Police cop backlash over GPS speedometer inaccuracy claim

The Luddite view of the police (who probably just want to keep things simple), has been comprehensively crushed:

The maker of a vehicle GPS navigation system is taking police to task over comments suggesting the units are unreliable.

Last week, a police spokesman said readings from GPS devices were not accepted as a reliable means of accurately proving a driver’s speed and therefore could not be used to disprove a speeding fine.

Software used by manufacturers was based only on straight-line driving, police said, and was less reliable because of signal loss through buildings, terrain or weather conditions.

Yesterday, former Navman engineer and current New Zealand development manager Sajeewa Dayaratne said GPS devices were susceptible to some interference but that was unlikely in New Zealand.

“The open roads in New Zealand where there are no tall buildings, and drivers are not frequently stopping, are perfect conditions for GPS speed readings.”

GPS speed readings are going to be more accurate than those on the car’s odometer.  Which is the problem really, as people with true knowledge of their speed are  likely to “speed” close to the limit.  

Automotive specialists last week swore by the accuracy of GPS devices for indicating speed.

Goodyear Timaru owner Carl Vaughan said using a GPS was the only way to get an accurate speed reading.

Dayaratne backed the accuracy of GPS units.

“If your GPS is receiving a signal from even just four satellites, current research shows dedicated GPS navigators have an accuracy of 0.2kmh. This is much more accurate than a car speedometer.”

Last week a Timaru driver approached The Timaru Herald after being told by police that his GPS reading, which showed he was travelling under the speed limit, would not be accepted.

Police said GPS devices were not recognised by or calibrated to the same accredited standards as speed guns and therefore were not a reliable means of gauging speed.

However, Dayaratne said GPS devices worked off a mathematical formula and did not need calibrating.

There is some lag, where you can accallerate or decellerate and the speed reading takes a few seconds to catch up.  But when you’re in the process of driving and you get clocked at going at X km/h, you can’t be certain your GPS will have you are that speed.

And this is why you need at least some tolerance.  It will be a complete waste of court and police time for people to argue over 1 or  km/h based on the radar gun reading being at 101 km/h, and your GPS saying you were at 100.


– The Timaru Herald


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  • sheppy

    I look forward to the first +1kmh being challenged in a court of law especially when the measuring device wasn’t calibrated the same day. I suspect the calibration has a far greater tolerance than 1kmh especially when factors such as tiny movements of the measuring device are taken into account.

    • axeman

      My understanding is that the reason why they operate with a 4km tolerance during holiday periods is that he margin of error / tolerance is + or – 3-4kph.
      What ever knuckle head came up with 1kph should be fired on the spot for gross stupidity.

    • symgardiner

      It will never get challenged in court. If a Police prosecutor thought for a minute that you had a half way decent set of data showing their gear was wrong, they would withdraw the ticket etc instantly. You would not want to be the Police prosecutor who caused that revenue stream to dry up. That would certainly be “career limiting”.

      • Damon Mudgway

        It’s a minor infringement, so the issuing officer would be required to give evidence, and it’s in front of a JP not a Judge. NO JP is going to allow a conviction over 1km/h. It’s the stupidest thing the police traffic heirarchy plonkers have come up with yet.

      • Bob D

        This is exactly what happened to me once, but in South Africa. I had a very good case lined up, and discussed it with the cops prior to the court case. They assured me they would demolish me in court, but they looked a little worried, since there was a similar case being heard that was all over the news.

        On the day of the case I turned up, and they waited literally until they saw I was about to step into court, then withdrew.

        Far too risky for them.

  • Orange

    Police claiming to be rocket scientists now?

  • metalnwood

    So they want everyone driving a 90km/h so they have no fear of getting a ticket, while others who cant stand being behind slow vehicles will take more risks to get past them.

    I wonder if the law of unintended consequences will come in to play.

    • Bob D

      Most certainly. And add to that the increased incidences of road rage.

  • ozbob68

    I have no doubt that GPS is more accurate than speedometers, but can it be used as evidence to have a speeding charge thrown out? Do GPS kits have a replay function matching speed to time-and-place?

    • Damon Mudgway

      The short answer at this time is no. The police radar detectors are contracted out yearly for calibration tests, and are recognised by the Minister in the Gazette (police practice). But as I replied to another post, no JP is going to enter conviction for 1km/h.

      • Brian of Mt Wellington

        We used to have to test the hawk radar at the start of every shift with tuning forks and record the info into a log book which was produced in court along with the annual certificate.

        • Mark C

          And I used to audit the processes used by the Calibration Unit to test the patrol car speedometers for deviation.
          Most people would be surprised by the high degree of checking that police employ to have confidence in their speed detection equipment (and for evidential purposes in court, too).

      • cmm

        According to the rules (provided to me by the police), the police equipment is supposed to be calibrated daily.

        • Damon Mudgway

          Yep, by each officer at the commencement of their duty. It’s a tuning fork test. There is a ton of training around their use, so coppers are normally pretty proficient at giving evidence as to their use if required.

          But the certification of accuracy of the unit is per annum.

          Hope this clarification makes sense?

  • Damon Mudgway

    Here’s the crux kids. I don’t know the exact circumstances of this alleged offence, but arguing over what measure you exceeded a posted speed limit is going to get you nowhere. I recall a motorist who took me to court saying my speed detector was faulty when I caught him doing 117 in a 100 zone. No biggies but he reckons he was only doing 110 cause that’s the speed he set his cars limiter to. So the JP presiding the matter asked him directly “so you are adamant you were only doing 110 km/h?”

    “Yes I am your worship” he steadfastly replied.

    “You realise you are here today to defend the offence of exceeds 100km/h in a 100 posted speed limit zone?” Chirps the JP.

    “Oh, riiiiight” says the defendant.

    Gets fined for doing 10km/h over plus demerits plus court costs.

    The moral of the story ‘always argue you were doing less than the speed limit, not the degree to which you were exceeding it.

    • Mav E Rick

      I appreciate your comment Damon and your reply to Symgardiner earlier on about the Police hierarchy coming up with the crap 1k tolerance idea. I find it hard to keep my car within a 4 k tolerance let alone 1k. Keeping your eyes continually on the speedo is so dangerous as it is the road you should be watching not the speedo. If the 1k tolerance became the nrom people would travel at between 90-95 to ensure there is no risk that they would cop a $30.00 fine and demerit points. This would slow traffic down hugely for the sake of a couple of K’s. I don’t promote speeding but I do promote logic and reasonableness. Seems that the JP’s and cops in NZ do as well.

      • Damon Mudgway

        Ex plod now Mav. Posting on WO while still a sworn member would almost certainly mean a DCM if you were caught :-)

        • Mav E Rick

          ha ha I wondered about that. Not sure if this is your nom de plume but if your real name and you’re still a plod – I could see you standing in front of your commanding office Monday morning doing a please explain! Good work.

      • Excitedly awaiting Whodunnit

        I see the “[email protected] under crowd” everyday now and the hold up on some roads is frustrating (I am forced to do 90kms max and if I am being held up by cars something is wrong). Worse still is that no one will pass these guys/gals because then they are in the gun for possibly speeding.

        Ever figured out how much clear road you need to stay under 100kms and pass someone doing 90kms? Its a lot, but a quick blip up to 110 to get past and then settle back quickly to 100 is safer I’d guess.

        I drive a truck so am limited to 90 (sorry guys I do try to move over if I can) so I see this crazy slow motion passing all the time and now its worse. Im not advocating stupid speeds but some discretion should be in play.

  • Hard1

    On the subject of the Police, is this statement for real ?.

    “Operation Yesterday has again demonstrated the extent of the illicit cannabis industry. Cannabis is still the leading illicit drug linked to hospital admissions and the harm it causes to our communities is immense,” Det Sen Sgt Leitch said.12/12/2014.

    Like the GPS claim, I din’t believe it.

    • Damon Mudgway

      I think DSS Leitch may have plucked those stats from where the sun don’t shine.

  • sin-ic

    1kph over is impossible to prove. GPS or police radar. The below comments will provide “reasonable doubt” of it goes to court.
    1. Most if not all car speedometers from new indicate 100kph at a true 95 kph.
    2. The reading will vary as the tires wear so the rolling radius reduces the number rotations per km to maintain a set speed will increase.
    3. Many cars will have fitted different tyres to the manufacturers original recommendation. Low volume sales of some sizes often gets them deleted from the tyre manufacturers catalogues and substitutes of as near similar size will be fitted. These will also affect the readout.
    That’s why the original 10kph ‘tolerance’ was accepted. The holiday reduced tolerance speed is difficult. Just don’t let the speedometer get over 105kph or pay the consequences.

  • elton_fred

    Given the technology that’s present in most new cars these days, you’d think speedos could be GPS driven, not reliant on a mechanical connection with the wheels.

    • Excitedly awaiting Whodunnit

      They are available from the US (for hotrods etc) and elsewhere too I’d imagine.

  • phronesis

    The issue with gps is not its accuracy as such but what it is measuring. GPS systems don’t generally take into account changes in elevation so speed is calculated as horizontal distance over time. If there is a significant vertical component (a hill) then the actual distance and thus speed may be higher than is indicated.

    • metalnwood

      Thankfully, when was the last time you ever saw a camera point towards you when going up a hill. Never. Going down a hill? Yes, that old police trick happens all the time..

      When going down a hill your gps may tell you that you are going slower than you actually are, which is the same for up a hill as you are travelling more distance than the GPS thinks you are.

      For your average policeman (person!?) perched in their hidey spot it wont make a lot of difference as an incline of 10 degrees only adds 20M more distance to 1Km travelled.

      • Hard1

        Are you thinking of that long slope coming down off the plateau into Waiouru ? How many tickets have been written at the bottom of that hill over the last decade or so ?. Cops should put an office there.

      • Excitedly awaiting Whodunnit

        As long as you have good satelitte coverage (good spread around you and at differing heights/angles) the slope speed will be accurate too.

  • Yeahright

    I fix Aircraft GPS units, in their software they have a thing called RAIM, Receiver autonomous integrity monitoring. This is due to “sometimes” GPS reception is not what it is made out to be, so when its “faulty” it tells the pilot, sorry buddy don’t use me.
    So in effect GPS’s are accurate 99.9% of the time, but the problem lies when is the 0.1% occurring. Your cars GPS isn’t going to tell you when this occurs!!

  • cows4me

    The police radars are made by the same crowd that makes thermometers used to measure global warming.

    • axeman

      If that’s the case we know beyond reasonable doubt they are 100% wrong now.
      Love it

  • Guest

    Translation: We gave up detective work years ago and for quite some period of time have lowered the bar on recruitment around gender, race, physicality and intelligence therefore do not present any new forms of technology to us as a defence as we just won’t go out of our way to understand it let alone interpret it.

  • Kimbo

    Translation: We gave up detective work years ago and for quite some period of time have lowered the bar on recruitment around gender, race, physicality and intelligence therefore do not present any new forms of technology to us as a defence as we just won’t go out of our way to understand it, more so if it saves you, the citizen money at the expense of us, the revenue collectors.

  • armotur

    I suspect that many who are ticketed for being just over the limit will take the issue to court to challenge, that’s going to tie up court and officers somewhat!

  • NoEyeDeer

    Should the Police have a register of tested accurate GPS units so that they can waive speeding tickets if the car has an ‘approved’ GPS if the incident is within tolerance ?
    If so who maintains the register and how often is it updated.
    If there is a say six month period between certification then do the Police have to check software version on site or clone the GPS for evidence purposes?
    These kind of scenarios get thrown at enforcement agencies all the time and while it sounds common sense it is practically unworkable.
    I think people forget that the laws (generally) are put in place as a line in the sand. Most folks stick near enough to the law for it not to be a problem. You’d hope the Police would focus on the souted up mitsi’s doing 120.
    When the cops are quibbling about 1km over then you know it’s revenue gatheringrowth.
    As for validating GPS units … nah the cost would be worn by the taxpayer anyway.

  • TreeCrusher

    If I get ticketed for anything less than 105 km/hr I will ask for all calibration certificates, proof of training of the officer and accreditation certificates of the person/organisation that undertook the calibration of the detector. I’ll then take it to court and pay the court costs if I loose. The police better hope there are no holes in their calibration and training processes.