Satellite Tile Layout
From Google Mapki
Google uses a different scheme for creating urls to fetch satellite tiles from the x,y,z scheme used for the mapping tiles. This page hopes to give a brief overview of how to interpret and use the satellite tile reference mechanism.
Satellite tiles are fetched using a query string that looks a bit like: http://kh1.google.co.uk/kh?n=404&v=5&t=trtq . All google satellite tile references start with a t.
In the URL above the trtqst is the tile reference, and each letter in the reference specifies a particular quadrant in turn, with the quadrants assigned a letter in clockwise order q,r,s,t, like so:
+------+------+ | | | | q | r | | | | +------+------+ | | | | t | s | | | | +------+------+
The diagram below shows how to resolve a reference like trtq.
+------------------+---------+---------+ | | | | | | | | | | trq | trr | | | | | | | | | | | | | | tq +----+----+---------+ | | | | | | |trtq|trtr| | | | | | | | +----|----+ trs | | | | | | | |trtt|trts| | | | | | | +------------------+----+----+---------+ | | | | | | | | | | tt | ts | | | | | | | | | | | | | +------------------+-------------------+
The satellite reference makes it fairly straight forward to determine the "zoom" level - it is given by the length of the reference string, so t is a tile at zoom 0, tr is a tile at zoom 1 and so on. You can also quickly and directly deduce the covering tile at the next zoom-out (trqsrtts is covered by trqsrtt) and the ids of the tiles at the next zoom in (trqsrtts covers trqsrttsq,trqsrttsr,trqsrttss,trqsrttst).
The satellite tile reference can also help when creating custom tiles. A problem sometimes encountered is that you end up with thousands of image files in a single directory which gives very poor performance. One solution to this is to slice the tile reference up into blocks of e.g. three letters so trqsrttsq becomes trq/srt/tsq.jpg doing this means that there will never be more than 4^3=64 image files in a single directory, keeping disk performance at a reasonable level.
