Map registration issue

Submitted: Sunday, Oct 05, 2014 at 08:10
ThreadID: 109717 Views:1022 Replies:3 FollowUps:6
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I use an android version of OziExplore CE with OzTopo 200 map and have noticed a funny registration issue.

When I travel down most roads the moving map tracks precisely down the road however if there is a railway next to the road. it always tracks down the railway. If the road moves away from the railway the tracking moves to the road but returns to the railway if the road goes back next to the railway.

What is going on here????????

It is not a problem as the map still gives me situation awareness but rather an interesting observation.

Alan
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Reply By: Member - Boobook - Sunday, Oct 05, 2014 at 08:27

Sunday, Oct 05, 2014 at 08:27
Alan can you give an example of a location or two where it happens, and do you have any other maps where it does or does not happen.

Oziexplorer maps are called raster maps. Ozi isn't aware of any features on the map like roads or railways. It just calculates your GPS derived position onto a calculated grid on the map and sticks you at that location. So it is strange that it relates to the proximity of railways.

AnswerID: 539937

Follow Up By: Member - Alan H (QLD) - Sunday, Oct 05, 2014 at 08:36

Sunday, Oct 05, 2014 at 08:36
An example was coming home through the granite belt (Stanthorpe) where the railway has a common fence with the road.

I have noticed this before in other locations.

It appears that OzTopo map gets the gps data but tries to find a feature at that location and the railway seems to have priority over roads if the two are in the "same" location.

Alan
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Reply By: WBS - Sunday, Oct 05, 2014 at 09:06

Sunday, Oct 05, 2014 at 09:06
This phenomenon is called feature displacement. Simply put, during the map production process, owing to the scale of the map there isn't enough room to show the railway line and the road in their true position due to the thickness of the lines and the symbols used for each feature, so one has to be displaced (off set). If this was not done the road would be on top of the railway line and this would be very messy looking and unclear to read. There is a priority for displacing features and in the case of railway lines and roads, the railway line has precedence so it is depicted as close to its true position as possible while the road is displaced.

Hypothetically, the cross tie on the railway symbol on the map might be perhaps 1 mm long. This equates to 200 metres on the ground if the map scale is 1:200,000. The road, depending on its classification might be from .50mm to 1.0 mm thick so again on the ground this measures between 100 and 200 metres wides. If in real life the railway and the road are separated by say 100 metres they would overlap.

The maps used in OziExplorer are image files, the data is not dynamic as say in a Tom Tom which uses vector data, so the features on the map are static. They don't move apart as you zoom in or close up as you zoom out. They do in vector data GPS such as TomTom and Garmin.

WBS
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Follow Up By: Member - Alan H (QLD) - Sunday, Oct 05, 2014 at 09:30

Sunday, Oct 05, 2014 at 09:30
Thanks WBS

What you say makes perfect sense to me. I should have thought about the scale of features etc.

Thanks again for explaining this observation. As stated, I do not find it a hindrance in anyway as I still have situational awareness.

Alan
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Follow Up By: Member - Boobook - Sunday, Oct 05, 2014 at 09:40

Sunday, Oct 05, 2014 at 09:40
Nice explanation and information WBS, and looking around Stanthorpe on my maps, that is exactly what has happened.

The road on the topo 250 maps, EO 200 Topo and Hema all appear to be about 350m to the wes from the true position. The NSW 25k which is a finer detail map is spot on.

I may be wrong WBS but I think railways are a higher priority as you highlighted because they are used for aircraft navigation.
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Follow Up By: Member - Boobook - Sunday, Oct 05, 2014 at 09:43

Sunday, Oct 05, 2014 at 09:43
Errrgh no edit...

I should add that the NSW 25k map still covers this area in Qld as it is near the border.
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Follow Up By: Allan B (Member, SunCoast) - Sunday, Oct 05, 2014 at 12:13

Sunday, Oct 05, 2014 at 12:13
I think that explanation from WBS must be about the clearest, on any subject, that I have seen on this forum.
Thanks mate.
Cheers
Allan

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Follow Up By: Member - Frank P (NSW) - Sunday, Oct 05, 2014 at 12:50

Sunday, Oct 05, 2014 at 12:50
x1 on Allan B's comment.

Thanks WBS
FrankP

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Reply By: TomH - Sunday, Oct 05, 2014 at 14:58

Sunday, Oct 05, 2014 at 14:58
In my travels when we were going past Port Stephens the road showed as being about 100m to the right which would have put us driving down a salt flat. Was like that for quite a long way.

Was using a Tomtom Go730.


Dont know why it was like that when generally it was spot on.

Any ideas
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