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

[Adam Dunn]

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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