On a recent trip to the Whitsundays to aid some students translate topography from the real world to the chart, I took some pics of some some obvious topographical features dolphin point, moss trooper peak etc and also backed them up with screen shots of the iPhone compass with gps coordinates. When these bearings and coordinates were plotted errors were obvious, several degrees and hundreds of meters. upon investigations the phone only uses minimal satellite input and relies on wifi tower triangulations. So no reception = dodgey positioning. The compass itself can be switched to true, which relies on the position fixing but true can be off and the compass works like a magnetic compass. the warning should be obvious to any navigators using phone / tablet built in gps

I tried to do some investigating into this a while ago. Android is better for GPS when out of reception I think. It really depends on the phone and the chip. Same as tablets.
I couldn't really figure out apple. It was a while ago. They use other phones, wifi towers etc to have really accurate GPS when in reception. Not sure about when out of reception.
Couldn't really find any info on GPS hertz on either.
I have checked my phone against the plotter on numerous occasions when out of reception and it has always been pretty accurate. Not something I would like to have to rely on though.

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tarquin1 said..
I tried to do some investigating into this a while ago. Android is better for GPS when out of reception I think. It really depends on the phone and the chip. Same as tablets.
I couldn't really figure out apple. It was a while ago. They use other phones, wifi towers etc to have really accurate GPS when in reception. Not sure about when out of reception.
Couldn't really find any info on GPS hertz on either.
I have checked my phone against the plotter on numerous occasions when out of reception and it has always been pretty accurate. Not something I would like to have to rely on though.

On my ancient non wifi iPad I use a bad elf gps plug in, it's very accurate. I never gave any thought to the apple gps capabilities, or lack of hence my post

I used Navionics for personal use on my iPhone when sailing from Port Phillip to Hobart twice. The boats chart plotter was a newish 15 inch touchscreen model. I have always fount it accurate even when out of range for phone coverage. In Port Phillip i have come across two issues. One was going from Sorrento to Port Arlington in foggy 4am dark conditions, I charted a course west of Chinamans Hat by about 50 yards. There used to be a tide gauge pile about 50 yards east of Chinamans Hat. As it happened Chinamans hat loomed out of the darkness straight ahead and if I wasn't keeping a keen eye out I would have hit it for sure. Turned out Navionics marked the Tide Gauge pile as Chinamans Hat!! .....and left the real Chinamans Hat un marked.
The other is a dog leg in the shipping Channel east of South Cannel Fort. In the middle of the dog leg there a Channel pile
that was unmarked on Navionics resulting in an unmarked pile placed about 50 yards inside the northern line of an otherwise relatively straight channel. That was a year or so ago and might have been corrected.

Guitz interesting you found the iPhone accurate across bass strait, what model iPhone ? I would have thought navionics was better than that in a well used area like port Phillip

Well the Port Phillip info is more detailed as I'm regularly getting about the bay from Blairgowrie to Docklands, Port Arlington, Brighton, Mt Martha , Queenscliff etc etc and I always have Navionics on my iPhone 8 and the Seiwa Marlin plotter with Cmaps. Consequently I'd pick up more detailed info about Port Phillip. Re Bass Strait I had it on regularly but not all the time and it checked out as accurate when I made observations as we passed various identifiable marks. Phone resorption was ok for most of the journey but patchy in certain spots. Deal Is reception was only at what they have called Telstra Corner. They have put a bench seat halfway up the path to the Caretakers residence where you get a signal from Wilson's Prom. The skipper had a newer model iPhone and always had reception which I found interesting.

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Guitz said..
Deal Is reception was only at what they have called Telstra Corner. They have put a bench seat halfway up the path to the Caretakers residence where you get a signal from Wilson's Prom.

Sorry to go off-topic, but there is a second mobile phone spot on Deal that gives better reception than the one you mentioned. It's roughly half-way along the track to Garden Cove and has a clear view to the mainland between Erith and Dover Islands. There's a bench to sit on while you enjoy the view as you chat or download.
Now back to OP's topic.......

Thanks for the reply's, I'm must say I'm a little gobsmacked that there's phone reception across bass strait, I can assure you there's zero on the eastern side of the Whitsundays and for that matter good parts of the NSW coast that's how I come to discover iPhone gps isn't really gps. Btw I was using iPhone SE

Yes, reception in Bass Strait with a decent phone and antenna is surprisingly good, and a major safety benefit.

Phone reception on Flinders Island in Bass Strait is great- but ONLY on Telstra network
Cheers

I did the walk to Garden Cove but my phone was in my pocket. Good to know because I will be back there end of January next year. Such a lovely walk and the water cress around the little creek was planted there by Mathew Flinders! The vegetation, tussoc grass and sheoak, give an insight into what Much of the western end of Mornington Peninsula looked like before the sheoak was cut down for the lime kilns and charcoal etc etc. There's a lot or remnant tussoc grass along the national park walk from Cape Shank to Ppint Nepean that is identical to Deal Island but the sheoak are gone except for the odd one.

Question - if I have a device on my boat such as a gps radio then I have nmea to wifi. Would navionics on my phone use the gps position from the phone or the nmea via wifi?

I'm no tec geek, far far from it, but I would hazard a guess that if you where to turn off the phone location then blue tooth or wifi the phone/ tablet to the gps radio then the phone would use the external gps. A gps radio ? A combo of a hand held gps & radio ? VHF or uhf ? If vhf it could be a rather useful device especially if it could also act as an AIS receiver