NAME¶
map::slippy - Common code for slippy based map packages
SYNOPSIS¶
package require
Tcl 8.4
package require
Tk 8.4
package require
map::slippy ?0.4?
::map::slippy length level
::map::slippy tiles level
::map::slippy tile size
::map::slippy tile valid tile levels ?
msgvar?
::map::slippy geo 2tile geo
::map::slippy geo 2tile.float geo
::map::slippy geo 2point geo
::map::slippy tile 2geo tile
::map::slippy tile 2point tile
::map::slippy point 2geo point
::map::slippy point 2tile point
DESCRIPTION¶
This package provides a number of methods doing things needed by all types of
slippy-based map packages.
API¶
- ::map::slippy length level
- This method returns the width/height of a slippy-based map
at the specified zoom level, in pixels. This is, in essence, the
result of
expr { [tiles $level] * [tile size] }
- ::map::slippy tiles level
- This method returns the width/height of a slippy-based map
at the specified zoom level, in tiles.
- ::map::slippy tile size
- This method returns the width/height of a tile in a
slippy-based map, in pixels.
- ::map::slippy tile valid tile
levels ? msgvar?
- This method checks whether tile described a valid
tile in a slippy-based map containing that many zoom levels. The
result is a boolean value, true if the tile is valid, and
false otherwise. For the latter a message is left in the variable
named by msgvar, should it be specified.
A tile identifier as stored in tile is a list containing zoom level,
tile row, and tile column, in this order. The command essentially checks
this, i.e. the syntax, that the zoom level is between 0 and "
levels-1", and that the row/col information is within the
boundaries for the zoom level, i.e. 0 ... "[tiles
$zoom]-1".
- ::map::slippy geo 2tile geo
- Converts a geographical location at a zoom level
(geo, a list containing zoom level, latitude, and longitude, in
this order) to a tile identifier (list containing zoom level, row, and
column) at that level. The tile identifier uses pure integer numbers for
the tile coordinates, for all geographic coordinates mapping to that
tile.
- ::map::slippy geo 2tile.float geo
- Converts a geographical location at a zoom level
(geo, a list containing zoom level, latitude, and longitude, in
this order) to a tile identifier (list containing zoom level, row, and
column) at that level. The tile identifier uses floating point numbers for
the tile coordinates, representing not only the tile the geographic
coordinates map to, but also the fractional location inside of that
tile.
- ::map::slippy geo 2point geo
- Converts a geographical location at a zoom level
(geo, a list containing zoom level, latitude, and longitude, in
this order) to a pixel position (list containing zoom level, y, and x) at
that level.
- ::map::slippy tile 2geo tile
- Converts a tile identifier at a zoom level (tile,
list containing zoom level, row, and column) to a geographical location
(list containing zoom level, latitude, and longitude, in this order) at
that level.
- ::map::slippy tile 2point tile
- Converts a tile identifier at a zoom level (tile, a
list containing zoom level, row, and column, in this order) to a pixel
position (list containing zoom level, y, and x) at that level.
- ::map::slippy point 2geo point
- Converts a pixel position at a zoom level (point,
list containing zoom level, y, and x) to a geographical location (list
containing zoom level, latitude, and longitude, in this order) at that
level.
- ::map::slippy point 2tile point
- Converts a pixel position at a zoom level (point, a
list containing zoom level, y, and x, in this order) to a tile identifier
(list containing zoom level, row, and column) at that level.
COORDINATE SYSTEMS¶
The commands of this package operate on three distinct coordinate systems, which
are explained below.
GEOGRAPHIC¶
Geographical coordinates are represented by
Latitude and
Longitude, each of which is measured in degrees, as they are
essentially angles.
Zero longitude is the
Greenwich meridian, with positive values
going
east, and negative values going
west, for a total range of
+/- 180 degrees. Note that +180 and -180 longitude are the same
meridian, opposite to greenwich.
zero latitude the
Equator, with positive values going
north
and negative values going
south. While the true range is +/- 90 degrees
the projection used by the package requires us to cap the range at +/-
85.05112877983284 degrees. This means that north and south pole are not
representable and not part of any map.
TILES¶
While
Geographical coordinates of the previous section are independent of
zoom level the
tile coordinates are not.
Generally the integer part of tile coordinates represent the row and column
number of the tile in question, wheras the fractional parts signal how far
inside the tile the location in question is, with pure integer coordinates (no
fractional part) representing the upper left corner of the tile.
The zero point of the map is at the upper left corner, regardless of zoom level,
with larger coordinates going right (east) and down (south), and smaller
coordinates going left (west) and up (north). Again regardless of zxoom level.
Negative tile coordinates are not allowed.
At zoom level 0 the whole map is represented by a single, putting the geographic
zero at 1/2, 1/2 of tile coordinates, and the range of tile coordinates as
[0...1].
To go from a zoom level N to the next deeper level N+1 each tile of level N is
split into its four quadrants, which then are the tiles of level N+1.
This means that at zoom level N the map is sliced (horizontally and vertically)
into 2^N stripes, for a total of 4^N tiles, with tile coordinates ranging from
0 to 2^N+1.
PIXELS/POINTS¶
pixel coordinates, also called
point coordinates are in essence
tile coordinates scaled by the size of the image representing a tile.
This tile size currently has a fixed value,
256.
REFERENCES¶
- [1]
- http://wiki.openstreetmap.org/wiki/Main_Page
KEYWORDS¶
geodesy, geography, latitute, location, longitude, map, slippy, zoom