# Beginner Question: Latitude/Longitude to x,y conversion - understanding output Classic List Threaded 6 messages Open this post in threaded view
|

## Beginner Question: Latitude/Longitude to x,y conversion - understanding output

 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+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
Open this post in threaded view
|

## Re: Beginner Question: Latitude/Longitude to x, y conversion - understanding output

Open this post in threaded view
|

## Re: Beginner Question: Latitude/Longitude to x, y conversion - understanding output

 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
Open this post in threaded view
|

## Re: Beginner Question: Latitude/Longitude to x, y conversion - understanding output

 AdamI 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=63781375733755.28      14193.25and it should have been:echo 0.1275 51.5072 | proj +proj=merc +a=6378137 +b=6378137 14193.24        6711506.71The local deformation of lengths at latitude LAT is approximately 1/COS(LAT)Regards José Gonçalves
CallSend SMS Add to SkypeYou'll need Skype CreditFree via Skype 2014-02-24 13:30 GMT+00:00 AdamDynamic : 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 -- View this message in context: http://osgeo-org.1560.x6.nabble.com/Beginner-Question-Latitude-Longitude-to-x-y-conversion-understanding-output-tp5105455p5105524.html Sent from the PROJ.4 mailing list archive at Nabble.com. _______________________________________________ Proj mailing list [hidden email] http://lists.maptools.org/mailman/listinfo/proj _______________________________________________ Proj mailing list [hidden email] http://lists.maptools.org/mailman/listinfo/proj