[Imports] [Talk-ca] NTS 1/10,000 Tile Proposal

[Sam Vekemans]

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 <dunnadam at gmail.com> 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
>
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>
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