speedometer or GPS?

[Never Compromised]

Maps are based AFAIK on maps published by the various government agencies. The Garmin keeps showing the copyright for the Queen's Printer.

I subscribe to a quarterly update for my Garmin, cost me $100 for 'lifetime of the unit" updates. If the unit gets stolen Garmin will only transfer the subscription to the same model.

Even with maps updated every 3 months, there are lots of areas in Brampton that are simply missing, changes to the 401 are not accurate, changes to interchanges (ie QEW and Hurontario) are not updated. The maps are sometimes iffy on one way streets.

As for accuracy, I've seen the Garmin think I'm in the collector when in the express and being on a ramp for 3 or 4 seconds after I passed the damn thing. Going up the 427, the Garmin gets very confused about collector/express and I am damn sure that it does not recognize Rathburn as an exit.

It only recently recognized the extension of the 427 above 7, and it is not listed as the 427 but as an RR route.

 

[simon482]

Maps are based AFAIK on maps published by the various government agencies. The Garmin keeps showing the copyright for the Queen's Printer.

I subscribe to a quarterly update for my Garmin, cost me $100 for 'lifetime of the unit" updates. If the unit gets stolen Garmin will only transfer the subscription to the same model.

Even with maps updated every 3 months, there are lots of areas in Brampton that are simply missing, changes to the 401 are not accurate, changes to interchanges (ie QEW and Hurontario) are not updated. The maps are sometimes iffy on one way streets.

As for accuracy, I've seen the Garmin think I'm in the collector when in the express and being on a ramp for 3 or 4 seconds after I passed the damn thing. Going up the 427, the Garmin gets very confused about collector/express and I am damn sure that it does not recognize Rathburn as an exit.

It only recently recognized the extension of the 427 above 7, and it is not listed as the 427 but as an RR route.

yes but is the speed accurate.

 

[Never Compromised]

Speed should only be considered accurate while travelling in a straight line over a period of 10 or so seconds. Obviously if the GPS thinks you are where you are not, the calculation of velocity must be considered inaccurate.

 

[simon482]

Speed should only be considered accurate while travelling in a straight line over a period of 10 or so seconds. Obviously if the GPS thinks you are where you are not, the calculation of velocity must be considered inaccurate.

hmmmmm!

 

[oldjones]

Speed should only be considered accurate while travelling in a straight line over a period of 10 or so seconds. Obviously if the GPS thinks you are where you are not, the calculation of velocity must be considered inaccurate.

Exactly. Speed being time over a measured distance, and—satellites having no tape-measures—in order to calculate speed the GPS is depending on externally-provided knowledge of the ground distance between the positions the satellites determine. Even a doppler calculation's gotta factor angular data from on-the-ground mapping of the Earth. Compromised pointed out the government maps which we assume to be the most accurate (they're used for satelllite launching and positioning after all) are not necessarily the most up-to-date. Only if you know the GPS unit's software can you know whether a particular innaccuracy affects its speed reading.

No measurement is 100% accurate; the acceptable standards for car speedos are known and mandated by law. Your GPS unit's speed accuracy could be determined, and I'd bet the manufacturer has done that. But will they tell?

Meantime, if you want accurate buy an aftermarket speedo, that counts known-size wheel revolutions over measured time.

 

[Mencken]

Try this with your GPS...go to a circular or oval track, set your cruise control at 30 or 40 (so you don't have to slide around any corners...lol), then see what your GPS tells you your speed is. My guess is that it will give you a nonsense result...on the curves anyway. Depends on how it is calculating speed (both the software and the actual sensing/measuring).

 

[night ride]

Speed should only be considered accurate while travelling in a straight line over a period of 10 or so seconds. Obviously if the GPS thinks you are where you are not, the calculation of velocity must be considered inaccurate.

Actually this is why you get a little break from police radar too, since it only measures your distance away at one point of time compared to your distance away at another point in time, so if you are going around a curve at 100Km/h the radar or GPS will say you are doing 95Km/h.