[OSM-newbies] Incorrect scale or longitude when exporting maps?

[Lauri Hahne]

The maps are in Mercator projection which doesn't preserve sizes but
preserves shapes and bearings. This is your "problem".

Some more information about it:

http://wiki.openstreetmap.org/index.php/Mercator
http://en.wikipedia.org/wiki/Mercator_projection


2008/6/30  <luke.angel at clipsal.com.au>:
>
> Hi,
>
> Maybe I am misunderstanding things, but something just doesn't seem to add
> up when choosing longitudes for exporting a map.
> If I want to export two maps at significantly different longitudes, but the
> same scale and map size for each (1280 x 1024), shouldn't the longitudinal
> width (number of degrees) be different for each map?  I'm getting the same
> for each.
>
> Here's the scenarios, both in Australia.
>
> #1. Adelaide.
> Select -34.6, 138.0, 139.61, -35.6536 at a scale of 1:500000
> Exporting this gives a 1280 x 1024 image.
>
> #2 Darwin.
> Select -12.3, 130.0, 131.61, -13.555 also at a scale of 1:500000
> Exporting this also gives a 1280 x 1024 image.
>
> The latitude range varies with each (1.0536 vs 1.255 degrees), which is as
> per expected because of Earth's eliptical bulge near the Equator.
> However, the longitude range is the same for each (1.61 degrees). Given
> this, I would expect the image width of each to be significantly different
> (Adelaide smaller as is further from the Equator where one degree longitude
> is less miles than nearer the Equator).
>
> The scale of the images seem proportional so it seems to me that the
> longitude values for the images must be incorrect. How can the width of both
> exported images equal 1.61 degrees of longitude when one image is 22.3
> degrees further North than the other?
>
> Kind Regards,
>
> Luke Angel
> Technical Communications Officer - Commercial Market
> Clipsal Integrated Systems
>
>
>
> Fifth Street
> Bowden.  SA  5007
> Telephone +61 8 8345 9509
> Facsimile  +61 8 8346 0573
> luke.angel at clipsal.com.au
>
>
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-- 
Lauri Hahne