[Proj] Beginner Question: Latitude/Longitude to x, y conversion - understanding output

Antti Castrén antti.castren at iki.fi
Mon Feb 24 09:37:28 EST 2014

2014-02-24 15:30 GMT+02:00 AdamDynamic <helloadamdynamic+proj4 at gmail.com>:
>> To output the map onto 900x1100 rectangle you have to scale the length
>> and width of your projected map coordinates to 900 and 1100. And then
>> you have to apply shift of origin of course.
> Am I correct in thinking that as I have the latitude of true scale as the
> Equator, then no shift of origin is required?

It depends on the area of interest and the coordinate system of your
map image (where your image's origin is). If you are mapping the
World, I think your image origin (0,0) will be somewhere else than in
the center. Or it could be there if your image coordinate ranges are
-450 to 450 and -550 to 550.

> In terms of scaling the output, my understanding is that the 'corners' of a
> projection based on latitude and longitude should be -180,-90 and 180,90 -
> when I convert these values using the PyProj library I get values of 1E30
> for each value (which seems to me clearly wrong). What is the best way to
> scale the output in terms of min/max values (in 'pseudo-meters') for each
> corner of my 900x1100 rectangle?

You simply can NOT project the poles (lat=90) using Mercator
projection, because the formulae give the infinity as result. Stop at
around 75N/S degrees in case you want something usable.


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