MapInfo Spatial Server Map Tiling Service - Product Documentation ...

Transforming a Pixel to a Map Coordinate
Transforming a Pixel to a Map Coordinate
To pan or zoom a map based on tiles from the Map Tiling Service the client must have the ability to
convert a pixel to a location on the Earth. To do this the client must create an affine transform that when
given a coordinate in pixels returns a coordinate in the projection of the tiles. You need to know the
projection of the tiles, the bounds of the map being tiled, and the dimensions of a tile. Given this information
you must do the following:
1. Calculate the maximum number of tiles across that represents the map.
2. Calculate the maximum number of tiles down that represents the map.
3. Calculate the maximum number of pixels across that represents the map.
4. Calculate the maximum number of pixels down that represents the map.
5. Create an affine transform based on the bounds of the map and the bounds of the map in pixels.
6. Feed your location in pixels into the affine transform. The result will be a location on the earth in the
projection of the map.
To perform these steps, and transform a pixel into your map's coordinate system:
1. Calculate the maximum number of tiles across the map using the following equation: tilesAcross
= 2 level
2. Calculate the maximum number of tiles down the map using the following equation: tilesDown =
2 level
3. Calculate the number of pixels across the entire map using the following equation: pixelsAcross
= tileWidth × tileAcross
4. Calculate the number of pixels down the entire map using the following equation: pixelsDown =
tileHeight × tileDown
5. Given the bounds of the map in the map's projection and the bounds of the map in pixels an affine
transform from pixels to map coordinates can be constructed. The pixel coordinate passed must be
in the map's pixel space. It may not be enough to directly feed a pixel coordinate from a mouse location
on the screen. For instance, you may have to convert the mouse location to a location in the
map's pixel space. This involves keeping track of the location in map pixels space of the upper righthand
corner of the image control and offsetting the mouse location with that location.
The getDescription method provides all the information needed to construct the affine transform like
tileWidth, tileHeight, map projection, and map bounds. See getDescription on page 29.
Transforming a Pixel to a Tile Coordinate
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To perform pan and zoom, and locate items in a particular map tile, you need to be able to determine
which tile a pixel is in. To do this the client must be able to calculate the column and row or the tile when
given a coordinate in pixels. To perform this step, and transform a pixel into a tile coordinate, you can
use the following calculations:
MapInfo Spatial Server 1.0