GPS speedo accuracy.

[muppet_man67]

I just had a thought and was wondering if it could be clarified. how accurate is a gps speedometer when traveling up or down hill?

Do the take into consideration altitude as well as longatude and latitude when determining distance between points on their speedo functions? if they dont then when traveling up or down hill at the speedlimit that the gps tells you, you may actually be speeding.

it would also call into doubt the accuracy of there trip meters if you were traveling through hilly terrain.

[+dj_hansen+]

GPS speed works via doppler shift, and is accurate to ~ 5cm/second.
This figure comes from my lecture at uni (assoc prof in geodesy/gps) and whilst in hilly terrain it probably decreases slightly due to changes in satellite geometry, multi-path and other small random errors, you can consider it accurate enough, more accurate than a speed camera or speedo in your car.

Without delving into GPS theory and datums too much, the elevation you get on your GPS will be different to that shown on the map because GPS uses a mathematical figure (an ellipsoid) for the shape of the earth, whereas elevations shown on maps are to AHD - derived from the mean sea level of 35 tide guages around the australian coast line. The difference between these 2 (known as an N value) can be interpolated using software from Geoscience Australia, but for all practical purposes wont matter (unless your using DGPS) as the error in your height reading at best will be ~10m.

Saying this, with GPS there it is very difficult to use it as evidence as there is no 'legal tracibility' because its very difficult, sometime impossible to produce the same result.... and 10km back down the road you may have been speeding before Mr Plod pulled you over, and when you use your GPS data in court they find that out and you shoot yourself in the foot.

[RN]

GPS speed works via doppler shift, and is accurate to ~ 5cm/second.
This figure comes from my lecture at uni (assoc prof in geodesy/gps) and whilst in hilly terrain it probably decreases slightly due to changes in satellite geometry, multi-path and other small random errors, you can consider it accurate enough, more accurate than a speed camera or speedo in your car.

Without delving into GPS theory and datums too much, the elevation you get on your GPS will be different to that shown on the map because GPS uses a mathematical figure (an ellipsoid) for the shape of the earth, whereas elevations shown on maps are to AHD - derived from the mean sea level of 35 tide guages around the australian coast line. The difference between these 2 (known as an N value) can be interpolated using software from Geoscience Australia, but for all practical purposes wont matter (unless your using DGPS) as the error in your height reading at best will be ~10m.

Saying this, with GPS there it is very difficult to use it as evidence as there is no 'legal tracibility' because its very difficult, sometime impossible to produce the same result.... and 10km back down the road you may have been speeding before Mr Plod pulled you over, and when you use your GPS data in court they find that out and you shoot yourself in the foot.

You know in the 30 years as a ( insert whatever you want here because I have heard them all and been called worse) , I have never worked with anyone called Plod, but I did work with a Senior Constable nicknamed 'Noddy'.

[bogged]

( insert whatever you want here because I have heard them all and been called worse)

a challenge?

[RN]

( insert whatever you want here because I have heard them all and been called worse)

a challenge?

Love a challenge...also if I get to know what new terms are floating around I will know what to do when someone utters it......

[RN]

( insert whatever you want here because I have heard them all and been called worse)

a challenge?

Bogged, you have an hour to cut and paste some appropriate terminolgy here.

Are your scissors blunt or have you run out of clag?

[Mowie]

Love a challenge...also if I get to know what new terms are floating around I will know what to do when someone utters it......

I can think of a few but that depends on whether or not you have rolled over in the past.

[RN]

Love a challenge...also if I get to know what new terms are floating around I will know what to do when someone utters it......

I can think of a few but that depends on whether or not you have rolled over in the past.

PM..me for shits and giggles.

Amnesty guaranteed.

Trust me.