Someone who was watching the Location feed on this site recently asked me on Twitter why I was travelling at a speed far under the speed limit. While I’ve certainly encountered construction, bad traffic, and bad drivers on this trip, in that instance I wasn’t. Here’s why:
First, GPS itself isn’t completely accurate, and what is reported may be off by 150ft or more. More importantly however is the application I’m using, an iPhone app called InstaMapper. In the InstaMapper settings is an option for minimum required accuracy with a range of:
- Any
- 2000 ft (any source)
- 600 ft (any source)
- 2000 ft (GPS)
- 1000 ft (GPS)
- 600 ft (GPS)
- 300 ft (GPS)
- 200 ft (GPS)
- 60 ft (GPS)
There is also an option for the frequency at which the app should post the location. I use 300 seconds, or 5 minutes. At anything greater than 60 seconds though, InstaMapper closes its connection to the GPS.
What this means is that every five minutes InstaMapper has to reacquire a GPS signal, but only to a 200 foot accuracy. It therefore isn’t getting an accurate enough signal to calculate the speed. I’d love to see them add an option for ‘150 ft (GPS)’ which I think would help improve the accuracy significantly. This assumes of course that the algorithms are correct, but when I’ve tested it at 30 second intervals where it doesn’t drop the GPS connection it is significantly more accurate.
Bottom line though, this is why I am at times reported to be travelling at 45 miles per hour when in fact I am on the posted nose of 75. Reportedly.
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