Why does my GPS do this?

General cycling advice ( NOT technical ! )
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Godlykepower
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Why does my GPS do this?

Postby Godlykepower » 6 Mar 2014, 2:54pm

I have recently purchased a Garmin Edge Touring and have only used it a few times.

I plotted a route on Garmin Connect today and I noticed when I started my ride, that it plots my position just to the left of the route on the map.

It's okay getting me out of Royston on small roads, but when I hit a main road, the planned route which is marked in pink is visible, but my position is displayed slightly to the left of this. You can see the blue route mark of where I have just travelled and it kind of zigzags slightly back and onto the pink to say that I did just travel on the highlighted pink route.

I'm not sure if I'm explaining this very clearly? It works okay on every other road type (as far as I noticed) but I expect there may be some simple explanation/setting as to why it's doing this otherwise. It's a bit annoying when you are clearly going down the A10 which is the only road there and it says "travelling on track."
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LondonBikeCommuter
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Re: Why does my GPS do this?

Postby LondonBikeCommuter » 6 Mar 2014, 5:21pm

I think I know what you mean as my Edge 810 did it as well. I believe its a settings thing but I'm blowed if I can remember what it was.

Gearoidmuar
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Re: Why does my GPS do this?

Postby Gearoidmuar » 6 Mar 2014, 6:25pm

I think I know what you're talking about...

You can get this kind of cutting corners appearance due to the route being reduced to corners only in areas, as guiding you totally on the only road there is is not useful. To have a point exactly on every bit of a winding road is too many points for the device. It only shows you the relevant turns..

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Godlykepower
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Re: Why does my GPS do this?

Postby Godlykepower » 6 Mar 2014, 6:57pm

It doesn't happen where there are turns, it does it on a dead straight road!

If I'm riding on the road that the unit wants me to go on (highlighted in pink) whilst I am doing so, the unit will say my position is travelling parallel to this road, even if no such road exists. Garmin units superimpose a blue line over the pink, when you have covered the ground of a route.

What is happening to mine, is that it will slightly zigzag the blue line backwards onto the pink, but my position will remain just to the left.
I don't have a gambling problem. I'm winning, and winning is not a problem for me. That's like saying AC/DC have an awesomeness problem.

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Mick F
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Re: Why does my GPS do this?

Postby Mick F » 6 Mar 2014, 8:02pm

Are you following a track created from a third party, or is the track created within the unit?

I ask because I would often create a track with BikeHikeUK and my 705 would beep and tell me I was off the course every now and then. It wasn't far, and you could see where I was and where the pink line was and you could see the separation.
BikeHike's map was inaccurate? - or was it Garmin's internal map in my 705?
I never did find out which, and there were many instances of inaccuracies.

I now use a Garmin Montana, and it doesn't complain even in the places the 705 used to beep, and now the Montana's pink line is in the "right" place.

Dunno for certain why there's a difference .......... except I reckon it was the mapping system in the 705.
Mick F. Cornwall

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Godlykepower
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Re: Why does my GPS do this?

Postby Godlykepower » 6 Mar 2014, 8:15pm

I created the course in Garmin Connect and sent it to the device.
It only ever does it on main roads though. When I got home and was uploading the ride details back to Garmin connect, I noticed that you can select between Google Maps, Bing Maps and Open Street Map. Perhaps this is partly to blame?

I've just been having a fiddle with the unit itself and I can't really see anything that looks like it could cause it.
I've changed it from North Up to Track Up, but otherwise map detail is set to Normal and units are in miles. Can't see anything else worth changing.
I don't have a gambling problem. I'm winning, and winning is not a problem for me. That's like saying AC/DC have an awesomeness problem.

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Mick F
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Re: Why does my GPS do this?

Postby Mick F » 6 Mar 2014, 8:30pm

I would suggest that there are "errors" between the maps.
I have never seen a Garmin Edge Touring, so I cannot comment with any specific knowledge.

If I zoom in on the map on my Montana, there is many an occasion that my position is off the mapped road. This isn't anything to do with a track I may be following on a pink line, it's just an error on the installed map.

The other day, I was riding along an empty :D road and my position was off to the left, so I moved over the the "wrong" side of the road and my GPS position ended up in the centre of the road!

When I'm out next - hopefully Saturday - I'll take a screenshot (yes, Montana's can do this!) and you'll see how the GPS position can be misaligned from the map. I'm sure I can find somewhere.
Mick F. Cornwall

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Mick F
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Re: Why does my GPS do this?

Postby Mick F » 6 Mar 2014, 8:42pm

Just struck me, I can see the track from today to show you what I mean.
174.jpg
Is this the sort of distance you are off?
Mick F. Cornwall

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Godlykepower
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Re: Why does my GPS do this?

Postby Godlykepower » 6 Mar 2014, 8:54pm

Right, Mick, you see in that picture how the pink route is slightly off? Well, my pink route is always on the actual road, but my position will be like the pink is in your pic (except on the left) and the blue line will zigzag away from the left where my current position arrow is, onto the pink as it should.

Perhaps it is just a small mapping error. When I next plot a route, I'll try plotting it on one of the other option I mentioned earlier. I don't trust anything from Bing, so I'll use Google or preferably OSM.
I don't have a gambling problem. I'm winning, and winning is not a problem for me. That's like saying AC/DC have an awesomeness problem.

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Mick F
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Re: Why does my GPS do this?

Postby Mick F » 6 Mar 2014, 8:59pm

That is the actual track I took today soon after 1pm.
GPS accuracy there - as it is open ground away from trees etc - would be 7ft.
I was wet and tired and hungry and only three miles from home. :oops:

I was heading south, so my position is off to the left of the Ordnance Survey map.
Mick F. Cornwall

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MikewsMITH2
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Re: Why does my GPS do this?

Postby MikewsMITH2 » 6 Mar 2014, 9:13pm

if you are downloading OS based mapping it is based on OSGB36 datum whereas GPS systems are based on WGS84. They differ by a few metres or a few 10s of metres depending on where you are in the country. Most GPS units can be set to different datums to accommodate this. I spoke to nice man from Garmin about this years ago and he put me right. I have a 14 year old Garmin, but for all I know modern ones can convert between the two automatically, but I doubt it is spot-on.
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Mick F
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Re: Why does my GPS do this?

Postby Mick F » 6 Mar 2014, 9:24pm

Are you saying the difference is "uniform" one way?
I reckon that mine varies EW and NS and sometimes spot on.

I reckon it's the map that's wrong, as afterall, they were drawn using theodolites and tape measures, plus the roads are shown wider than they actually are.
Mick F. Cornwall

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honesty
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Re: Why does my GPS do this?

Postby honesty » 6 Mar 2014, 9:28pm

I believe that there's a setting like snap to roads or something within the garmin menu. Give that a try.

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MikewsMITH2
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Re: Why does my GPS do this?

Postby MikewsMITH2 » 6 Mar 2014, 9:40pm

No I said it varies depending on where you are in the country. It is not uniform - lat varies different to long. Mapping the curved surface of the earth onto a flat surface is very complicated and there is more than one way of doing it. The OS method was adopted in 1936 and the WGS in 1984, so I assume WGS is more accurate.

I am just trying to make you aware that the GPS unit should be set to the same datum as the map you load onto the unit. I have no doubt there are mapping errors too. For the purposes of navigating a bike it is of little imporatnce, but if you are locating buried treasure then in certainly does :lol:
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Mick F
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Re: Why does my GPS do this?

Postby Mick F » 6 Mar 2014, 9:42pm

Here's some more screenshots from my ride today.
170.jpg
244.jpg
294.jpg
312.jpg
419.jpg
As you can see, sometimes the map is correct.
All shots are at the same scale.

All these shots are within a radius of no more than ten miles from each other.

I still say the maps are "wrong".
Mick F. Cornwall