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convert lat/lon to tile x/y and back

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This commit is contained in:
Daniel Barlow 2025-05-27 23:48:32 +01:00
parent 1deaea2948
commit dc95c2252e
2 changed files with 45 additions and 1 deletions
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10
pkgs/maps/assert.fnl
View File

@ -17,8 +17,16 @@
(.. "\nexpected " ve# "\ngot " va#)
))))
(fn expect-near [actual expected]
`(let [delta# (math.abs (- ,actual ,expected))
e# ,expected
a# ,actual]
(assert (< delta# 0.00000001)
(.. "expected " e# " to be approximately " a#))))
(fn define-tests [& body]
(when _G.RUNNING_TESTS
`(do ,(unpack body))))
{ : define-tests : expect : expect= }
{ : define-tests : expect : expect= : expect-near }

36
pkgs/maps/tiles.fnl
View File

@ -2,6 +2,7 @@
(local { : dict_to_query } (require :http.util))
(local json (require :json))
(import-macros { : define-tests : expect : expect= : expect-near } :assert)
(local { : view } (require :fennel))
@ -33,3 +34,38 @@
(print (view headers))
(print (view (json.decode (stream:get_body_as_string))))))
(fn sinh [x] (/ (- 1 (math.exp (* -2 x))) (* 2 (math.exp (- x)))))
(expect (< (math.abs (- (sinh 2) 3.626860407847)) 0.001))
(fn tile->latlon [xtile ytile zoom]
(let [n (^ 2 zoom)
lon-deg (- (/ (* xtile 360) n) 180.0)
lat-rad (math.atan
(sinh (* math.pi (- 1 (/ (* 2 ytile) n))))
)]
(values (/ (* lat-rad 180) math.pi) lon-deg)))
(let [(lat lon) (tile->latlon 0 0 0)]
(expect= lon -180)
(expect-near lat 85.05112877)
)
(let [(lat lon) (tile->latlon 232798 103246 18)]
(expect-near lon 139.699401855)
(expect-near lat 35.6595278648)
)
(fn latlon->tile [lat lon zoom]
(let [n (^ 2 zoom)
x (* n (/ (+ lon 180) 360))
t1 (/ (* lat math.pi) 180)
t (math.log (+ (math.tan t1) (/ 1 (math.cos t1))))
y (* (- 1 (/ t math.pi)) (/ n 2))]
(values x y)))
(let [(x y) (latlon->tile 52.1234 -0.53 17)]
(expect= (math.floor x) 65343)
(expect= (math.floor y) 43221))