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

[Bernhard Ströbl]

Hi Matthias,

In your mail I attached two files (not for the list). One is North 
America in a Lambert Conformal Conic projection, the other one in WGS84. 
Both show a grid with 10 degrees distance between parallels and meridians.

Am 24.06.2014 14:02, schrieb Matthias Kuhn:
 > Hi Bernhard,
 >
 > On 24.06.2014 11:06, Bernhard Ströbl wrote:
 >> Hi Matthias,
 >>
 >> probably this is academical...
 >>
 >> Am 24.06.2014 10:42, schrieb Matthias Kuhn:
 >>> Hi Bernhard,
 >>>
 >>> I wouldn't say no sense at all. It strongly depends on the context, but
 >>> if you have for example a lesson for geography students and are
 >>> introducing CRS/projections and their properties one could want to 
add a
 >>> scale bar in degrees.
 >>
 >> But would that scalebar show the degrees for lon or lat?
 >
 > Maybe I am wrong, but I assume that there is no difference. One unit
 > (degree) will represent the same amount of pixels/points horizontal and
 > vertical.

Well, you are wrong because one degree in lat is always ~110km whereas 
one degree in lon is ~110 km at the equator and e.g. in Zurich ~75km 
(for calculation see [1]). So how many pixels are 1 degree? Depends on 
the projection; in WGS84 it is the same amount no matter if for lon or 
lat, in Lambert it is not.

 >
 >> If the first (lon) for which latitude?
 > It doesn't matter in degrees. But it really matters when trying to put a
 > scalebar in meters.

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.

 >> Either of the two: how do you want to tell people that this scalebar
 >> is only true for North-South (lat) or East-West (lon) measurements and
 >> must not be used in any other direction? IMHO a scale bar is to enable
 >> readers to use their ruler to measure a distance on the map in _any_
 >> direction.
 >>
 >>> I agree that it's not very common and most people
 >>> are probably unused, but if you explicitly state the fact that the map
 >>> is in degrees you might even avoid confusion and prevent people from
 >>> trying to compare distances.
 >>
 >> But adding a scale bar encourages users to compare distances! The fact
 >> that a map is suitable to measure and compare distances is not decided
 >> by the map units but by the used projection and the covered area. If
 >> your map is in degrees just enable the graticules and (if useful) add
 >> a scalebar in m/km/miles (does that work with degrees? I have not
 >> tried. If not this would be a feature request.)
 >>
 > Doesn't really make sense to me. Graticules are just another reference
 > for distances (in degrees in this case) and an alternative or addition
 > to scale bars. What problem exactly would the combination of a grid in
 > degrees and a scalebar in meters solve?

a scale bar makes distances measurable while a graticule helps 
localizing a point. In certain cases (projections) the graticule could 
be used for measuring, though.

 >
 >> BTW: for which point of a map is the scale bar currently created
 >> (thinking of non-distance-true projections and large areas e.g.
 >> continents)?
 > No idea.
 > But if there should be proper support for scalebars in meters on
 > degree-based maps, then it has to be configurable. And also the two
 > different scalebars (horizontal vs. vertical) that you mentioned. Then
 > it could be that there is a small enough area that this can be
 > considered accurate enough to be useful. And there should be warnings to
 > inform the mapper that he might be misleading readers and should
 > consider to reproject.

I cannot imagine any use case for measuring distances in degrees. If you 
look at either of my maps you see that the south of Greenland is 
apporximately 40 degrees north of Cuba and that Canada covers almost 90 
degrees in east-west direction. But why should someone measure this in 
degrees and not in km or miles? Would you measure the distance between 
your place and your favourite bar in degrees?

all the best

Bernhard

[1] http://en.wikipedia.org/wiki/Longitude


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