[OSM-dev] tile calculation

[Rob Aerts]

 i try to calculate the x and y tile url in c

the example on the wiki (in perl code) looks like
my $xtile = int( ($lon+180)/360 *2**$z ) ;
my $ytile = int( (1 - log(tan($lat*pi/180) + sec($lat*pi/180))/pi)/2 *2**$z
) ;

i can't figure out the sec formula in the $ytile

wiki : http://wiki.openstreetmap.org/index.php/Slippy_map_tilenames
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