GPS or speedo, which is more accurate?
Discussion
As the title says really. Just a quick question after doing 1000miles at the hands of the sat-nav and it telling me that my speedo is reading about 5mph under what i am really doing. I would imagine the GPS is correct as i have heard that manufacturers set up the speedo to over read your speed by a small amount. It makes sence i guess, better over than under then face a law suit!!
The GPS might not have taken the readings properly at the beginning after a stop - which might account for losing a bit of the speed.
The GPS is always more accurate than the speedo, but Garmin do allow their devices to read slightly higher than the TomTom ... But that is probably only due to rounding up for display.
The GPS is always more accurate than the speedo, but Garmin do allow their devices to read slightly higher than the TomTom ... But that is probably only due to rounding up for display.
Mr Will said:
Mr Gear said:
If it;'s completely flat, the GPS should be more accurate, but add a hill, and the GPS can't account for any uphill or downhill movement, because it sees the word in two dimensions.
Not true, GPS can (and does in most sat-navs) work in 3 dimensions.Mr Gear said:
If you dropped a GPS off a cliff, there is absolutely no way it could account for the downward movement.
Still not true, assuming it could see enough satellites it would perfectly happily record the speed until it went smash on the rocks below. GPS works in three dimensions, it doesn't rely on having the altitudes saved as part of the map.Mr Gear said:
If you dropped a GPS off a cliff, there is absolutely no way it could account for the downward movement.
I think they can do altitude if they can see enough satellites, but they tend to be shockingly inaccurate for it and I don't think the car ones typically take it into account when measuring speed, although I'm not sure. Mr Will said:
Mr Gear said:
If you dropped a GPS off a cliff, there is absolutely no way it could account for the downward movement.
Still not true, assuming it could see enough satellites it would perfectly happily record the speed until it went smash on the rocks below. GPS works in three dimensions.kambites said:
I think they can do altitude if they can see enough satellites, but they tend to be shockingly inaccurate for it and I don't think the car ones typically take it into account when measuring speed, although I'm not sure.
They only know altitude if they have a terrain map overlay. They can't tell altitude from the GPS signal itself, it needs to cross-reference this with a map to determine altitude.
Mr Gear said:
They only know altitude if they have a terrain map overlay. They can't tell altitude from the GPS signal itself, it needs to cross-reference this with a map to determine altitude.
Wrong, wrong, wrong, wrong, wrong.They are perfectly capable of triangulating in all three dimensions from the signals alone.
GPS works by calculating the distance between the unit and the various satellites that it can see. In a very simple example, your GPS unit can see 5 satellites; one directly north, one south, one east, one west and one directly above. Suddenly, the distance between it and the one directly above it increases, but the distance between it and all the others is staying the same. Which direction is it moving, and what part of this would the GPS unit not be able to figure out itself?
Mr Gear said:
If it;'s completely flat, the GPS should be more accurate, but add a hill, and the GPS can't account for any uphill or downhill movement, because it sees the word in two dimensions.
Gradient makes little difference. As an example take a seriously steep incline, driving 2 miles (as seen from above in 2 dimensions) and climbing 0.5 miles (that's driving to the top of Scafell Pike from Wasdale Head). Simple trig shows the distance actually driven on the incline to be 2.06 miles.Gassing Station | General Gassing | Top of Page | What's New | My Stuff