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

[AdamDynamic]

Hi,

I've been scouring the internet for a couple of days now and getting
absolutely nowhere - I appreciate this is probably a simple question once
you know the answer but any help on this would be appreciated!

I have a series of latitude/longitude coordinates that i want to plot as xy
coordinates (i.e. a flat, rectangular map).

I'm using the PyProj Python library to perform the conversion, the problem
is I can't determine what the output of the function means or represents? 

e.g. Converting [51.5072,0.1275] (what Google tells me is the lat/long for
London in the UK) returns [5733755.276187301, 14193.246790158479] for a
Mercator projection - my question is '5733755.276187301 of what'? I
understand that the library converts to Proj.4 notation (?) but I can't seem
to find what the units are, or how I'd map the output onto a 900x1100
rectangle?

I'm using the following conversion formula which I found  here
<http://osgeo-org.1560.x6.nabble.com/Google-Mercator-Projection-td3841191.html>  

+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0
+k=1.0 +units=m  +no_defs

Any assistance would be great as I haven't been able to get any information
from anywhere else. Please let me know if more information would be useful.

Thanks,

Adam



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