Thanks Adam, Im forwarding this explaination to the import@ at list, and those who might be able to help make this possible, to include as part of the new canvec2-to-osm conversion script routeen.
Cheers, Sam On Tue, Nov 17, 2009 at 10:41 PM, Adam Dunn <[email protected]> wrote: > 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 > > _______________________________________________ > Talk-ca mailing list > [email protected] > http://lists.openstreetmap.org/listinfo/talk-ca > >
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