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Error in UTM/ED50 coordinate system

 
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nicolase



Joined: 21 Oct 2014
Posts: 3
Location: PYRENEES - FRANCE

PostPosted: Tue Oct 21, 2014 7:48 pm    Post subject: Error in UTM/ED50 coordinate system Reply with quote

Hello,
I use Trekbuddy V1.26 with a samsung B2710 phone to go hiking in mountain on french-spain boundary (Pyrenees moutains).

Two close countries, France and Spain, but two UTM systems used on the maps : WGS84 for french maps and ED50 for spanish maps.

That situation requires to change UTM system on trekbuddy as soon as I pass the boundary line for a correct use of the two UTM maps systems.

The problem is if I set trekbuddy in UTM/ED50 system, then the UTM coordinates point returned by trekbuddy is still in UTM/WGS84 system.

Thanks for this precious work.

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nicoe
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Ranger



Joined: 12 Dec 2006
Posts: 6030

PostPosted: Wed Oct 22, 2014 6:26 pm    Post subject: Reply with quote

First of all - it is not app bug Smile
WGS84 and ED50 are slight different systems, with distance difference up to 100 m.
I don't know how maps you use are calibrated, but if you would use ED50 grid, maps should be calibrated in ED50 for TB.
Country versions can be different a bit, fortunately needed ellipsoids, International 1924 or Hayford 1909, are available in TrekBuddy, see description:
http://www.trekbuddy.net/wiki/index.php?title=Customization#Map_Datums
ED50(Spain and Portugal) is already definied in TB, for France you can use custom definition in datums.txt file, ie
ED50(France){Hayford 1909,-87,-96,-120}=map:European 1950 (Mean France)
TB uses coordinates system based on loaded map, means that it doesn't discover country borders, but loaded maps.
Hope it helps.
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nicolase



Joined: 21 Oct 2014
Posts: 3
Location: PYRENEES - FRANCE

PostPosted: Thu Oct 23, 2014 11:28 am    Post subject: Reply with quote

Hello, and thanks for your answer.
I add a precision : I don't use trekbuddy with atlas or calibrated map, but only to obtain my current UTM position, my focus is just to know where I am and be able to situate that information on a PAPER map.

In my case : For example, I am physically on a point named "Puerto de Bielsa", exactly on french-spanish boundary.
The coordinates of this point are : Latitude 42deg 42'03 N, Longitude 0deg 36'12 E.

I set Trekbuddy in UTM/ED50, then the point returned by Trekbuddy is : 31T / 303674 E /4730406 N.
If I try to situate this point manualy on the Spanish PAPER map (PAPER map in UTM/ED50 system), I find a point [u]which is not [/u]the point that where I am (it is about 240m at SW of my real position).
Then if I try to use this UTM/ED50 coordinates given by Trekbuddy directly on the french map (PAPER map in UTM/WGS89 system), I found a point [u]which is my real position[/u].

My conclusion is that Trekbuddy gives a good position in WGS84 system for a point. But if I set Trekbuddy in ED50 system, the point returned by trekbuddy is still in WGS84 system, the coordinates stays the same. May be there is a problem in the conversion to ED50 (in this case there is no conversion because coordinates are same), and why not, a bug.

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nicoe
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Ranger



Joined: 12 Dec 2006
Posts: 6030

PostPosted: Thu Oct 23, 2014 12:17 pm    Post subject: Reply with quote

@nicolase
You are just using build-in world map in app ? It is WGS84/UTM map.
How you set ED50/UTM in TB ? It isn't possible without map ...
BTW, as I remember ED50 Spain uses International 1924 and Gauss-Kruger grid, means TM type. Or I'm wrong ? Smile
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kruch
Site Admin


Joined: 02 Jul 2006
Posts: 5675

PostPosted: Thu Oct 23, 2014 12:23 pm    Post subject: Reply with quote

I guess nicolase may have set Datum: ED50 and Coordinates: UTM in Settings->Basic... ?

It does not work. Actually I just found that Datum option has no effect in TB whatsoever Confused I could just remove the option.

If there is any way TB can help to solve this use case, I will be happy to implement it.
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Ranger



Joined: 12 Dec 2006
Posts: 6030

PostPosted: Thu Oct 23, 2014 12:53 pm    Post subject: Reply with quote

Default datum setting just can give datum shift between WGS84 and others, but nothing more. Can't solve ie maps grid definition problems, or automatic changing of map systems on borders, like in this case was expected.
This option just helps to list info about built-in and added by user datums (are visible in app as recognized), but still correctly calibrated map in defined system should be used in TB. Even empty Smile
As I remember datum shifts are not the same for ED50/GK country variants & ED50/UTM, so it is hard to be sure without map checking.
To be exact:
ED 50{International 1924,-87,-98,-121}=map:European 1950 (for UTM)
ED 50(Spain and Portugal){International 1924,-84,-107,-120}=map:European 1950 (Spain and Portugal) (implemented in TB)
So nicolase used what is available in app, but ...
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nicolase



Joined: 21 Oct 2014
Posts: 3
Location: PYRENEES - FRANCE

PostPosted: Fri Oct 24, 2014 10:19 am    Post subject: Reply with quote

Hello,
(...) nicolase may have set Datum: ED50 and Coordinates: UTM in Settings->Basic : Yes that's right.

May be I will try to implement it in a CMS to display the two UTM coodinates.
Thanks for your answer.

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nicoe
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Ranger



Joined: 12 Dec 2006
Posts: 6030

PostPosted: Fri Oct 24, 2014 11:03 am    Post subject: Reply with quote

Good idea, to see two systems coords Smile
Second possible solution - if you can find ie small region map in ED50/UTM as picture and use it like ie TB's world build-in, it will be possible to change used system by loading such map. Adapted map should be exactly in the same system as paper ones you use.
BTW, do you have detailed system description in paper map legend ? What kind of grid is visible ?
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