Can I trust my iPad as a chart plotter?

Emjaytoo

Emjaytoo

Well-Known Member
Joined
23 Jan 2005
Messages
808
Location
Us: Kent; Emjaytoo: Holland; Kate: Conyer Creek
www.emjaytoo.net
Last week, whilst cruising the inland waterways of the Netherlands, we ventured in to a narrow and shallow channel. The channel entrance was marked and once in, it was eyeball navigation.

TheOmloop.jpg


The Garmin chart plotter accurately showed the channel, the markers and our track.

TheOmloopGarmin.jpg


The iPad (with Navionics) - well I don't know what that was doing!!:eek:

TheOmloopiPad.jpg


The channel markers are all over the place and our track, well say no more!

If the channel had been through rocks awash in poor visibility and we were relying on the iPad we would now have a wrecked boat!
 
When my iPad nods off, the track gets picked up from where it wakes up which might explain some of the positions but otherwise can not help. Not had this happen with mine, ever.

At very slow speeds, can get some oddities but .... I've used another app "GPS Status" to double check Navionics simply because I don't entirely trust it and, like you, have a Garmin.
 
My android tablet has always been absolutely spot on for navigation and it has never given me a track like you show above, I can see from the picture that you didnt have it on charge, does the ipad gps go to sleep or run in a low power state when its on battery?, that could account for the inaccuracies shown
 
It's not unusual for maps, both paper and electronic to be a long way off. The navionics maps on the ipad don't seem any better or worse in this regard than any other.

There is a lot of benefit in having a couple of different mapping systems running. Even better is to put a radar overlay on top of the map this imediatly shows any discrepancy, but this is not possible with the iPad.

I have a suspicion the iPad internal GPS is a bit worse than some of the others, but most users feel its on par. People do seem generally happy to accept poor gps position as "good enough" for some weird reason.

I understand the iPad is one of the few GPS units that will work with the alternative satellite navigation systems that are coming on line.
 
I was using an android phone and an iPad up the west coast of Scotland last month, both with Navionics. Going into Tinkers Hole (where I'd never been before) both gave me very accurate positions to within feet. What I noticed about the iPad is that the gps is much slower to get a fix when started up. Apart from that, no real differences in accuracy.
 
marklucas said:
You did check the datum of the charts and the iPad - didn't you?
Click to expand...
If there had been a datum error, the track would be displaced a constant XY distance from the actual. I have had an error in a map (of Fanny Bay Donegal) but the chart itself said there were GPS errors. Never happened again.
 
I am one of the generation that started sailing using mostly chartplotters.

I have got used to depending on them and no longer work out GPS waypoints for each course change when using inshore passages!

Lesson 1 - was about 5yrs ago when I found the navionics chart for River pontrieux showed me sailing 500m inside the granite shore most of the way up to Pontrieux! Softway for Navionics does contain some errors IMHO (possibly more on rivers than shoreline). I reported it to Navionics

Now always give a wider berth to isolated rocks etc when approachiong Guernsey and Aldernay etc. Chartplotters are not always correct.

Lesson 2 - when doing inshore passage to Guersey this year it was v dark (no Moon) windy, raining and SWMBO was below I realised that I was completely dependant on the boat chartplotter to identify my turning points. It was too dark to make out exact details but could see some darker masses in various places.

Lesson 3 - sailing back from cherbourg last weekend my chartplotter started flickering, quick check revealed boat voltage had dropped to 9V. started engine then all OK. Bad to just rely on just one chartplotter they can fail.

After Lesson 2 I investigated an i Pad as backup. For others info you need to buy the one with 3G as this is the one with enhanced GPS capability. Then buy Navionics package. I have no SIM card in my i pad and it works well. I left on plastic sleeve it was sold in to give it better splash proof protection in addition to i pad case. If just for backup same package on i phone works but screen is small.

If you want accurate info on French coast unlike Reeds Almanac (chartlets not to be used for navigation!) the French Block Marine chart pages are completely accurate - covers the whole of the French Atlantic/channel coastline.
 
Last edited:
In actual fact, the errors in the track are of the order of 150ft or so, which is better than my astronav.
It's not that uncommon for gps to be this far out. Sort of the top end of the range of expected errors. TomToms can be this far out but make an intelligent guess that you are on the nearest road.
An internal gps aerial operating inside a case won't help, the trees won't help.
If the gps is losing track, it may make assumptions that you hold course and speed, with variable results.

More concerning is the way the channel markers are not in the right places on the map, it looks like the app is not working properly or has some errors in the data.

It's not great, but any plotter can do this to you.
Plotters are sometimes too good and people believe them to the nth degree, even in the face of contradictory info from depth sounders, channel marks etc.
 
Sailfree said:
I am one of the generation that started sailing using mostly chartplotters.
Click to expand...

I am one who began with paper and a walker's log. I have just returned to cruising and visiting places I last saw 20 years ago. When I think back to how I used to happily navigate in bad visibility it now gives me the shudders ...
 
lw395 said:
More concerning is the way the channel markers are not in the right places on the map, it looks like the app is not working properly or has some errors in the data.
Click to expand...
I got free charts from Imray after pointing out a similar error in Brittany on their paper charts. Somebody has pinned a photo to the Navionics chart which shows the island forested and whoever did so, couldn't have failed to point the error out to Navionics .... I hope ...
 
causeway said:
Wonky barometer in the ipad? I know a lot of these new units have a barometer to increase accuracy.
Click to expand...

Say what !!!!!

No gps system 'uses a barometer' to increase accuracy. GPS may defer to barometric pressure to measure altitude because GPS is so bad at fixing the z dimension ( geometry issues ) but as to increasing the accuracy of a fix. Sorry not on!
 
lw395 said:
In actual fact, the errors in the track are of the order of 150ft or so, which is better than my astronav.
It's not that uncommon for gps to be this far out. Sort of the top end of the range of expected errors. TomToms can be this far out but make an intelligent guess that you are on the nearest road.
An internal gps aerial operating inside a case won't help, the trees won't help.
If the gps is losing track, it may make assumptions that you hold course and speed, with variable results.

More concerning is the way the channel markers are not in the right places on the map, it looks like the app is not working properly or has some errors in the data.

It's not great, but any plotter can do this to you.
Plotters are sometimes too good and people believe them to the nth degree, even in the face of contradictory info from depth sounders, channel marks etc.
Click to expand...

hi,
I just checked my ipad for the same spot and it appears to be accurate with the navigation marks in the right place

Regards

Rab
 
You must log in or register to reply here.

Share this page

Top