Reference:
http://wiki.openstreetmap.org/wiki/ Slippy_map_tilenames#Zoom_levels
Brief overview:
OSM tiles are 256x256 pixel PNG files.
With each additional zoom-level,
the number of OSM tiles that
cover the (non-polar) Earth goes up
a factor of 2 x 2 = 4.
Filename (URL) format is ... tile.openstreetmap.org/zoom/x/y.png
where
zoom=zoom-level-integer (from 0 to 19),
x=longitude-locator-integer (non-negative column number),
y=latitude-locator-integer (non-negative row number).
In the link above, see sample code to calculate x and y
for a given zoom-level and a given latitude and longitude.
The following Earth map
is done with OSM map tiles
at OSM zoom level 3.
Each OSM tile is a 256x256 pixel PNG file.
Each zoom-level-3 tile is one image of
2^3 x 2^3 = 8 x 8 = 64 images
which are used by OpenStreetMap to
cover most of the Earth --- except for
near the North and South poles.
These zoom-level-3 tiles go from
x-numbers 0 to 7. (8 columns)
These zoom-level-3 tiles go from
y-numbers 0 to 7. (8 rows)
Total: 8 x 8 = 64 tiles
Image width: 8 x 256 = 2,048 pixels
Image height: 8 x 256 = 2,048 pixels
Each Level-3-tile in the x-direction represents
360 / 8 = 45 degrees of longitude.
Each Level-3-tile in the y-direction represents
roughly 170 / 8 = 21.25 degrees of latitude.
Hence, the centers of each of the 8 columns of tiles
in the x-direction are (in degrees)
1) -180 + 22.5 = -157.5
2) -157.5 + 45 = -122.5
3) -122.5 + 45 = -67.6
4) -67.6 + 45 = -22.5
5) -22.5 + 45 = 22.5
6) 22.5 + 45 = 67.5
7) 67.5 + 45 = 112.5
8) 112.5 + 45 = 157.5
These are longitudes.
And, the centers of each of the 8 rows of tiles
in the y-direction are (roughly) in degrees
1) 85 - 10.625 ~ 74.4
2) 74.4 - 21.25 ~ 53.2
3) 53.2 - 21.25 ~ 31.9
4) 31.9 - 21.25 ~ 10.6
5) 10.6 - 21.25 ~ -10.6
6) -10.6 - 21.25 ~ -31.9
7) -31.9 - 21.25 ~ -53.2
8) -53.2 - 21.25 ~ -74.4
These are latitudes.
(These latitude numbers need some adjustment --- especially near
the extremes of +85 and -85 degrees. This is a linear
interpolation to specify the center of each tile when it should be
more like a logarithmic interpolation.)
Using these longitude and latitude coordinates,
the approximate center of each of the 64 level-3 tiles
--- in latitude and longitude degrees --- can be seen in this array :
0 1 2 3 4 5 6 7
0 74.4,-157.5 74.4,-122.5 74.4,-67.6 74.4,-22.5 74.4,22.5 74.4,67.6 74.4,122.5 74.4,157.5
1 53.2,-157.5 53.2,-122.5 53.2,-67.6 53.2,-22.5 53.2,22.5 53.2,67.6 53.2,122.5 53.2,157.5
2 31.9,-157.5 31.9,-122.5 31.9,-67.6 31.9,-22.5 31.9,22.5 31.9,67.6 31.9,122.5 31.9,157.5
3 10.6,-157.5 10.6,-122.5 10.6,-67.6 10.6,-22.5 10.6,22.5 10.6,67.6 10.6,122.5 10.6,157.5
4 -10.6,-157.5 -10.6,-122.5 -10.6,-67.6 -10.6,-22.5 -10.6,22.5 -10.6,67.6 -10.6,122.5 -10.6,157.5
5 -31.9,-157.5 -31.9,-122.5 -31.9,-67.6 -31.9,-22.5 -31.9,22.5 -31.9,67.6 -31.9,122.5 -31.9,157.5
6 -53.2,-157.5 -53.2,-122.5 -53.2,-67.6 -53.2,-22.5 -53.2,22.5 -53.2,67.6 -53.2,122.5 -53.2,157.5
7 -74.4,-157.5 -74.4,-122.5 -74.4,-67.6 -74.4,-22.5 -74.4,22.5 -74.4,67.6 -74.4,122.5 -74.4,157.5
NOTE:
There may be some adjustment to these values
in the future, to position the 'slippy maps'
in a better initial position (for example,
over a land area,
rather than in the midst of an ocean).
Each one of the 8x8 = 64 tiles below is a link to an
OSM (Open Street Map) service that provides a 'slippy map'
that is located at that tile --- at an appropriate zoom-level
that positions you for exploration of that area of the Earth.
Simply click on a tile to bring up a 'slippy map' in a
separate window (or tab) of your web browser.
Note that about 18 of the 64 tiles are over 'heavily' populated areas.
So about 18 / 64 ~ 30% of these tiles will receive the most clicks.
About 64 - 18 = 46 of these tiles are expected to receive
few, if any, clicks.
Since the circumference of the Earth
is 40,075.16 kilometers (24,901.55 miles),
for OSM tiles at the equator, the width
of each tile represents:
40,075.16 kilometers (24,901.55 miles) at OSM zoom-level-0
20,037.58 kilometers (12,450.77 miles) at OSM zoom-level-1
10,018.79 kilometers ( 6,225.39 miles) at OSM zoom-level-2
5,009.40 kilometers ( 3,112.69 miles) at OSM zoom-level-3
2,504.70 kilometers ( 1,556.35 miles) at OSM zoom-level-4
1,252.35 kilometers ( 778.17 miles) at OSM zoom-level-5
626.17 kilometers ( 389.09 miles) at OSM zoom-level-6
313.08 kilometers ( 195.54 miles) at OSM zoom-level-7
156.54 kilometers ( 97.27 miles) at OSM zoom-level-8
78.27 kilometers ( 48.64 miles) at OSM zoom-level-9
39.14 kilometers ( 24.32 miles) at OSM zoom-level-10
19.56 kilometers ( 12.16 miles) at OSM zoom-level-11
9.78 kilometers ( 6.08 miles) at OSM zoom-level-12
4.89 kilometers ( 3.04 miles) at OSM zoom-level-13
2.45 kilometers ( 1.52 miles) at OSM zoom-level-14
1.22 kilometers ( 0.76 miles) at OSM zoom-level-15
0.61 kilometers ( 0.38 miles) at OSM zoom-level-16
305 meters ( 334 yards) at OSM zoom-level-17
153 meters ( 167 yards) at OSM zoom-level-18
76 meters ( 83 yards) at OSM zoom-level-19
(a little less than the length of a fusbol/football field)
To construct the table above,
we took half of the previous width
at each zoom-level.
NOTE:
At the equator, each tile (above) at zoom-level-3
represents about 5,000 kilometers (or 3,100 miles)
in width (x-direction) and height (y-direction)
on the surface of the Earth.
Page history:
Page was created 2019 Feb 21.
Page was changed 2019 Sep 16.
(Added a note on the need to adjust the degrees used to
initially position the 'slippy tiles' in the latitude
direction --- especially the tiles near the south and north poles.)