How to Calculate Tiles Coordinates in Leaflet to Request TMS Server?

Question

In order to cache tiles for off-line use, I tried to calculate tiles coordinates according to a certain zoom level. Calculated x coordinates were correct but the y coordinates Were not.

This Old example compares actually received coordinates with that calculated. (click in the map to display results)

I was using map.project(latlng,zoom) to get the projected coordinates and then divide by tileSize which is 256. is this approach even correct ?

EDIT : Thanks to Ivan Sanchez for the orientation about y inversion in TMS. Actually after projecting the point with map.project(latlng,zoom) you need to inverse the y coordinate as follow :

1. You calculate _globalTileRange(zoom) for the corresponding zoom level, then
2. InvertedY = _globalTileRange(zoom).max.y - y ;

Here is another Link that shows the correct calculation of y coordinates for the current zoom of the map, for other zoom levels the globalTileRange need to be recalculated accordingly.

Regards,

Answer 1

Your approach is correct. However:

• In order to get the tile coordinates loaded by Leaflet, you are looping through all the loaded images and outputting the min/max of those values.

  The problem with this approach is that Leaflet doesn't immediately unload off-screen tiles. See the keepBuffer option, bug #4039 and PR #4650.

  In order to fetch the bounds of tiles visible within the map bounds, see the private methods used internally by L.GridLayer around this line of code.

• In TMS, the y coordinate goes up, and in non-TMS tiles it does down. This is because TMS was done by geographers, where the y coordinate is the northing, and non-TMS tiles were initially done by computer programmers, who interpret the y coordinate as downward pixels.

  For more background, read https://wiki.openstreetmap.org/wiki/TMS#The_Y_coordinate and https://wiki.osgeo.org/wiki/Tile_Map_Service_Specification#TileMap_Diagram and https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#X_and_Y