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

[Ted]

Hi Jorge Tornero, Bernhard, Matthias, Nyall, All

Thanks for taking up the scalebar issue.

I agree that in most cases, the scalebar is more of an academic /
customary. It hardly gives a "true" measurement.

Users generally use that as means of comparison and not to measure the
exact distance.

IMO, the scalebar makes sense when you have maps in large scales like 1:100
better. Otherwise, its just an estimation.

Hope QGIS can provide a solution.

Regards
Ted


On Fri, Jun 27, 2014 at 1:27 PM, Ted <tiruchirapalli at gmail.com> wrote:

> Hi Richard, Matthias, All
>
> Yes, the crash problem is gone in master (nightly built) version. Thanks a
> lot for the fix.  Tested it on Win 7 x64 and also Win XP x32.
>
> Just for note, that issue is reported in v2.0.1 and v2.2 releases.
>
> Will wait for the v2.4 release in July.
>
>
> Regards
> Ted
>
>
>
>
>
> On Wed, Jun 25, 2014 at 5:02 PM, Bernhard Ströbl <bernhard.stroebl at jena.de
> > wrote:
>
>> 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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