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

[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