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.stro...@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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