[OSM-talk] JOSM and scale

[Lars Aronsson]

Juan Lucas Dominguez Rubio wrote:

> If you use JOSM with the Mercator projection and draw a perfect 
> circle in the North of Sweden, you are actually drawing a 
> horizontal ellipse (east-west oriented) in the real world.

Not true.  The very idea of (the transversal) Mercator is that 
shapes such as circles are preserved, at least on the finer 
scales.  JOSM's other mode (EPSG:4326) is a plain "lat=x; lon=y" 
and that does distort shapes, unless you're at the equator.  
Mercator maps longitudes to x, and then extends latitudes to a 
higher y so that shapes are preserved.  Mercator is what the 
slippy map uses, so it's the natural setting to use in JOSM.

Andy wrote:

> > Vector editing software such as CAD packages are normally 
> > scale free at the editing level.

JOSM is as scale free as any CAD program, of course.  The only 
irritating bug is the number that is printed next to the little 
measurement scale in the upper left corner.  At least when I'm 
using the Mercator setting, this needs to be multiplied with the 
cosine of the latitude (right?) to become correct.

David wrote:

> I disagree. I often dictate 'postbox 10m after junction' or '... 
> set back 30m from road' by my estimate,
> 
> In Mercator distance is constant independent of orientation, 
> isn't it?

At the local level (high zoom) this is true, yes.  If 150 
horizontal pixels on screen is 150 metres east-west, then 150 
vertical pixels on your screen is also 150 metres north-south.

On the world or continental map, it's a different thing.  
Mercator makes Iceland (103,000 km², all north of 63°N) look as 
big as Spain (504,000 km², all south of 44°N), which we all know 
actually isn't true.

The optimal solution for computerized maps is to use Mercator on 
the local level and a picture of a rotating globe on the world 
map, just like Google Earth does.  But that's a lot harder to do 
on the web with today's technology (HTML, AJAX and a tile server).


-- 
  Lars Aronsson (lars at aronsson.se)
  Aronsson Datateknik - http://aronsson.se