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

Jose Gonçalves jagoncal at gmail.com
Mon Feb 24 09:27:47 EST 2014


I think you have to change the order of your coordinates. The default input
for PROJ is (lon,lat) and the default output is (Easting,Northing).
I used PROJ in the command line to check.

You have done:
echo 51.5072 0.1275 | proj +proj=merc +a=6378137 +b=6378137
5733755.28      14193.25

and it should have been:
echo 0.1275 51.5072 | proj +proj=merc +a=6378137 +b=6378137
14193.24        6711506.71

The local deformation of lengths at latitude LAT is approximately 1/COS(LAT)


José Gonçalves

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2014-02-24 13:30 GMT+00:00 AdamDynamic <helloadamdynamic+proj4 at gmail.com>:

> Ah ok, thanks for that. Reading your answer, the notation makes a bit more
> sense (a risk of using something you find on the internet without actually
> understanding it)
> > 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?
> 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?
> Thanks again for the help!
> Adam
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