Mapsource and shonky maps

[under10kg]

Struggling to get myself up to date on GPS!!!

I did load shonky maps onto Mapsource and Base camp and downloaded some tracks from http://www.wikiloc.com
The tracks are out by about 100 meters compared to actual features on the shonky maps.
Does anyone have any ideas to fix this?
Do I need topo maps?

I also had issues downloading tracks from http://www.everytrail.com to mapsource.

[photohiker]

Is your location accurate on the GPS in Shonkymaps?

What datum do you have set?

Here's something to test. I have a local GPX Track available here: Yurrebilla2-3

Loaded up on Basecamp and using Shonky no contours, it should look like this:

yurrebilla2-3.jpg (44.45 KiB) Viewed 17540 times

[photohiker]

Also, bear in mind that Shonky is based on Geoscience Aust. 250k geodata. Don't expect metre accuracy out of it, but 100m seems too far off. It's generally pretty accurate, but every now and then I find the GPS thinks I'm walking in a paddock instead of along a road.

[under10kg]

Here's something to test. I have a local GPX Track available here: Yurrebilla2-3

Opened up and this track is fine.
Not sure why downloaded tracks from other sites are funny.

Why do I need to set datum if I download gpx tracks?
I understand I need to set this correctlly on my GPS unit but not on mapsourse?

[tastrax]

Is it consistently out (like a parallel path adjacent to where you think you should be) or just out occasionally (crosses to the left and then the right of a road)? This will gives us idea of the overall errors.

[phan_TOM]

I've been using shonky too which is okay but a bit vague as photohiker says but combine it with contours Australia 5m and its a great combination and unbeatable when backed up with a topo. All for the princely sum of $10 to get the CD sent to you (or even free if you have fast downloads?).

heaps of info out there regarding datums and how they affect your position accuracy. Heres a thread which might help http://bushwalk.com/forum/viewtopic.php?f=5&t=2660

basically the current datum in Aust. is GDA94, if your gps doesn't have that listed then you should use WGS84 which is only a few centimetres different (and your gps will be around 5m accuracy at best so it won't matter). If you use GDA66 it will be about 200m difference but it doesn't sound like thats your problem.

You should set your datum to the map you're using though, its written on the map so if you're using an old map, first editions etc it may be GDA66.

[tastrax]

Import the attached file of the Overland track into Basecamp (File, import) and you will see the general error rate for the Shonky maps.

Its a KMZ file that can also be used in Google Earth

[under10kg]

Lets try this a different way.
Can someone download this track

http://www.wikiloc.com/wikiloc/view.do?id=536622

I opened it with Mapsource and the track was about 100 meters out with shonky maps full topo.
I can see the start and end of the track do not line up with the road.

[photohiker]

Can you post the gpx here? You have to join that site to download...

[under10kg]

How do I attach a file to a post?

[photohiker]

Never mind, I've just checked, and "The extension gpx is not allowed."

I'll send you a pm with my email.

[Son of a Beach]

I've just added "gpx" to the list of allowed extensions (in the "Navigation" category). Let me know if it doesn't work again.

[Azza]

Opened up and this track is fine.
Not sure why downloaded tracks from other sites are funny.

Why do I need to set datum if I download gpx tracks?
I understand I need to set this correctlly on my GPS unit but not on mapsourse?

The datum matters for everything... Just looking at a couple of GPX files I have there is no datum information set in the file.
So they'll just appear in whatever projection system your using. If its wrong then your points will out.
Most things use WGS84 or GDA94 - as mentioned they are the same thing (a few cm's different).
I would have thought that if you set everything up for WGS84 / GDA94 then you should be okay.

It's always a bit tricky, I've written some software to manipulate maps and there is no end of frustration dealing with different coordinate systems and transformations.

[photohiker]

under10kgs sent me the GPX, here is the result on Shonky on my system:

Is that different to your result under10kgs?

[photohiker]

I've just added "gpx" to the list of allowed extensions (in the "Navigation" category). Let me know if it doesn't work again.

Thanks Nik, I've added my Yurrebilla track to this post, and it seems to work.

[under10kg]

Is that different to your result under10kgs?

Yep that is the same.
You can see the start and end of the track are not correct?

[tastrax]

Looks pretty good at my end - try the same file in Google Earth and you will see that its not too bad. Its all about scale - shonky is from 1:250,000 base data so it wont be perfect. It may also be the settings that you have in your gps for the rate of data collection (once per second or every 20 metres or automatic etc). At each step in the process there can be errors or anomalies .... including poor gps reception in gullies etc

[photohiker]

Is that different to your result under10kgs?

Yep that is the same.
You can see the start and end of the track are not correct?

Yes. Could be the GPS that created the track was wonky, or perhaps shonky is not very accurate in that area.

The last test is for you to go walk the track, recording it on your GPS. Once recovered, load it up on your computer and see what the difference is.

According to the schema, the GPX datum should be WGS84:

GPX schema version 1.1 - For more information on GPX and this schema, visit http://www.topografix.com/gpx.asp GPX uses the following conventions: all coordinates are relative to the WGS84 datum. All measurements are in metric units.

Software should therefore respect that, but the only way we have of checking is to compare.

[under10kg]

I friend looked at that laminton track with topo australia v2 maps using BaseCamp and the track was perfect.
It was not so good with shonky as illustrated above so this proved that the shonky map is a bit out at this location.
Looks like I need topo maps!!