stop returning invalid x tile values in pointToTile

[tcql]

Currently pointToTile on coordinates with +180 longitude will result in invalid X tile coordinates. IE:

tilebelt.pointToTile(180, 0, 0);

yields [1, 0, 0] instead of [0, 0, 0]

cc @mourner

[mourner]

You probably meant y = on this line.

[mourner]

I think we'd want to go with a more sophisticated approach here:

• wrap x instead of clamping (so that 190 longitude is equivalent to -170, not -180)
• clamp negative y as well (if (y < 0) y = 0)

[mourner]

Any updates here?

[DenisCarriere]

coordinates with +180 longitude will result in invalid X tile coordinates.

Might be useful as reference material, global-mercator doesn't have the -180 or 180 longitude issue using lngLatToTile([x, y], zoom).

const mercator = require('global-mercator')
mercator.lngLatToTile([-180, 0], 0)
//=[ 0, 0, 0 ]

wrap x instead of clamping (so that 190 longitude is equivalent to -170, not -180)

Reversing longitudes over the dateline isn't overly complicated, however if you need another reference, I built bbox-dateline for that exact purpose.

CC: @tcql @mourner

[DenisCarriere]

I've made a wrapTile method that can easily be added:

/**
 * Wrap Tile -- Handles tiles which crosses the 180th meridian or 90th parallel
 *
 * @param {[number, number, number]} tile Tile
 * @param {number} zoom Zoom Level
 * @returns {[number, number, number]} Wrapped Tile
 * @example
 * wrapTile([0, 3, 2])
 * //= [0, 3, 2] -- Valid Tile X
 * wrapTile([4, 2, 2])
 * //= [0, 2, 2] -- Tile X 4 does not exist, wrap around to TileX=0
 */
function wrapTile (tile) {
  var tx = tile[0]
  var ty = tile[1]
  var zoom = tile[2]

  // Maximum tile allowed
  // zoom 0 => 1
  // zoom 1 => 2
  // zoom 2 => 4
  // zoom 3 => 8
  var maxTile = Math.pow(2, zoom)

  // Handle Tile X
  tx = tx % maxTile
  if (tx < 0) tx = tx + maxTile

  return [tx, ty, zoom]
}

You could also include these test cases:

test('tilebelt.pointToTile -- cross meridian', t => {
  // X axis
  t.deepEqual(tilebelt.pointToTile(-180, 85, 2), [0, 0, 2], '[-180, 85] zoom 2')
  t.deepEqual(tilebelt.pointToTile(180, 85, 2), [0, 0, 2], '[+180, 85] zoom 2')
  t.deepEqual(tilebelt.pointToTile(-185, 85, 2), [3, 0, 2], '[-185, 85] zoom 2')
  t.deepEqual(tilebelt.pointToTile(185, 85, 2), [0, 0, 2], '[+185, 85] zoom 2')

  // Y axis
  t.deepEqual(tilebelt.pointToTile(-175, -97, 2), [0, 0, 2], '[-175, -95] zoom 2')
  t.deepEqual(tilebelt.pointToTile(-175, 95, 2), [0, 3, 2], '[-175, +95] zoom 2')
  t.end()
})

http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/

Benchmark results

wrapTile x 30,352,603 ops/sec ±2.05% (87 runs sampled)

CC: @tcql @mourner

[tcql]

@DenisCarriere the X wrapping looks good, but Y should definitely clip, not wrap. -90 and 90 latitude don't touch (whereas -180 and 180 longitude do, so wrapping makes sense)

[DenisCarriere]

👍 You're correct about the Y clip, I added it there last minute.. but it was mostly the X wrapping that was the main one.

@tcql Do you want me to make a PR for this? It's already good to copy-paste the example I posted (minus the Y clip).

[mourner]

Superseded by #32