Nav or Speedo? What do you believe when setting your Speed?

[CairnsDuc]

With the whole Nation going Speed crazy, and considering in about 3 weeks I will be spending a
great deal of time driving across it, I have been pondering something with regards to Speed and
GPS/Nav vs Speedo.

We have all noticed that our Speedo's have a certain amount of error built in, The speed on the
GPS/Nav systems is very accurate, So what do you set your speed to?

I have always set my speed on the Highway to the 100 KPH shown on the Nav system when out on
 the highway, which means my Speedo says close to 108 KPH, now I have had Highway patrol cars
pass me with no problems, and I have gone past a few Speed Camera Vans also without an expensive
Kodak Moment.

Now, this is of course in Queensland, where the Speeding tolerance is not as tight as NSW, or worse again
Victoria, where I notice they give you a Tolerance of 3k's, after that bend over and lube up.

So which would be safer to believe? What do you use to set your speed?

Stick to the Car's Speedo? which if I use that should give me 5 or 6k's of buffer
or
Trust the GPS? which will get me there a tiny bit faster, but the margin of error is just about nil.

[ozducati]

i tend to trust my GPS when i have it on the bike, done quite a few trips with it and never had an issue, i think it is more accurate than the speedo... same as you, i have passed H/way Patrols with no issues, the fixed camera's i tend to use the speedo just in case... i have a Garmin Zumo 550, which is a brilliant bike GPS, comes with a cradle for the bike and one for the car as well...

[ungeheuer]

What do you use to set your speed?

I generally trust the GPS and so far even down here in the People's Republic of Victoria it hasnt cost me.

So which would be safer to believe?

You already know the answer to this 

[bikeless]

never trust a gps all the time with gps availability allways changing they can have a error of ten precent for civilian use when the arability changes is set to change at certain time per hour its so we cant use this to send rockets at the yanks or track the forces accurately how do i know all this when i was involved in tracking 3d sounding images of the continal shelf and marking the gradients we had a licence and a little device to tell us what distance the gps signal was deveated by and how much , servaours use the same unit   you gps will be accurate some times and wont be other times  not noing when it isnt accurate is the problem  to prove this to you brive down a road and set your gps on tracks and soom in, drive for a coulple of kays then turn around and repeat the route and you will find the tracks wont overlay that time availability doing its thing tho australia has had availability lessened you should not bet your left nut on it cheers bazz 

[avizpls]

  amazing run-on. Simply stellar.

[CairnsDuc]

Interesting Reading Bazz! Cheers 

Might be safe and stick with the Speedo in the Police State of Victoria, and err on the side of Caution going into NSW. 

[bikeless]

it written on shipping gps not too use this gps for navigation solely in most cases they are fine but ive seen my own gps show me i was steaming along a road instead of being up the shipping channel when the war broke out with the towl heads i couldnt find my fishing bouys my gps was out over a hundred meters and stayed that way for a week and then got a letter from the local gov saying so they could have warned us trying to find a little bouy in the dark is hard enough with a good gps but neer impossible in rough seas with out one cheers bazz

[CairnsDuc]

I sell Nav systems (TomTom, Garmin, Navman, Etc) and we regularly hear complaints of people saying that there GPS says they are off driving in a field
and then it will correct itself and put the car Icon back onto the road, We just got into the habit of saying to people that there is a margin of error of approx
20 meters, that will increase if there is heavy rain/cloud cover, lots of trees covering the road or if the Receiver in the Nav can not find enough Satellites to  
lock on to.

The Satellites are 20 Kilometers above us with 24 of them floating around up there* I have spent a bit of time in the last day or 2 reading up about how
the system works, Fascinating reading (for the Nerd in me)
* with 3 spares making 27.

The Mathematics involved had me going a bit cross eyed for a while  

http://en.wikipedia.org/wiki/Global_Positioning_System

[bikeless]

there is actully 42 gps sats now but we can only tap into 27 what happens is they keep falling back to earth as there compressed air boosters run out of air and they canot be cept in there proper alignment so they turn them of and they fall back to earth in america they use a digital posting signal to take away the variable availability that work of there phone towers we were suposed to get it here but never did it gave them accuracy to less than one meter ,clever little things tho cainsduc , are you heading straight to hobart when you get here ,im taking the first load to melb on the 16 th then we all leave here on the 29 th for bairnsdale   

[suzyj]

http://geography.about.com/library/weekly/aa050400a.htm

I work in this field.  GPS used to have "selective availability", whereby the timing of some of the signals was deliberately dithered by a pseudo-random sequence.  The idea was that civilian users of the service would be able to get a +/-100m fix on their location, but those with the cryptography gear (ie military users) would be able to undo the dithering and get much better (<10m) accuracy.

That worked all well and fine, until the US went to war in the gulf.  They didn't have enough of the crypto-equipped GPS units to go around, so the easy fix was just to turn off the dithering.  They did that in 2000, and it's been off ever since.

GPS is much more accurate than your speedo, as long as you get a decent signal on enough satellites in the constellation to accurately triangulate your position.  The GPS signals are at 1.2 and 1.5 GHz, so they're very easily attenuated by buildings, trees, and even sometimes car windscreens.

The issue with people's GPSs showing them driving through fields is mainly bad maps, where the position of the road on the map doesn't match reality.  Mapping software often tries a bodgy fix for this, by moving you to the nearest road if your position puts you in a field (or in the water), hence the fishing problem.

In any case.  Trust the GPS.  Especially on country roads.

[bikeless]

no suzy wasnt talking about map overlays was talking about track overlays and there still is variables built in  i had  a new variable decoder fitted on a new police boat that was just built and that shows how much variable there is and in what direction , while there is a war on it will stay apparently and then there time lasp it must be taken into account it takes time for the gps to settle so if you can hold an exact speed for a time frame you will get a corect speed unto signal change over happens from the satellites on most occasions 
 a speedo is more accurate in measuring speed  a gps will tell you the difference between real speed and what your car or bike is doing but never as accurate as the car speedo as signal loss can happen from many thing as you are a where , trees buildings , hills , where its fitted  which satellites are being used  , external anteners all have too be taken into account so they are not as accurate as a normal speedo only what your average speed is cheers bazz

[monstermick58]

It does'nt matter for me cos I'm usually speeding anyway ('cept in towns and never through school zones) so do I need to know which one to look at when the copper sez 'you're nicked 12 over' and I say 'bollocks, its only 8 by my speedo/gps'.






                            Mmick 

[auntymal]

GPS more accurate. just changing the profile/pressure or using a worn front will 'change' the reading. Aunty

[craigo]

+ 1 GPS being more accurate. I've had three separate GPS's in the same car at the same time, they all stated the exact same speed. Almost every car I have been in is about 8-10% *under* (speedo vs GPS). The exception is my Commodore, which is pretty much on the button.

I have a Zumo 550 as well, I go by that on my bike a lot and I've never been pinged. I find that I cruise past the traffic at 115km/h GPS Speed (in a 110km/h zone) on the Hume Hwy on the way back from Kanga Valley, but my Monster speedo says ~125. I figure I am within the coppers 5% tolerance for overspeed @ 115 in a 110 zone. In 80 zones I go about 90 and GPS still indicates I *should* be ok.

I read something somewhere about some legislation that states that Speedometers in vehicles can NEVER be over, but can be under by a certain percentage in Australian vehicles. So the manufacturers err on the side of caution and run under. Heaps of cars measured rated well under (so when it looks like they are bang on 60, they are actually 57 for example). This might explain why I am occasionally* frustrated in Sydney traffic when people slow down to ludicrously low speeds when going past Speed Cameras.

GPS for hardcore stuff like boats and smartbombs and stuff are probably a different kettle 'o fish, but IMHO the GPS will always be more accurate than the vehicles speedo.

Serious Groper knows a bit about road and other GPS stuff as well. Dunno if he's around anymore, haven't seen him for a while.

* where occasionally equals every.friikkkken.day

[CairnsDuc]

http://geography.about.com/library/weekly/aa050400a.htm
SNIP:
The issue with people's GPSs showing them driving through fields is mainly bad maps, where the position of the road on the map doesn't match reality.  Mapping software often tries a bodgy fix for this, by moving you to the nearest road if your position puts you in a field (or in the water), hence the fishing problem.

The interesting part about that comment, I would have to agree, 99% of the complaints I get about driving in fields is from people owning Navman Nav systems.
(I had 1 complaint about a new Garmin, but we replaced it and he never complained again, so I think it was faulty from day one)
I have found up here in Cairns the Navman maps are so out of date and inaccurate, they have streets missing everywhere, Points of Interest that are missing or
wrong, The Navman has an interesting Glitch we mention to every customer now, If you are navigating into the City and you drive past Stockland Shopping Center
on Mulgrave road, for no reason and no warning, it will tell you to turn right, Across a Traffic Island!!
There is no right turn for at least 300 meters, and if you are going to the City, you would not be turning right at this point (it's a dead end) Had a lot of complaints
about that little Glitch!

Never had Map complaints about TomTom or Garmin, But Navman cause us lot's of headaches. They use Navteq maps exclusively, where Garmin and Tomtom use
Sensis and Whereis Maps, mind you tallking to the Garmin Rep recently, they have access to Navteq maps, but they very rarely use them because of the amount
of complaints they get in regional areas. Seems Navteq are great overseas and in the Aussie cities, but get into the Regional areas and they are pretty average.

My Favorite map problem I keep striking, is of intersections and streets that don't exist because they are new, so using "Report Map error" I submit the details
to TomTom and sure enough within a week or 2 they appear on the map, one intersection that is near me with new traffic lights and access roads I have reported
6 times and it is still not listed, So I asked the Rep to check up on what was the issue with that Map error, about 2 weeks later she rings me and passes on what
the drama is, According to both Mainroads, and the Cairns city Council, there are no traffic lights there, and no road extension there, I am wrong. WTF?!?!?
So they have to follow the rules they have in place, if a map error is reported and can not be confirmed by Main roads or the local council, they are not allowed
to add it to the maps, even though they have confirmed the lights and intersection physically are there, because Government says it's not, they can't show it.