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Coordinate Systems

There are three coordinate systems involved (and a projection!).

  • Latitude and longitude - how we identify a location on Earth,
  • Screen points - the distance in pixels from the top left of the screen,
  • Tiles - the number of tiles (or fraction of tiles) across or down the screen.

Latitude and Longitude​

This is how we identify a position on earth. Thinking of the planet as a ball, the coordinates describe two circles; one around the equator, and one around the poles. Longitude describes the ring around the equator, from -180 degrees to +180 degrees (i.e. 360 degrees in total). Latitude is an arc from the north to the south pole, from 90 to -90 degrees. The point at which these rings intersect is arbitrary, but by agreement is near the Greenwich Observatory in London England.

To represent the sphere of the earth on a two dimensional surface we need to stretch or compress the original spherical surface. This is known as a projection.

Screen Points​

These are the actual pixels on the screen, represented as a vertical and horizontal grid. There are a fixed number of dots in the horizontal and vertical direction. The points start at zero at the top left and increase right and down.

Tile Points​

The final map is composed of a grid of one or many image tiles. A point on the earth of a given latitude and longitude may be viewed from a range of distances (or zoom levels). the bigger the zoom, the closer and more detailed the image.

Typically a tile is 256 pixels square. So an image of 1024 wide by 512 high (in screen points) would comprise of 4 tiles across by 2 down. The coordinates in latitude and longitude this would depend on the zoom level.

The tile provider only serves tiles for integral zoom levels. To handle intermediate zooms we can scale the tiles by applying a simple scaler.