Hi Matthias,
Am 24.06.2014 16:46, schrieb Matthias Kuhn:
On 24.06.2014 16:10, Bernhard Ströbl wrote:
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.
For Lambert neither one nor the other makes sense. The only appropriate
solution here would be some kind of "Lambert unit" whatever that may be.
But for the WGS84 map you sent the original statement above holds true:
the same amount of pixels matches the same amount of degrees everywhere
on the map (In terms of lat/lon, not in terms of degrees on the sphere
though). While a scalebar in km as provided is subject to distortion for
the exact reason you noted.
Agreed. But a scale bar is used to measure distances (and IMHO distances
are in miles, km,..., not in degrees) If a scale bar makes sense depends
on the projection and the area covered (as I stated some mails ago: "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 it does not make sense one should not put a scale bar on the
map.
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.
The question absolutely makes sense but I don't know the answer :)
Could you check? or whom would we have to ask?
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.
You are right here. It's not a replacement but a bit of a different
thing (because graticules are not necessarily required to be straight).
So for the ship navigation example before, graticules in degrees for
localization (non-straight) combined with a meaningful scalebar (given a
suitable projection) make absolutely sense. My main point was, that a
graticule doesn't compensate for a scalebar on an unsuitable projection.
OK, total agreement, all is related to the projection, so the maker of
the map has to use care to choose a suitable one.
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?
That's not the right question to ask. Instead it should be: "Why would
someone want to measure distances on an unsuitable projection, with a
ruler that he has no idea if it has any meaning for the location he
tries to measure?".
Again, its up to the maker of the map to provide a scalebar if it is
meaningful and none otherwise, same with the graticule.
The result is the same as when a friend measures the distance to his
favorite bar, find out that it's 2.8 miles and then telling me that it's
2.8 km. I'd rather know that it's 2.8 "units" and therefore be aware of
the requirement to be careful with the interpretation of the result.
Or even better: have somebody warn my friend that he might be using the
wrong tool for the job he tries to accomplish :)
or even better: go with him to the bar to show you the way :-)
Bernhard
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