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January 29, 2007

Edge versus Vista (2)

Filed under: GPS — mtbguru @ 12:52 am

This is not a rematch, but some additional data highlighting the difference that the SiRFStar III chip (Edge) makes.

Previously, we noted that the accuracy of the Vista goes down significantly when under canopy or in narrow canyons, whereas the Edge was holding up very well. Most of the route in the previous test consisted of trails in open meadow land with some relatively short forested sections.

But when the trees are getting really dense, things become more pronounced. The Vista may start to lose signal – you’ll notice this when the track log is broken up in several sections. I took both units on another local ride (Alpine Road in Portola Valley to Skyline Blvd), this time mostly under dense canopy and/or skirting a fairly narrow canyon.

The first graph shows longitude versus latitude – the Edge recorded again a very clean track, whereas the spread on the Vista was large, and included several drops in signal. The route was done out-and-back.


The image below shows a section of the track recorded by the Edge, overlayed on a satellite image in Google maps on which the trail can be seen, hinting at the Edge’s accuracy. Of course, this isn’t saying much since this section of trail was obviously visible by a satellite (which would not be the case in really dense sections), but it does give an idea.

Alpine Rd

Interestingly, looking at cumulative distance over time (graph below), a lot of the spread in the signal is again averaged away, as mentioned in the earlier post. Except for the signal drops which may show up as small anomalies, the Vista produces pretty much the same data as the Edge. So even a GPS having crappy reception can still make a decent odometer… but not a good tracker.


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