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talaj/spherical-mercator.lisp

Created January 4, 2018 09:10
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Envelope of a tile in Spherical Mercator
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spherical-mercator.lisp
(defconstant EARTH_RADIUS 6378137.0)
(defconstant HALF_OF_EQUATOR (* PI EARTH_RADIUS))
(defun tile-size (zoom) (/ (* 2 HALF_OF_EQUATOR) (expt 2 zoom)))
(defun tile-envelope (x y z)
(let ((size (tile-size z)))
(list
(+ (- HALF_OF_EQUATOR) (* x size))
(- (+ HALF_OF_EQUATOR) (* (+ y 1) size))
(+ (- HALF_OF_EQUATOR) (* (+ x 1) size))
(- (+ HALF_OF_EQUATOR) (* y size)))))
(format t "~{~6$~^ ~}" (tile-envelope 112 672 11))
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