Virtual Earth Tile System
Virtual Earth offers a map of the whole world that can be directly manipulated. Zoomed and explored by the users. To make this educative interaction very fast and responsive, the maps are pre rendered at various different levels of details. Each map is cut into tiles for displaying and retrieving the map quickly. This article talks about the co ordinate systems, projections and the addressing scheme of the map tiles. These are jointly known as the Virtual Earth Tile System.
Now first let us focus on the Map Projection. It is always better to use a single projection for the whole world to make the map flawless as well as to make sure that the aerial images coming form the different sources line up properly. The Mercator projection has two important properties that outweigh the distortion of the scale. One is the conformal projection that preserves the shape of the smaller objects. This is of great significance wile displaying the aerial images, as we are willing to avid the distorted shape of the buildings. The rectangular buildings should appear rectangular and not square. And the other one is the cylindrical projection. Here the north and the south are always pointed straight upwards; whereas the east and west always point towards the straight right and left.
Tile Co ordinates and Quad Keys- for optimizing the display and the map retrieval performance, the rendered map is cut into tiles of about 256 x 256 pixels each. The number of tiles differs according to the number of pixels at each level. To optimize the storage and indexing tiles, the 2-dimensional XY co ordinate tile is combined into a 1-dimensional string that is known as quad keys. Each quad key identifies a single tile at specific levels of details in a unique way. This can be used as a key in some common database indexes.
For converting the tile co ordinates in a quad key, the bits of X and Y co ordinates are interleaved. These quad keys have many interesting properties. Firstly, the length of the quad key i.e. the number of digits on it is always equal to the level of details of the corresponding tile. Secondly, the quad key of any tile begins with the quad key of the parent tile. The parent tile is the tile that contains the tile at the previous level.
Thus, these quad keys offer a one dimensional index key that conserves the proximity of the tiles in the XY space. These tiles should be kept in the same disk blocks to reduce the number of disk reads.
Another Virtual Earth tile system is the Pixel Coordinates. By selecting the projections and the scale to be used at every level of details, you can change the geographic co ordinates into pixel co ordinates. As the width and the height of map varies at all the levels, so does the pixel co ordinates. The pixel towards the upper left corner of the map has (0, 0) pixel co ordinates; whereas towards the lower right corner, the pixel co ordinates are (width-1, height-1). Referring the given latitudes and longitudes and the level of details, you can calculate the pixel XY co ordinates.[tags]Microsoft, virtual earth, features of virtual earth, [tags]
Tags: Microsoft Virtual Earth
