BiB’s view on accuracy (MPH) of hand-held GPS?

[jj.]

I’ve had my Garmin hand-held GPS for many years now, and used it successfully for a variety of uses. Including tracking back a 150 miles route to a friends house across France, by just following a previous ‘bread-crumb’ trail alone – very impressed.

Now several times I’ve monitored the speed on this, and found that the speedo’s on most cars are below the speed readings on my GPS.

How accurate are the GPS readings, compared to what the police use for speed detection, do you use GPS based monitoring equipment.

As an example 70 MPH on my GPS is an indicated 77/78MPH on my speedo. I assume I’m fine to use the GPS reading, as these should be more accurate than the speedo- correct…? Does GPS still have an element of error built into it for personal use..?

Any BiB’s with a speed detection camera in the MK/Northants area (Super Sausage debrief café) – want to test it against a GPS, to see if the readings are correct – or has someone else done this before.
jj

[Streetcop]

Hi JJ..

I have a Etrex vistra...and it's very accurate...

[pdV6]

jj. said:

How accurate are the GPS readings, compared to what the police use for speed detection, do you use GPS based monitoring equipment.

Can't possibly be as accurate as a properly calibrated device being used in accordance with the manufacturers instructions and in ideal conditions.

jj. said:

As an example 70 MPH on my GPS is an indicated 77/78MPH on my speedo. I assume I’m fine to use the GPS reading, as these should be more accurate than the speedo- correct…?

Speedometers are permitted to over-read but not under-read. Therefore most manufacturers err on the side of caution. Differing wheel/tyre fitments will make a tiny amount of difference as well, so its sensible to have a bit of leeway in the speedo.

jj. said:

Does GPS still have an element of error built into it for personal use..?

I believe that this has been turned off, but the US military reserves the right to put the fudge factor back in whenever it suits them.

In general, GPS speed will be pretty accurate, but can only ever be a calculated average, so the instantaneous "speed now" is likely to be a bit out whilst accellerating/decellerating but may settle towards a decent figure on a long straight at steady speed.

[Edwards]

Hi jj,

If it helps GPS is dead accurate, well within reason, obviously the U.S military has more accurate ones than the civillian GPS satellites. Bill Clinton personally ordered the removal of inaccuracies in the GPS satellites.

So they are dead accurate right?....

Well not entirely so, keep in mind these satellites are very high so there is a delay of 1-2 seconds for the received signal to be received by the GPS unit and be calculated into a speed reading, so for example you could do 44 mph partially break to 40 and speed up to 44mph again and it would continually read 44mph, due to it taking an average speed between two lat/long points.

It has also been known for surrounding ground radio/terrain interference to interfere with the signal, so dont entirely rely on the speed reading given.

Hope this helps

[sadako]

You also have to take into aco going uphill/downhill will give a lower speed reading as it diesnt take into account changes in height, just coordinates

[Edwards]

Actually some do this is known as a 3D fix, but not all units can calculate it.

[Mr E]

I think the straight answer is
"A crap load more accurate than the average car speedo"

[pmanson]

I've got gps in the car via TomTom and an iPaq its not too bad. There is definatly a lag though between the change of speed.

Also if you only have 1 satellite locked it won't change the speed.


Always makes me think how they expect GPS tracking/speed limiting to work!

[TonyToniTone]

sadako said:

You also have to take into aco going uphill/downhill will give a lower speed reading as it diesnt take into account changes in height, just coordinates

I thought most triangulated the reading..