[Qgis-developer] QGIS Crash - Serious problem in 2x

[Bernhard Ströbl]

Hi Nyall,

thanks for clarification!
However I would say that even if using a projected coordinate system 
(map units m) scalebars are not neccessarily accurate all over the map: 
if your map covers a small area this may hold true but not if you look 
at continents.

Bernhard

Am 25.06.2014 00:13, schrieb Nyall Dawson:
> On 25 June 2014 01:20, Bernhard Ströbl <bernhard.stroebl at jena.de> wrote:
>
>>>> It does also matter in degrees, depending on the projection. same in
>>>> meters: 1 cm on the map represents always a certain distance in
>>>> reality (though this distance varies troughout the map depending on
>>>> the projection and the area covered). If you look at the Lambert map,
>>>> you realize that the distance between two parallels (10 degrees!)
>>>> increases towards the pole, although in reality it is always (10*110km
>>>> =) 1100 km. In the WGS84 map the distance between the parallels is
>>>> constant but so is the distance between the meridians, but this is
>>>> false as the distance gets less towards the pole in reality. So a
>>>> scalebar (in m) being accurate in the middle of the map becomes less
>>>> accurate towards the edges. Hence my question on which base the
>>>> scalebar is calculated.
>>>
>>>
>>> The question absolutely makes sense but I don't know the answer :)
>>
>>
>> Could you check? or whom would we have to ask?
>
> It's calculated this way:
>
> If you're working in a projected coordinate system (ie, map units are metres):
>
> - Take the current extent of the map, calculate the width (x max - x
> min), divide this by the width on paper of the map
>
> If you're working in a geographic coordinate system (ie, map units are degrees):
>
> - Convert the width of the map (map's extent x max - x min) from
> degrees to metres, using a variant of the Haversine formula, and
> treating the current latitude as the MIDDLE LATITUDE from the map's
> extent
> - Convert this distance to a scale by dividing by the width on paper of the map
>
> So, yes, scalebars using m/km/miles/etc are only an approximation when
> map units are degrees, and are very inaccurate when used with maps
> covering a large area or for areas far from the equator.
>
> Nyall
>
>


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