[QGIS-trac] Re: [Quantum GIS] #1433: measurements and scale broken in latlon

[Quantum GIS]

#1433: measurements and scale broken in latlon
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        Reporter:  msieczka                          |         Owner:  timlinux                                      
            Type:  enhancement                       |        Status:  closed                                        
        Priority:  major: does not work as expected  |     Milestone:  Version 1.2.0                                 
       Component:  Projection Support                |       Version:  HEAD                                          
      Resolution:  duplicate                         |      Keywords:  scale, projections, measure line, measure area
Platform_version:                                    |      Platform:  All                                           
        Must_fix:  Yes                               |   Status_info:  0                                             
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Comment (by lutra):

 Replying to [comment:10 homann]:
 > I don't know how many bugs I have seen submitted on this issue.



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 about projections, scale and measures? many. See here

 http://www.qgis.org/wiki/Bugs

 actually the page is not available cause the problems with the qgis/osgeo
 servers.


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 > If OTF is off, the selector sets the unit of the source layer.

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 This seems to me not true. If I open qgis and the load the alaska
 shapefile (from the qgis sample dataset), that has a projection in feet,
 map units do not changes from degrees. If I open a shape that has a
 projection in meters and set map units to meters, then close the project,
 open a new one and open a shape with a projection in feet, map units
 remain in meters, and so on... this can be puzzling for many users.




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 > So, if you change from feet to meter, the reading should be the same.
 >
 > So a)and c) is not a bug.


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 Actually when describing the point A) and C) I wasn't enough detailed. In
 fact the map scale and the measure tools are both right if the project was
 defined with the same unit of the layer projection, otherwise they are
 both wrong (but give same values). So in general it would be useful/enough
 to '''not''' allow selecting the "wrong" map unit in the project
 properties.







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 > In b) and d), changing the map scale with OTFP enabled DOES NOT
 recalculare the resulting areas/distances from meters to feet.
 >
 > Map scale is a plugin, so behaves differently. It might be buggy.


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 Also in the B) and D) cases I could have been more precise (and probably I
 was wrong in at least one case).

 In the D) case, if the map unit is in degrees than the map scale is
 *right* but the measure tool gives wrong values.

 If the map unit is in meters or feet, than the map scale is always wrong
 regardless the unit of the layer projection (meters or feet), and the
 measure tool gives right results '''only in meters''', even if the
 projection unit is feet.

 So it is exactly as described in B).

 Other words have to be spent in the case we have layers with a projected
 coordinate system and we enable OTFR using a projected CRS in the project
 properties (for example the same CRS of the layer or at least a CRS
 defined with the same unit as the layer).

 1) if the layer has a projected system in meters and map units are in
 meters, than map scale and measure tool are right. If map units are feet
 than map scale and measure tool are wrong.

 2) if the layer has a projected system in feet and map units are in feet,
 than map scale is *right* but '''measure tool is wrong'''. If map units
 are meters then map scale is *wrong* but '''measure tool is right'''.


 [[BR]]
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 > Simple rule: If OTFP is off, there is no relation between the data in
 the file and any physical size in the real world. Every distance and area
 is unitless, and the unit selector only slaps on the chosen unit as a
 friendly.


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 This should be documented in the user manual.


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 > Please suggest a way how it *should* behave on the dev mailing list.


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 Hope this observations will help taking a decision about this matter.

-- 
Ticket URL: <http://trac.osgeo.org/qgis/ticket/1433#comment:11>
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