lat_long_to_merc_and_bucket.rb

#!/usr/bin/env ruby

ASPECT = 0.77539 # target map width / height
PI = 3.14159165358

MAP_BUCKET_SIZE = ENV['BUCKET_SIZE'] or raise "no BUCKET_SIZE env set!"
MAP_BUCKET_SIZE = MAP_BUCKET_SIZE.to_f

STDIN.each do |line|
	time,lat,lon = line.split "\t"
	lat,lon = lat.to_f, lon.to_f

	# bucket time into a 10 min chunk of day
	time =~ / (\d\d):(\d\d):\d\d \+00/		
	time_slot = ($1.to_i * 60 + $2.to_i) / 10

	# convert to mercator
	x = (180 + lon) / 360
	lat = lat * PI / 180 # deg -> rad	
	y = Math.log(Math.tan((lat/2) + PI/4)) # mercator project
	y = ASPECT/2 - (y/(2*PI)) # rescale for map

	# allocate to buckets
	left = (x / MAP_BUCKET_SIZE).to_i * MAP_BUCKET_SIZE
	top = (y / MAP_BUCKET_SIZE).to_i * MAP_BUCKET_SIZE

	puts [time_slot,left,top].join "\t"
end