GPS Speed accuracy
GPS Speed accuracy
I have a Tom Tom Sat Nav and a Talex speed camera alert. Both will show current speed and give the same reading at any time. Depending on which car they are in, there is a 5 to 10% difference between GPS's and the speedometer. This is understandably due to the crudeness of the speedometer and also tyre wear. In general, I stick to the GPS speed as it reads lower and this means I can drive 10% faster without actually breaking the speed limit.
Then I started thinking about how GPS works. If it works via satellites then presumably it is measuring horizontal speed. So, if I am going down or uphill I am actually traveling faster than the GPS reading. Pythagoras had a theory about this and came up with the formula to calculate the difference between the distance traveled on the flat and the distance traveled on a slope. I don't think he was thinking about GPS at the time, though.
So, is GPS speed only accurate when traveling on a flat road ?
Then I started thinking about how GPS works. If it works via satellites then presumably it is measuring horizontal speed. So, if I am going down or uphill I am actually traveling faster than the GPS reading. Pythagoras had a theory about this and came up with the formula to calculate the difference between the distance traveled on the flat and the distance traveled on a slope. I don't think he was thinking about GPS at the time, though.
So, is GPS speed only accurate when traveling on a flat road ?
...a good question and one I've often thought about. GPS certainly works on the triangulation principle in which case I think you are right in that it can only calculate horizontal distance accuractely and hence speed. I suspect it will have error going up and down hills and the steeper the hill the worse it will get. But I might be wrong????So, is GPS speed only accurate when traveling on a flat road ?
Sapphire Black 02 S54 M Roadster
And corners? I don't know, I've thought about this often, myself, but I'm pretty sure it copes with three dimensions OK, just not sure about curvature ... my guess is that it calculates the distance from one three dimensional point to another and assumes a straight line between them.
That's how I'd do it if I was writing the software, anyway.
That's how I'd do it if I was writing the software, anyway.
Ciao,
Spokey
Spokey
jackal on PH wrote:i love your profile... an endless pornographic paroxysm of the letters BMW
do you actually like driving at all or are cars to you just a manifestation of some sort of pathological mother complex ?
Nice theory, but I think that unless you are driving vertically, it will make very little difference. GPS satellites are quite a few miles up - roads, generally, don't climb more than a few hundred feet per mile. So surely any error caused by a change in height would be very very small. I suspect that the reported speed may be much less accurate if you are driving on a bendy road - it all depends upon the time interval between triangulations and the error in the positioning (2-10m depending upon the system used). Or am I barking up the wrong tree?
Arctic Silver '99 Z3 1.9 & Black '59 Frogeye 1275cc
how did you get 0.5% - I thought a 1 in 10 slope would give you about 10% error, a 45' slope would give you 50% error etc (assuming that if you were travelling at 90' to the earths surface (i.e straight up) it wouldn't measure your speed at all).Driving on a slope will make hardly any difference - example 1 in 10 slope the GPS will read 0.5% under
The more I think about it though the more I wonder if triangulation would infact read a change in altitude so maybe there's no error at all?
Sapphire Black 02 S54 M Roadster
That's what I said.SpunkyM wrote:The more I think about it though the more I wonder if triangulation would infact read a change in altitude so maybe there's no error at all?
So ...The Global Positioning System, usually called GPS, is the only fully-functional satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio, allowing any GPS receiver (abbreviated to GPSr) to accurately determine its location (longitude, latitude, and altitude) in any weather, day or night, anywhere on Earth.
Ciao,
Spokey
Spokey
jackal on PH wrote:i love your profile... an endless pornographic paroxysm of the letters BMW
do you actually like driving at all or are cars to you just a manifestation of some sort of pathological mother complex ?
GPS works by calculating the position against several satalites, it needs atleast 3 and upto 10 at a time to get a reading, so long as the GPRS can get a reading it will be accurate, if it can't be accurate enough it will display no signal in the software.
Due to this, I can't see how It will ever loose any accuracy due to hills or corners.
Due to this, I can't see how It will ever loose any accuracy due to hills or corners.
No longer a Zed owner.... been there, done it, disliked it. (sorry)
GPS needs 4 good signals for a 3D fix on your position, but most receivers need 5 or more to get the accuracy down to 10M in built up areas. There are only about 12 satellites in view in a perfectly flat area such as the ocean IIRC.
Your speed in one direction is calculated correctly using Pythag if you have a good 3D signal. Remember though the speed displayed on your device may not be updated fast enough.
Your speed in one direction is calculated correctly using Pythag if you have a good 3D signal. Remember though the speed displayed on your device may not be updated fast enough.
Use the Search button before posting newbie questions about hard tops and fitting kits, footwell speaker amps, water in the boot, hood maintainance and those horrific angel eyes. We get like 10 threads a week on the same subject, it's obvious that you haven't searched.
GPS is pretty good at pinpointing your location in 3 dimensions. My Garmin GPS12 gives a height meaurement that is pretty accurate.
GPS calculates your speed by measuring your change in position against time. There is no reason it cannot measure speed in a vertical direction just as it does along the ground, but I suspect your average tomtom is not programmed for it.
I suppose the ideal test would be to take your tomtom skydiving and see what it tells you as you fall out of the sky!
GPS calculates your speed by measuring your change in position against time. There is no reason it cannot measure speed in a vertical direction just as it does along the ground, but I suspect your average tomtom is not programmed for it.
I suppose the ideal test would be to take your tomtom skydiving and see what it tells you as you fall out of the sky!
Here's a little thing I've done as an Excel DB showing the difference between the hypoteneuse (representing the road in reality) and the base (the road on a map).
If GPS Sat Nav works solely from assessing your position on a map, where your altitude is ignored in the calculations, then the GPS speed reading error difference between your actual speed on the road and your horizontal position on a map would be the percentage difference between the hypoteneuse and the base of a Right Angled triangle.
The column on the extreme right of the table shows the percentage error: e.g. on slopes less than 1 in 10, the error is less than 0.5%. When travelling on gradients of more than 1 in 5 it's unlikely that your speed will be at "warp speed", therefore, the error is hardly likely to have any significant error on your travelling within speed limits by using your GPS to check your speed.
If GPS Sat Nav works solely from assessing your position on a map, where your altitude is ignored in the calculations, then the GPS speed reading error difference between your actual speed on the road and your horizontal position on a map would be the percentage difference between the hypoteneuse and the base of a Right Angled triangle.
The column on the extreme right of the table shows the percentage error: e.g. on slopes less than 1 in 10, the error is less than 0.5%. When travelling on gradients of more than 1 in 5 it's unlikely that your speed will be at "warp speed", therefore, the error is hardly likely to have any significant error on your travelling within speed limits by using your GPS to check your speed.
^^Does the above assume you are 0M AMSL?
I have to say chaps, that when the US military designed this system, they took the 3D position into account, your speed will be calculated by the GPS receiver and transmitted in the NMEA messages in the form PVT (position, velocity, time).
I have to say chaps, that when the US military designed this system, they took the 3D position into account, your speed will be calculated by the GPS receiver and transmitted in the NMEA messages in the form PVT (position, velocity, time).
Use the Search button before posting newbie questions about hard tops and fitting kits, footwell speaker amps, water in the boot, hood maintainance and those horrific angel eyes. We get like 10 threads a week on the same subject, it's obvious that you haven't searched.
...and in plain English?estocks wrote:^^Does the above assume you are 0M AMSL?
P.S. I don't like your little danasoft.com window which tells me about what IP address, ISP Computer and browser I'm using: I find it a bit "Big Brother is watching you!". For this to work, is any spyware installed onto my computer?
Zero meters above mean sea level - at a guess!exdos wrote:...and in plain English?estocks wrote:^^Does the above assume you are 0M AMSL?
Nowt to do wi me, but if you look at danasoft.com you will see how it works. No it doesn't use spyware. It simply requests an image from their server and your HTTP request provides the info to put on it. What you see is not what we see - you only see your own details. I thought it was pretty neat.exdos wrote:P.S. I don't like your little danasoft.com window which tells me about what IP address, ISP Computer and browser I'm using: I find it a bit "Big Brother is watching you!". For this to work, is any spyware installed onto my computer?
Arctic Silver '99 Z3 1.9 & Black '59 Frogeye 1275cc
Does anyone know how often the measurements are taken? I think this would contribute to the accuracy of the speed.
The speed can only be the average distance over a fixed period, and the accuracy will be determined by the length of the fixed period.
Near Bradford (on the A650) there is a measured mile. I believe Plod use it to calibrate their speedos.
Set cruise control to 60mph and timed the mile. I measured it at 60.25 secs. My son measured it at 60.38. Obviously, we can't measure it to two decimal places, but these measurements convince me that the BMW speedo is VERY accurate.
I suggest you could do the same with a GPS to measure its accuracy.
The speed can only be the average distance over a fixed period, and the accuracy will be determined by the length of the fixed period.
Near Bradford (on the A650) there is a measured mile. I believe Plod use it to calibrate their speedos.
Set cruise control to 60mph and timed the mile. I measured it at 60.25 secs. My son measured it at 60.38. Obviously, we can't measure it to two decimal places, but these measurements convince me that the BMW speedo is VERY accurate.
I suggest you could do the same with a GPS to measure its accuracy.
Pingu
My calculations are based on simple geometry and the relationship to sea-level, or any other altitude level, is irrelevant. My purpose of doing the calculations was basically to show that in the real world of motoring and using a Sat Nav for speed recording, has a relatively small error at most commonly occuring gradients for motoring; far less than the speedo on your car.Robert T wrote: Zero meters above mean sea level - at a guess!
Just shows though how bloody simple it is for the spooks at Menwith Hill to monitor every bit of internet traffic and record the information for their databases. They can then get the Police to come and confiscate specific computers for much closer inspection of the HD if they wish.Robert T wrote:Nowt to do wi me, but if you look at danasoft.com you will see how it works. No it doesn't use spyware. It simply requests an image from their server and your HTTP request provides the info to put on it. What you see is not what we see - you only see your own details. I thought it was pretty neat.
GPS was originally a military system intended for better targetting of missles etc. As such it takes into account all 4 dimensions (x, y, z & time).
The GPS data is more than capable of allowing suitable receivers to calculate speed, whatever direction you're travelling in, and altitude. TomTom satnav's don't display height, but others (especially ones like Garmins that are intended for orienteering or whatever) certainly do.
The more recent uses of GPS, for route calculation, are nothing to do with GPS per se, they are just making clever use of the GPS data. When GPS was invented, this wasn't on the cards, simply because the digitised mapping data just didn't exist. So the speed displayed by the device takes no account of bendy roads, because it doesn't know about them - The speed displayed is simply a function of the difference in position between two samples, and the time interval of that sample. In theory, if you could drive in a circle fast enough so that in every time interval you were back in the same position, your GPS-indicated 'speed' would be zero, because you wouldn't have moved (as far as the GPS position sees it).
That's why most built-in satnavs also take a speed signal from the axle, which increases the accuracy of both the speed and distance displays, and also allows these functions to work where there is no signal, such as tunnels etc. That's why such devices need to be calibrated by driving certain distance forward, backward etc. Even my after-market Becker in-dash satnav has menu options for putting in circumference of tyres etc.
Portable satnavs can't do this, so will be less accurate in theory, although in practice they are accurate enough for their intended use.
The GPS data is more than capable of allowing suitable receivers to calculate speed, whatever direction you're travelling in, and altitude. TomTom satnav's don't display height, but others (especially ones like Garmins that are intended for orienteering or whatever) certainly do.
The more recent uses of GPS, for route calculation, are nothing to do with GPS per se, they are just making clever use of the GPS data. When GPS was invented, this wasn't on the cards, simply because the digitised mapping data just didn't exist. So the speed displayed by the device takes no account of bendy roads, because it doesn't know about them - The speed displayed is simply a function of the difference in position between two samples, and the time interval of that sample. In theory, if you could drive in a circle fast enough so that in every time interval you were back in the same position, your GPS-indicated 'speed' would be zero, because you wouldn't have moved (as far as the GPS position sees it).
That's why most built-in satnavs also take a speed signal from the axle, which increases the accuracy of both the speed and distance displays, and also allows these functions to work where there is no signal, such as tunnels etc. That's why such devices need to be calibrated by driving certain distance forward, backward etc. Even my after-market Becker in-dash satnav has menu options for putting in circumference of tyres etc.
Portable satnavs can't do this, so will be less accurate in theory, although in practice they are accurate enough for their intended use.
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- Joined: Sun 09 Oct, 2005 15:59
- Posts: 2
Thank god we got that sorted!
Purely academic now, but if we pretend that GPS cannot in fact measure altitude and purely works in two dimensions, Exdos - what answer do you get if you take your little tablle to the extreme and put in a slope of say, 1 in 1.001 (i.e. near vertical), logic says you should get very close to 100% error?
Purely academic now, but if we pretend that GPS cannot in fact measure altitude and purely works in two dimensions, Exdos - what answer do you get if you take your little tablle to the extreme and put in a slope of say, 1 in 1.001 (i.e. near vertical), logic says you should get very close to 100% error?
Sapphire Black 02 S54 M Roadster
Blimey! I came in on the end of this and we seem to have some pretty high theory here.
I have a Navman SatNav and also an Origin B2, and there are two observations I would make:
The speed reading is always delayed by approx 3 seconds. Try driving at a constant 70, and then doing a brake test type stop. By the time you actually come to a complete halt, your GPS will probably still be reading 30mph!
Also, I am sure that when you corner around tight corners, the readings are wrong (like on the slow side). Given the point above, it's a hard one to check up on, but try cornering fast round a tight corner with a passenger watching the speedo and the GPS simultaneously, and I reckon you'll find that the GPS says your speed dipped to, say 50mph, while the slowest the speedo read throughout the hole operation was, say 60mph.
This one is almost impossible to check on your own though!
Aren't we all just a bit anal about this?
I have a Navman SatNav and also an Origin B2, and there are two observations I would make:
The speed reading is always delayed by approx 3 seconds. Try driving at a constant 70, and then doing a brake test type stop. By the time you actually come to a complete halt, your GPS will probably still be reading 30mph!
Also, I am sure that when you corner around tight corners, the readings are wrong (like on the slow side). Given the point above, it's a hard one to check up on, but try cornering fast round a tight corner with a passenger watching the speedo and the GPS simultaneously, and I reckon you'll find that the GPS says your speed dipped to, say 50mph, while the slowest the speedo read throughout the hole operation was, say 60mph.
This one is almost impossible to check on your own though!
Aren't we all just a bit anal about this?
Best regards,
Sam Lever.
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Sam Lever.
Anyone who says money isn't everything hasn't found the right classic car yet.
Z1 - 3.0csl - 987 Boxster - Austin 7 Special - Rolls 20 - '72 911 - '95 318is
Black Sport Evo M3 & SG M3 CSL - now someone else's pleasure
Blue 2800cs - now someone else's pain
Old cars - the original "Not for Profit" organisation
Independent Financial Adviser in Buckingham
My Financial Blog
Thanks to everyone for all the replies.
I think the best I can do is to check the car speedo against the GPS on a flat straight road and recalibrate accordingly.
The main reason for the question is because the Z1 reads in kph. I bought the Talex Lite from Aldi (£60) and so I not only have mph but also speedcamera alerts.
I think the best I can do is to check the car speedo against the GPS on a flat straight road and recalibrate accordingly.
The main reason for the question is because the Z1 reads in kph. I bought the Talex Lite from Aldi (£60) and so I not only have mph but also speedcamera alerts.
SpunkyM,SpunkyM wrote:Exdos - what answer do you get if you take your little tablle to the extreme and put in a slope of say, 1 in 1.001 (i.e. near vertical), logic says you should get very close to 100% error?
If you consider the slope where the base is 1 and the vertical height of the triangle changes, so that you now get gradients such as; 1 in 1, 2in 1 and 20 in 1 etc., then the error will exceed 100% at all gradients over 1.733 in 1. As you can see from the table below, when the gradient is 100 in 1, the error is 9900% etc.
I use TomTom on a Palm PDA and when travelling in my MC in a straight line on the level at a given speed, then with OEM 17" wheels the speedo under-records by 7% and with 18" wheels it under-records by 3%.
I've also got a couple of other GPS applications on my Palm which also give altitude readings, although my house is at an established 28.4 metres above sea level, one application shows my house to be at anything between 75m - 85 metres ASL and another shows it to be 120m - 135m ASL between readings taken every few seconds. Clearly, the altitude calculation by low spec consumer end Sat Nav is not very accurate.
Last edited by exdos on Tue 07 Nov, 2006 20:33, edited 1 time in total.
Let me throw another little spanner into the works. You are looking at the analogue speedo. Whilst this is a reasonably precise instrument the scale that you read off on may be marginally out of line. Also as the needle is a few mm above the scale, so you get parallax error. On top of that, there are no individial graduations and 10 mph doesn't cover more than 1 cm, making it hard to judge exactly say 32 mph. With all that, you are going to be pushed to get within +/- 2 mph of your actual speed.
I performed an interesting test on my car. I used the odometer self-test and put it on speed (in km/h). I then drove down the road at a steady 50 mph and glanced down at the odometer and it was reading 77 km/h. So my
analogue speedo reads at least 3 km/h over the digital readout (50 mph = 80 km/h).
I performed an interesting test on my car. I used the odometer self-test and put it on speed (in km/h). I then drove down the road at a steady 50 mph and glanced down at the odometer and it was reading 77 km/h. So my
analogue speedo reads at least 3 km/h over the digital readout (50 mph = 80 km/h).
Arctic Silver '99 Z3 1.9 & Black '59 Frogeye 1275cc
Basically, all I am saying is that even if a consumer Sat Nav system, such as TomTom, totally ignores the change in altitude of a car on its travels,to all intents and purposes, it is good enough to give a reasonably accurate speed reading on most commonly encountered road gradients in the UK and is probably more accurate than the speedo on the car with which it's being used.
If you want to go in for aerial warfare and bombing using GPS then you want something a bit better :
If you want to go in for aerial warfare and bombing using GPS then you want something a bit better :
You are quite correct, Exodus, we need to take into account acceleration due to gravity when dropping smart bombs. It varies significantly around the world. Perhaps we better not pursue this line of discussion though.exdos wrote: If you want to go in for aerial warfare and bombing using GPS then you want something a bit better :
Fg=G(M x m)/(R squared)
Fg = Force due to gravitational attraction (force felt on the soles of your feet)
G = Gravitational constant (6.67 E -11)
M = Mass of object 1
m = Mass of object 2
R = distance between objects' centres of density
If I was stood at sea level R would be 6111km (based on 24000 mile circumference)
If I go to the top of Everest (8848m), R would be 6120km
As G, M and m are unaffected by the change in height (ignoring loss of body fat due to the climb), Fg would now be 0.3% less than it was at sea level
Conclusion - if you want to loose weight, climb a mountain.
Now, errors due to GPS...
Fg = Force due to gravitational attraction (force felt on the soles of your feet)
G = Gravitational constant (6.67 E -11)
M = Mass of object 1
m = Mass of object 2
R = distance between objects' centres of density
If I was stood at sea level R would be 6111km (based on 24000 mile circumference)
If I go to the top of Everest (8848m), R would be 6120km
As G, M and m are unaffected by the change in height (ignoring loss of body fat due to the climb), Fg would now be 0.3% less than it was at sea level
Conclusion - if you want to loose weight, climb a mountain.
Now, errors due to GPS...
Pingu
- whiteminks
- Joined: Tue 26 Sep, 2006 09:58
- Posts: 2768
- Location: Lincoln
Guys,
I am not known for being daft ( surprisingly some of you might think )
But don't you think there's some poetic beauty in not having a bleeding clue where you are going?
I love getting lost in the Zed and having to go cross country to get home........... bendy roads ...yum yum
I am sure Pythagorus and Issac Newton would be immensely proud of you all........
And yes travelling up a mountain might give an interesting twist to this discussion ........... but I suspect that you would have limited cognition as you probably would have difficulty breathing and be hypoxic unless you had access to RAF spec pressure assisted breathing apparatus?
Just a thought!
Cheers P
P.S If your satnav leads to to a disused airfield ........... in the words of Tasmin Archer ( sort of ) ........'blame the sleeping satellite
I am not known for being daft ( surprisingly some of you might think )
But don't you think there's some poetic beauty in not having a bleeding clue where you are going?
I love getting lost in the Zed and having to go cross country to get home........... bendy roads ...yum yum
I am sure Pythagorus and Issac Newton would be immensely proud of you all........
And yes travelling up a mountain might give an interesting twist to this discussion ........... but I suspect that you would have limited cognition as you probably would have difficulty breathing and be hypoxic unless you had access to RAF spec pressure assisted breathing apparatus?
Just a thought!
Cheers P
P.S If your satnav leads to to a disused airfield ........... in the words of Tasmin Archer ( sort of ) ........'blame the sleeping satellite
- whiteminks
- Joined: Tue 26 Sep, 2006 09:58
- Posts: 2768
- Location: Lincoln
Guys,
I am not known for being daft ( surprisingly some of you might think )
But don't you think there's some poetic beauty in not having a bleeding clue where you are going?
I love getting lost in the Zed and having to go cross country to get home........... bendy roads ...yum yum
I am sure Pythagorus and Issac Newton would be immensely proud of you all........
And yes travelling up a mountain might give an interesting twist to this discussion ........... but I suspect that you would have limited cognition as you probably would have difficulty breathing and be hypoxic unless you had access to RAF spec pressure assisted breathing apparatus?
Just a thought!
Cheers P
P.S If your satnav leads to to a disused airfield ........... in the words of Tasmin Archer ( sort of ) ........'blame the sleeping satellite
I am not known for being daft ( surprisingly some of you might think )
But don't you think there's some poetic beauty in not having a bleeding clue where you are going?
I love getting lost in the Zed and having to go cross country to get home........... bendy roads ...yum yum
I am sure Pythagorus and Issac Newton would be immensely proud of you all........
And yes travelling up a mountain might give an interesting twist to this discussion ........... but I suspect that you would have limited cognition as you probably would have difficulty breathing and be hypoxic unless you had access to RAF spec pressure assisted breathing apparatus?
Just a thought!
Cheers P
P.S If your satnav leads to to a disused airfield ........... in the words of Tasmin Archer ( sort of ) ........'blame the sleeping satellite
The chipping rate* on the encoded system the US use for war is 10 times that of the free domestic system and the absolute 3D position accuracy is 1m.exdos wrote:If you want to go in for aerial warfare and bombing using GPS then you want something a bit better :
*rate at which the scrambling code is repeated in the signal to spread it across the channel bandwidth.
I can't find any idea of how often the calculations of speed are made, I would suppose one or twice a second, although each bit of navigation software will be different in how often it updates the screen with the new value.
I suspect the problem seen where two different programs give different heights will be rounding error in the conversion from degrees to minutes and seconds, or a rounding off of the XYZt coordinates before calculation.
If you are bored one day open Hyperterminal and connect to your GPS (bluetooth) serial port and watch the NMEA signals rushing past. It's riveting.
Use the Search button before posting newbie questions about hard tops and fitting kits, footwell speaker amps, water in the boot, hood maintainance and those horrific angel eyes. We get like 10 threads a week on the same subject, it's obvious that you haven't searched.
estocks, I expect you're fluent in POP3, SMTP and IMAP as well! At one time I was also able to understand IIOP messages just by looking at the raw data.estocks wrote:If you are bored one day open Hyperterminal and connect to your GPS (bluetooth) serial port and watch the NMEA signals rushing past. It's riveting.
Nowadays I have a life!
All this talk of satnav. I still follow signposts - at least until they evaporate - then I just head in what I think is the right direction. It's much more exciting, as you never know quite where you are going to come out. Found some interesting roads that way - trouble is I can't find them twice!
Arctic Silver '99 Z3 1.9 & Black '59 Frogeye 1275cc
I reckons you read the Matrix encrypted really. :pRobert T wrote:estocks, I expect you're fluent in POP3, SMTP and IMAP as well! At one time I was also able to understand IIOP messages just by looking at the raw data.estocks wrote:If you are bored one day open Hyperterminal and connect to your GPS (bluetooth) serial port and watch the NMEA signals rushing past. It's riveting.
Nowadays I have a life!
Use the Search button before posting newbie questions about hard tops and fitting kits, footwell speaker amps, water in the boot, hood maintainance and those horrific angel eyes. We get like 10 threads a week on the same subject, it's obvious that you haven't searched.
You've made your mistake Professor MoriartyRobert T wrote:Gadzooks, I've been found out! It's my screensaver.estocks wrote:I reckons you read the Matrix encrypted really. :p
Use the Search button before posting newbie questions about hard tops and fitting kits, footwell speaker amps, water in the boot, hood maintainance and those horrific angel eyes. We get like 10 threads a week on the same subject, it's obvious that you haven't searched.
That seems about right for Tom Tom, looks like it updates roughly once a second for the speed calculation...my Driftbox updates at 10Hz and is accurate to +-0.1km/hI can't find any idea of how often the calculations of speed are made, I would suppose one or twice a second, although each bit of navigation software will be different in how often it updates the screen with the new value.
www.driftbox.com