradient Profile

Towards the end of 2004, Richard got a new “toy”—a hand-held GPS receiver; mine, a Garmin Etrex Vista C is a device of similar size to a late-1990s mobile phone.

[PHOTO: nighttime view of marker stone and GPS device: 33kB]

This is a marker stone on the north side of the road just to the east of Toft; it marks the Greenwich Meridian(1). My GPS receiver reckons that I’m 0 degrees and 0.002 minutes West (which, at 52 degrees 11.120 mins North, is about 7 feet away if measured with the Ordnance Survey British Grid). (It also reckons that I’m at an altitude of 17 metres.)

It’s owning-up time—this is a composite photo, because the area containing the GPS display was wildly over-exposed making it unreadable; so I took another that was exposed correctly for the display (but was otherwise similar, although too dark), and combined them.

Photo taken on 1 December 2004 at 22:00.

GPS receivers

Very briefly, a GPS receiver calculates its position on the Earth by triangulating synchronised radio signals that have travelled different distances (and so arrive at different times) from 32 satellites in low-earth orbit. Nowadays receivers determine their position every second, to an accuracy of ±15 feet in latitude and longitude (subject to a reasonably clear view of the sky round about.)

Recording a route using GPS

[PHOTO: nighttime blurry shot of bike handlebars: 39kB]

The ‘dashboard’ of my Audax bike shows the handlebar-bag (with these route-notes in map-holder for re-checking); speedometer to right of stem; GPS receiver to left of stem; and 10-Watt headlight mounted to the fork. Towards the (easier) end of such a ride, I often end up finding activities like this to amuse myself! Apologies for the blurry view, caused by the 1/13-second exposure and the bumpy road through Comberton at 22:09 on 1 December 2004.

With the aid of the manufacturer’s pricey plastic handlebar-mount, my GPS unit was fitted (with quick-release function) to a spare spot on my handlebars, where I could see & operate it if required whilst on the move. My device automatically records the times and locations of many thousands of points through which it has travelled; additionally mine has a barometric altimeter which will record the elevation at each point. All these data can be uploaded to computer via a USB cable, and interpreted with proprietary software. The result is a virtual 3-dimensional record of the journey.

When I rode the route in both directions in December 2004, I had the unit switched on and recording the route. I calibrated the barometric altimeter to a known spot-height before setting off. The unit recorded about 1,800 points in the 84 miles, in each direction.

GPS track data of this cycle route

After correcting the height data (see below) I made these data available via the links at the top of this page; I chose the Oxford-bound track, for what it’s worth. I created a GPX version of the recorded track, for the benefit of riders with GPS units who would be able then to follow my track directly; and a KML version for users of Google Earth) who can follow the route virtually. I would welcome feedback from interested readers as to the usefulness of these files.

Generating the Gradient Profile

The Garmin software on the computer will plot a gradient profile, and this would have been fine; unfortunately, barometric altimeters are not of themselves an exact instrument: they suffer from “drift” in which a zero-error creeps in because of changes in ambient air pressure (particularly noticeable after a descent at dusk, as I found out!). For instance, the Cambridge-bound profile wound up as a flat line at zero metres for the bit from Barton into Cambridge, as it had lost about 16 metres’ height and was thus negative! Although I didn’t wish to spoil the “natural” data, it was going to be necessary to introduce a fudge factor to make any gradient profile accurate and repeatable.

(Did you notice how, in the photo of the Greenwich Meridian above, the lower part of the display says ‘17’? That was what it thought the Altitude was; but from the Landranger map we know it should really have been 32m.)

So, I painstakingly went through the Landranger maps, and noted every spot-height (sixty-one of them!) and determined what amount of zero-error had accrued in the course of both the recorded tracks. By exporting the GPS data to a spreadsheet and manipulating them to introduce suitable corrections for the zero-error, I ended up with a two-column table containing corrected heights and cumulative mileages. A quick Google search found me a good, free, no-frills graph-plotting prgramme—the aptly-named Graph by Ivan Johansen.

The thumbnail below is a link to the full-size version of the gradient profile. Two traces are shown, one for each journey made (in opposing directions); they are suitably colour-coded according to University colours.

[GRAPH: plotting elevation in metres against distance in miles: 64kB]

(1) – The Greenwich Meridian is an imaginary line between the north and south poles which passes through Greenwich, and from which all longitude distances to East and West on the earth’s surface are measured.