getTiles: Get tiles from a tile server

Description Usage Arguments Details Value Author(s) Examples

View source: R/getTiles.R

Description

These are functions for loading and displaying map tiles from a tile server or a local cache of tiles.

Usage

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getTiles(xlim,ylim,zoom,path,maxTiles=16,cacheDir=tempdir(),timeOut=5*24*60*60,verbose=FALSE)
getTilePaths(xlim,ylim,zoom,path)
nTiles(xlim,ylim,zoom)
tileBbox(x)
## S3 method for class 'tiles'
print(x,...)
## S3 method for class 'tiles'
image(x,...)

Arguments

xlim

Length-2 vector of min and max X coordinate (latitude)

ylim

Length-2 vector of min and max Y coordinate (longitude)

path

URL of root directory of tile server, or file path to a local tile store.

zoom

Zoom level

maxTiles

Limit on the number of tiles to get, to avoid sending too many requests accidentally.

cacheDir

A directory for caching tiles retrieved over the web. Not used if tiles are coming from a local directory.

timeOut

Tiles in the cache directory older than this (in minutes) are re-retrieved.

verbose

If TRUE, give information on where tiles are coming from, cache hits, etc

x

A tiles object

...

Extra arguments passed to image or print

Details

These functions download, display, and manipulate map tiles stored in a standard way either on a web server or a local folder.

Map tiles are a set of PNG images that span the world at a set of zoom levels. Zoom level 0 has one tile that covers the whole world at a resolution of 256x256 pixels. Zoom level 1 has four 256x256 pixel tiles in a 2x2 pattern over the whole world. In general, zoom level n has 2^n by 2^n tiles. Zoom levels go up to about 17 or 18 depending on the tile server.

The function getTiles() retrieves tiles to cover a given latitude and longitude range at a given zoom level. Note that since tiles are generated at a fixed set of lat-long ranges the returned tiles will overlap the requested area. There may be some way of clipping to the requested area in a future revision.

The return value from getTiles() is an object of class "tiles", which is basically a list of SpatialGridDataFrame objects with three bands (for RGB colour).

The image() method for tiles objects creates a blank plot and draws the tiles. It can be given the parameter 'add=TRUE' to overlay them on an existing plot. Transparency is currently not supported.

The nTiles() function returns the number of tiles needed to cover the given area at the given zoom level. Use it to check how many tiles you'll need to get before getting them. It does not need a source of tiles since this can be computed from the tile geometry.

The getTilePaths() function returns information about tiles in a region at a zoom level. The tile source path is given but tiles are not requested. A list is returned with one element per tile giving the path, the I and J coordinates, the zoom level, and the geographic bounds for the tile.

The tileBbox() function returns the bounds of the tiles by simply constructing the bounding box of all the constitutent SpatialGridDataFrame objects.

http://wiki.openstreetmap.org/index.php/Tile_usage_policy

Value

Various

Author(s)

Barry Rowlingson

Examples

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## how many tiles cover the world at zoom 17?
nTiles(c(-180,180),c(-89,89),17)

## realistic example:
nTiles(c(-2.842,-2.7579),c(54.0295,54.063),12)

## tile Info
tileInfo = getTilePaths(c(-2.842,-2.7579),c(54.0295,54.063),12,"http://server/")
tileInfo[[1]]


lancs = getTiles(c(-2.842,-2.7579),c(54.0295,54.063),12,path="http://tile.openstreetmap.org/")


image(lancs)

webmaps documentation built on May 31, 2017, 2:24 a.m.