map::slippy - Mapping utilities

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# map::slippy(n) 0.5 tcllib "Mapping utilities"

## 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 ?0.5?

- ::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
- ::map::slippy fit geobox canvdim geobox zmin zmax

## 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.

::map::slippy fit geobox canvdim geobox zmin zmax

  Calculates the zoom level (whithin the bounds zmin and zmax) such that geobox (a 4-element list containing the latitudes and longitudes lat0, lat1, lon0 and lon1 of a geo box, in this order) fits into a viewport given by canvdim, a 2-element list containing the width and height of the viewport, in this order.

## 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