A great proposition! - About NTS tiling: Just take into account in your algorythm that the width of NTS tiling double at latitude 68 and 80 (don't use fixed longitude values because you will end up with 400 tiles for an NTS up north !) - About naming convention, I would propose to use a number as suffix instead of a letter. It would be easier (for me) to find neighbour tiles. Example: 021E05-18 (with a 5X5 window) Neighbours are 12,13,14,17,19, 22,23,24 ([18-6,-5,-4],18-1,18+1,[18+4,+5,+6]) Example: 021E05R (with a 5X5 window) Neighbours are L,M,N,Q,S,V,W,X (I have no magic to do that!) - Is it going to work with Canvec as well? Cheers, Daniel
________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Adam Dunn Sent: 18 novembre 2009 01:42 To: talk-ca Subject: [Talk-ca] NTS 1/10,000 Tile Proposal This applies only to the band of tiles represented by the 7 southernmost major tiles. That is to say {00-06, 10-16, 20-26, etc}. Once you go north of that, the tile naming scheme changes, and the following no longer applies (and is likely no longer needed). Though many people know how the NTS naming scheme works, it will be summarized here. NTS tiles are first given a major number, such as 010 or 093, progressing south to north and east to west. Each of these tiles is four degrees high (latitude) and eight degrees wide (longitude). These tiles are then split up into 16 tiles, the so-called 1/250,000 tiles, and given a letter as a name, starting from the south-east corner then going west and zig-zagging north. Each of these tiles is one degree high (lat) and two degrees wide (long). Each of these tiles are split once again into the 16 1/50,000 tiles, each with a number, following the same zig-zag pattern as the 1/250,000 tiles. These numbered tiles are 0.25 degrees high and 0.5 degrees wide. So far, the tile divisions have worked out to nice round numbers. For 1/250,000 tiles the least significant digits are: lat: {0, 1, 2, 3, ...} and long: {0, 2, 4, 6, 8} For 1/50,000 tiles, least sig digits are: lat: {.00, .25, .50, .75} and long: {.0, .5} This system works well, but for the purposes of OSM in high-density areas it becomes necessary to split NTS tiles up further. Splitting the 1/50,000 tiles up into 16 tiles would follow the NTS splitting scheme, but would result in very strange numbers: lat: {.0, .0625, .125, .1875, .25, ...} and long: {.0, .125, .25, .375, .5, ...} While there's nothing wrong with this and computers could handle it easily, it's a little hard on human eyes ;) If the tiles were instead split into 25 tiles (5x5), the numbers would work out a little nicer to the eye: lat: {.0, .05, .1, .15, .2, .25, ...} and long: {.0, .1, .2, .3, .4, ...} Thus, the proposal is to split NTS 1/50,000 tiles into 1/10,000 tiles such that each tile is 0.05 degrees in latitude and 0.1 degrees in longitude. The naming would take on letters A through Y, in a zig-zag pattern similar to the pre-existing NTS scheme, starting from the south-east corner. This letter would go in the last position of the tile name, eg. 042I/12A or 092H/04Y. Figure 1 shows an example for 092H/04 (the figure is twice as wide as it is tall, following an equirectangular projection. In real life, and most projection methods, the length ratios change with latitude.) Fig 1: http://www.mediafire.com/imageview.php?quickkey=mmmymj42yw4&thumb=4 Adam
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