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

[luke.angel at clipsal.com.au]

Thanks guys for your replies to my email earlier in the week.

Lauri, does this mean that the exported images in my example in fact will 
be the actual latitude and longitude as specified (top, left and bottom, 
right coords), that the vertical scale would be 1:500000, but the 
horizontal scale would be variable depending on how far from the Equator? 
If so then the fact that the images "appear" proportional then would be 
that I/we are used to seeing Mercator projections of the world?

Luke.

2008/6/30 Lauri Hahne wrote:

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


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

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