View Full Version : Calibrate GPS?

16th October 2008, 08:57 AM
Is there a way to calibrate the GPS?

My Google Map says I am 4k's away from where I actually am.
I live on Thursday Island so it has me 4k's out to sea.

I live 200 metres from the hospital, Google Maps says no driving route available, and the "Around Me" app says I am 15k's away?

16th October 2008, 09:30 AM
Is there a way to calibrate the GPS?

My Google Map says I am 4k's away from where I actually am.
I live on Thursday Island so it has me 4k's out to sea.

I live 200 metres from the hospital, Google Maps says no driving route available, and the "Around Me" app says I am 15k's away?

How strange. I'm not sure you can calibrate them but I might be wrong. What you could do is get the GPS coordinates by taking a photo with the location services switched on, then looking at the exif data in preview (and then changing east for west as I think it still reverses them). Then check those coordinates on a non-google map as it could be that your location is being reported correctly by the iPhone but the Google Map is incorrect - which wouldn't surprise me - it's brilliant in the cities but not that accurate out in the country, so perhaps they've got the whole of Thursday Island 4km off...

Have you tried it on the mainland?

16th October 2008, 10:22 AM
Haven't tried it on the mainland yet.

I'm going to Cairns next week so I'll try then.


16th October 2008, 10:35 AM
A GPS doesn't need to be, and can't be, calibrated. If the GPS is operating correctly, the measured co-ordinates will vary with a small (10-15m) error around the true location, but will not be permanently offset. If you have the patience you can sample the GPS co-ordinates over time to get an increasingly accurate location measure. The GPS receiver can calculate the approximate error, and when it's sufficiently small it will report a "fix".

One thing that can trip you up is the way the measured co-ordinates are presented to you - unfortunately the Earth is not a simple shape, and there are a number of models of the Earth used to account for local variations. These models are called 'datums'. Most GPS units out of the box, and Google Earth, use the global WGS84 (http://en.wikipedia.org/wiki/WGS_84) datum. i.e. the GPS co-ords taken directly from a GPS should show the same location on Google Earth (excepting G. Earth errors, as Tom125 noted they can be a bit off, up to around 100m in my experience).

Old (anything not updated since 2000) Australian maps will use the old datum (AGD66/GDA84), which has an offset of around 200m from WGS 84. The new Australian datum (http://en.wikipedia.org/wiki/Geodetic_system#Australian_Geodetic_Datum_1966_.5B AGD66.5D_and_Australian_Geodetic_Datum_1984_.28GDA 84.29), GDA2000, is essentially the same as WGA84. i.e. new maps won't need conversion. I don't believe there is any

Sooo, assuming that you can't change the map datum on the iPhone, and it is actually reporting a fix, the only options left are: the iPhone's faulty, or Google Earth is.

Can you compare the iPhone w/ a dedicated GPS?

PS: in some parts of the world (e.g. North America) the satellite GPS signal can be augmented by a ground-generated signal to help improve accuracy. This still isn't "calibration".

PS2: all of the above refers to a real GPS, not the faux "GPS" of the original iPhone.

16th October 2008, 10:59 AM
I can compare it to 3 other GPS.

I have:
Tom Tom, running on a Palm Treo 750 for turn by turn nav (my old phone),
Magellan Explorist XL for 4WDing, and
Navman GPS/Chart Plotter in my boat.

They all work on the WGS84 system and are spot on.

16th October 2008, 06:57 PM
Possibly the internal clock is not synchronising properly with the incoming signal - this is what gives the GPS receiver its accuracy. Maybe a software glitch - try reloading the firmware. If it is hardware (have you dropped it at any time?), not much you can do


16th October 2008, 07:06 PM
Never dropped, not very old. Clock seems ok, same time as other phones on the same network.

How would I reload the firmware?

Would I lose my data, settings etc?

16th October 2008, 07:56 PM
Try turning off 3G, my GPS is more accurate with it off.

16th October 2008, 08:15 PM
I would do that, but I live in a 3G only area:(

Plus, I shouldn't have to:mad:

But thanks anyway!

25th October 2008, 03:35 AM
Just a pic to show how inaccurate google maps is in my area:thumbdn:

I live on Thursday Island


25th October 2008, 10:09 AM
Is GPS definitely turned on?

Silly question, but if it isn't, it will still work but not accurately. It used the cell towers unless 3G is turned on.

Do you see the blue dot?

25th October 2008, 10:43 AM
I have always found the GPS on the iPhone to be finicky. Usually it will locate me within about a 5km radius at a location about 15 km away.

Here's what I do to get it working.

Turn of WiFi and 3G.

Go outside and stand in a pot where I have a clear view of the sky.

Launch maps.

Wait. It can take time for it to get an accurate GPS lock, the more you wait the more accurate it becomes.

If I get the the cross-hairs and not the dot I hit the home button and re-launch maps.

After i get a GPS lock I go and turn WiFi and 3G.

It seems to be that the iPhone has a problem establishing a GPS lock. Once it has an actual lock it has no problem whatsoever. Even if exit out of maps and re-launch.

From what I understand the iPhone uses assisted GPS. This is where the phone will use the phone towers to get an initial approximate position and then use the GPS to get a proper lock. It seems though that using the phone towers (well with Telstra at least) confuses the phone and throws everything out of whack as the initial location is so far off.

I wish there was a way to turn off AGPS, or even better I wish Telstra would fix the bloody problem with AGPS on their towers.

25th October 2008, 11:34 AM
Is GPS definitely turned on?
Do you mean "location services"? If so, yes it's on.

25th October 2008, 11:35 AM
Turn of WiFi and 3G.
How do I turn 3G off?

I live in an area where I only have 3G so this will not be suitable all the time.

25th October 2008, 11:47 AM
From the manual,

"iPhone uses Location Services to determine your current (approximate) location.
Location Services uses available information from cellular network data, local Wi-Fi
networks (if you have Wi-Fi turned on), and GPS (if you have an iPhone 3G; GPS not
available in all locations). The more accurate the information, the more precisely your
location is indicated. This feature is not available in all areas."

I guess they shouldn't call it GPS, the G stands for Global, which iPhone is not.

"If you have an iPhone 3G, a blue marker indicates your location when GPS is used
to find your location. Otherwise, a circle is used to show your approximate location.
As you move around, iPhone updates your location, adjusting the map so that the
location indicator remains in the center of the screen. If you tap again or drag the
map, iPhone continues to update your location but stops centering it, so the location
information may move off the screen. "

Mine is showing the blue circle so I guess it is using mobile towers.

Given that I live on a remote island I probably only have 1 tower available so triangulation would not be possible.

I have other GPS here which work fine so I don't get why the iPhone doesn't?

Just doesn't make sense to me that a Global system is not available in some areas, I know the satellites are up there:confused:

25th October 2008, 12:28 PM
Turn of WiFi and 3G.
I tried this and it said "your location could not be determined", but I guess this is because 3G is the only network in my area so the phone to go off.

23rd December 2010, 02:14 AM
To af3556,
Actually GPS receivers should be calibrated. For personal commercial use (such as phones) this is not normally done, but for complete accuracy a GPS receiver requires calibration. It has been an arguement for some time if this is neccessary because the GPS signal is an absolute standard in itself. However, any receiver device can be in error or malfunction, then the absolute signal received becomes questionable. Jacke's problem is probably line of sight or number of satelites locked onto but could be a gps receiver problem (the phone) as well. I calibrate GPS receivers in my line work, Metrology. A GPS receiver must receive and display the 10 MHz signal to the correct resolution, if it cannot do that, it is defective or out of calibration. Calibration of phones is not seen as a neccessity because you can find a city or street even with a small amount of error. Calibration becomes critical only for non commercial applications, such as space shuttle launches.