Gps Speed Measurement - How Accurate ?

[MonkeyHanger]

I have a nice little PDA Based GPS / Sat Nav "toy", with the TomTom Navigator talking software.

Now i'm finally getting a chance to play with it, does anyone know how accurate GPS indications are in comparison with the Speedo ?

[robmawer]

I'd say much more accurate, as they calculate how far you've travelled in a set period of time,

while a speedo works off how many revolutions the tyres are doing...

Rob.

[redbaron]

gps should me lots more accurate.. the tvr and the skud both overread by 3-6mph speed dependent...

[Stimps]

In theory, the GPS should be a lot more accurate. Although as "domestic" GPS products are accurate to 10? 100? metres, there must be a slight margin for error somewhere.

Speedos usually under-report the true speed anyway, don't they?

[redbaron]

all gps is now accurate to the same point.. bill clinton turned off the "smudger" a few years back

[Stimps]

OIC!

Carry on...

[Kentish]

Originally posted by ffelan in this post

all gps is now accurate to the same point.. bill clinton turned off the "smudger" a few years back

Typical American miltary paranoia, but does anyone know if WE still print deliberate mistakes in our maps like we used too?

Oh yes my Road Angel says my Superb is about 4/5 mph over optimistic and my VW Transporter 3/4 mph over, but the hand book does state that they may not be deadly acurate, just more acurate than your speedo.

Cheers

Andy

[devonutopia]

Looking at Jon's on Saturday - I'd say a figure of around 7% or 8% is a good over-estimated reading from the speedo as opposed to the GPS. I'd like to see if mine is the same some time.

[DGW]

Garmin quote a velocity accuracy figure of 0.05 meter/sec steady state for their eTrex Vista. That scales up to a steady state velocity accuracy figure of 180 metres/hour, or about 0.1mph if my calculations are correct. I use my GPS when out walking and believe the average speed figures it gives are correct, ie circa 2.8 mph on an average walk with 2 dogs off lead, or 3.5 mph on a brisk walk.

[flyingscot]

Originally posted by ffelan in this post

all gps is now accurate to the same point.. bill clinton turned off the "smudger" a few years back

Was up on the Bridge of a Caledonian McBrayne Ferry 2 years ago on January looking at their GPS systems. The thing was stilll 500 yards inaccurate, so much so that if you docked correctly at the pier according to the GPS you'd have been beeched on rocks, 500 yards north of the pier. That was a useful system!

[Quinten]

or 3.5 mph on a brisk walk

Did anyone register Briskwalk.net yet?

[Dutch4x4]

Have used a Garmin too to compare with my speedo too, and using the gear ratio and wheel circumnavigation trick I learned here on Briskoda I was able to ascertain that the Garmin is very accurate...

[DGW]

Any significant inaccuracies I have seen have been due to problems with maps not with the GPS-indicated position as such. My GPS regularly reports an accuracy of 25 feet or less, and can distinguish the difference in position of the front and back doors of my house. The more I use it, the more I am amazed at what it can do. (It's a Garmin eTrex Vista).

[philc]

A friend of mine used to work for a GPS company, and reckons that accuracy is a function of the number of sattellites in "view" versus "fix time" -so, if you keep the thing switched on, without moving, with a good lot of satellites available, you'll get the best results.

Makes me nostalgic for the laser gyro systems we used in Boeing 767's. When you switched them on, they would immediately work out which way up they were, where north was, and their approximate latitude on the Earth's surface -all from the earth's rotation! Spooky!

Phil

[KLB]

Originally posted by Kentish in this post

Typical American miltary paranoia, but does anyone know if WE still print deliberate mistakes in our maps like we used too?

The Ordnance Survey do make deliberate mistakes, but the reasons are rather mundane - not a hint of cloak'n'dagger.

They will sometimes "move" a feature to make it visible (for instance, if it's close to a major road - the road on the map is much wider than it is on the ground).

The other reason is simply to help them enforce copyright - they can identify a ripped-off map by the pattern of deliberate mistakes.