[OSM-talk] JOSM and scale

[Juan Lucas Dominguez Rubio]

Yes, they are probably using the same scale bar for WGS84 and Mercator.
 
As far as I know, in WGS84, the scale bar is correct vertically (lat), but in Mercator, that scale bar is wrong both vertically and horizontally if you are far away from the Equator.
 
As you say, one easy way would be to consider the latitude of, for example, the center of the map when calculating the length of the scale bar.
 
Lucas

 
________________________________

De: talk-bounces at openstreetmap.org en nombre de Lars Aronsson
Enviado el: mar 25/03/2008 12:41
Para: talk at openstreetmap.org
Asunto: [OSM-talk] JOSM and scale




At the equator, each degree of latitude or longitude represents a
distance of 111 km.  At latitude 60° (N or S), each latitude
degree is still 111 km but since cos 60° = 0.5, each longitude
degree is just half of that or 55.5 km.

In JOSM, when I use the Mercator projection method, look at the
equator and zoom out until the little scale bar in the top left
corner is 111 km, the difference in longitude between the ends of
this bar is one degree.  That's fine.

But if I look at 60° N (e.g. Stockholm) and repeat the exercise,
the end points of the 111 km scale bar is still one degree. 
That's an error, it should be two degrees.  Or rather, when the
bar represents one degree, the label should say 55.5 km instead of
111 km.

I've never cared too much about that scale, but now I know that
these apartment buildings along Syrengatan are 11 x 32 metres and
not 22 x 64 metres as JOSM would lead you to believe,
http://www.openstreetmap.org/?lat=58.407&lon=15.600&zoom=17&layers=0BFT

Also, returning to cycle lanes, the "secondary" road
Malmslättsvägen is now marked with cycleway=lane, but this doesn't
show on the map here.  And how can I indicate that this bus stop
is only on the southern side of the street (buses going east)? 
Buses going in the other direction stop at another place.

"Micromapping" is fun.  I want zoom=18 now.  Hmmm... and higher
GPS accuracy.


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

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