Navionics App.

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Ian_Rob

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I downloaded the Navionics App on Friday - basically because I forget to take my Navionics card for the chart-plotter with me for the planned weekend sail. Very impressed but why does the triangular boat symbol keep changing orientation? It seems spot on position-wise but not heading-wise.
 
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Just taking a wild guess but perhaps you have to be moving in a direction for it to determine heading.. If you are more or less stopped it might get confused as to exactly which "direction" you are going based on the GPS signals it's receiving..

Were you stopped or moving at the time you noticed it changing orientation?
 
Not noticed that before.

I've used it for a couple of weeks-worth of sailing recently and it has always behaved for me. Do you enable Wi-Fi on your device? I get a warning when Wi-Fi is turned off as it reduces accuracy. I wonder if it also affects heading calculations?

just a thought.......

Andy
 
wipe_out said:
Were you stopped or moving at the time you noticed it changing orientation?
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I first noticed it when I was stopped and thought that was the explanation but whilst it was certainly better when moving the orientation still periodically flicked. I think (but would like to confirm this) that the heading line always disappeared when it flicked. I was running without the wireless but there wouldn't have been any connection anyway in the middle of Poole Bay?
 
It is the gps from the attached device. It depends on how good it is and how often it updates. If it is a bit poor then the faster you go the better the direction of the arrow. When you are going slow small anomalies will update the arrow and make it flick from direction to direction. It will still be right in general on location but will be wrong on current cog and sog but right on average sog
 
I took this up with some similar issues direct with Navionics. The app doesn't really function well at very slow speeds, below 3 knots the GPS chip only gives intermittent data to the app and so you get some anomalies like the moving heading. The app doesn't update position very well at all below 2 knots though because you're travelling slowly it's not such an issue. It causes a few problems when we're nosing up rivers.
 
Sounds as though it is the app rather than an issue with the IPad. I don't think it is significant issue in practice but something to be aware of. Thanks for the responses.
 
I dont know about Navionics however some GPS apps will show the heading determined by the heading sensor, if the device has one. Otherwise, it will show heading derived from subsequent GPS positions which may, as above, give random readings when standing still or moving slowly. If set by heading sensor, the orientation of the device will influence it.
 
It will probably be down to gps update rate or accuracy. I use mine a lot offshore with no phone signal (or wifi) and get good accuracy and heading all the time. If gps is working I cant see how wifi can help accuracy, as a wifi zone can be 30m radius and gps normally gives 4-5m.
 
QUOTE=Neil_Y;4265673]It will probably be down to gps update rate or accuracy. I use mine a lot offshore with no phone signal (or wifi) and get good accuracy and heading all the time. If gps is working I cant see how wifi can help accuracy, as a wifi zone can be 30m radius and gps normally gives 4-5m.[/QUOTE]

Neil,

In short, the GPS system in mobile devices works independently of Wi-Fi & 3G, but they use Wi-Fi and 3G to improve the speed and confidence of locating services when in range. As you say - offshore it does not change anything.

I found this on the Apple support site...

"Depending on your device and available services, Location Services uses a combination of cellular, Wi-Fi, and GPS to determine your location. If you're not within a clear line of sight to GPS satellites, your device can determine your location using Wi-Fi3. If you're not in range of any Wi-Fi, your device can determine your location using cellular towers." (http://support.apple.com/kb/HT1975)

Apple forums discuss Wi-Fi as being used to help the device get a quicker fix, assisting better GPS initial location. This is the same with 3G by using towers to identify rough location more quickly than with GPS alone. Clearly if one is outside 3G and Wi-Fi range then it is down to GPS only.

Another report I found (albeit from 2009) debated the accuracy of an iPhone GPS receiver versus a purpose built GPS system. It talked of accuracy of tens of meters (~30-50m) versus single figures (~8m) for purpose built GPS devices (notwithstanding general concerns over civilian GPS accuracy). Not surprising really although this has probably improved in recent years.

Andy
 
With Apple's IOS Core Location package, the GPS functionality is fully programmable. So it may poll for location (using any or all methods discussed above) at set intervals. The class also returns the level of accuracy so the app can decide how to handle - for example on the standard maps you will get a blue circle of uncertainty. I don't believe the Navionics app shows the accuracy but could be wrong.

When I have used Navionics on the iPad, slow speeds (couple of knots) is not good for showing direction. This could be because the app is polling very frequently and it seems like nothing has changed between two reading. Certainly at five knots it is much more stable. I've written a couple of apps that had that issue and I had to damp the polling times at slow speed to make the screen look stable.

So... I don't think you have a problem compared to anyone else.
 
3G and wifi give speed of fix but can't help accuracy (as they have a range of more than 10m you can be anywhere in that 20m+ dia circle) was what I was trying to say, GPS will take longer to fix as it needs more data, but once a fix has been achieved it should be in the region of 5-10m accuracy. Assuming you get 5 or 6 satelites.
 
Neil_Y said:
3G and wifi give speed of fix but can't help accuracy (as they have a range of more than 10m you can be anywhere in that 20m+ dia circle) was what I was trying to say, GPS will take longer to fix as it needs more data, but once a fix has been achieved it should be in the region of 5-10m accuracy. Assuming you get 5 or 6 satelites.
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It appears that we vehemently agree ;)

Andy
 
Bear in mind that GPS is measuring course over ground, so you need a sufficient speed over the ground in order for it to calculate your heading.

You might be making knots into the tide but barely moving over the ground and in that scenario, it's very easy for the GPS to erroneously calculate heading - especially if running it on a hand-held which you're actually holding - you can often move about in the boat quicker than the boat is moving in relation to the ground.
 
scotts_mist said:
Bear in mind that GPS is measuring course over ground, so you need a sufficient speed over the ground in order for it to calculate your heading.
Click to expand...

This is if the heading is being determined by GPS. I don't know what Navionics does, but the last few versions of iPhone have included a magnetic compass sensor so it's possible that it reads heading (of the phone if not the boat!) directly.

Pete
 
prv said:
This is if the heading is being determined by GPS.
Pete
Click to expand...

I run Navionics on a Samsung tab, and it definitely just seems to use the GPS on that rather than the compass or accelerometer. Don't believe Navionics can take an external feed on the app version.

Besides, the built in device compass on a mobile device wouldn't tell you which way you were heading, just which way the device was pointing?
 
scotts_mist said:
Besides, the built in device compass on a mobile device wouldn't tell you which way you were heading, just which way the device was pointing?
Click to expand...

Yep, presumably why they use the COG. Though if I was writing it, I think I'd fall back to compass if speed was too low to give COG.

Pete
 
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