Base map images for worldKit must be JPEG images. The images can be of any dimension; worldKit will scale them to the specified width and height. Map images can cover the entire Earth, or a few blocks of the neighborhood.
There are four types of map images supported. The type is specified in the <displaytype>
- day: A normal rectangular projection. Specify the url of the map image in the <dayimg> configuration option. If <dayimg> is set to blank, no image is loaded.
- daynight: The current solar illumination of the earth is displayed. This requires both a <dayimg> and <nightimg> specified in the config.xml.
- zoomify: For very large map images, Zoomify tiles the image and only retrieves sections necessary for the current Zoom and Pan. Read more about Zoomify here
- polar: A polar projection of the Earth, centered on the North or South pole. Here is an example. Must specify a <polarimg> configuration option.
The "bounding box" of the map image must be known. This is specified in the configuration options <north>
, and <west>
. By default, worldKit assumes that the map covers the entire Earth (north=90, south=-90, east=180, west=-180). For "polar" maps, it is only necessary to specify the northern or southern extent of the projection.
It is permissible for the bounding box to cross the line of longitude at 180 east, -180 west. If the value of <east> is the same as <west>, then it is assumed that the map crosses this line.
Here are a few pointers for finding basemap images.
is a free service, retrieving maps for any US address, zip code or lat/long, or International city, and providing the N/S/E/W boudning box for that map. Maps are available as aerial photographs (some in high quality color), topographic maps, or street maps.
There are many interesting whole Earth images in Flatplanet Maps
contains a "Zoomify" image of the high resolution Blue Marble satellite image. Read about using Zoomify here
This South Pole Image
can be used with the "polar" projection. North is -60.
Cropping Earth Images
satellite images are stunning, highly detailed whole earth images produced from satellite data. Two Blue Marble images are included with the worldKit download.
One strategy for producing basemaps on the scale of continents or countries is to download a large 8192 by 4096 Blue Marble image (located here
: warning! very large 26.5 MB), crop it to the desired area, and save as a JPEG.
In the cropping, it is necessary to determine the exact pixel bounding box of the crop, in order to set up the N/S/E/W values in configuration. Assuming the origin is in the upper left hand corner, use the following formulas. "Hieght" and "Width" refer to the original image.
North = ( 180 * (Height - TopCrop) / Height ) - 90
South = ( 180 * (Height - BottomCrop) / Height ) - 90
East = 180 - ( 360 * (Width - RightCrop) / Width )
West = 180 - ( 360 * (Width - LeftCrop) / Width )
Another option is to use the <maxzoom> or <initialzoom> options
, to focus a whole Earth image on the area of interest.
Instead of a small shape, it is possible to a small image to mark annotations. Supply the url of an image in the configuration option <icon>
, and that image will be used as a marker. This is especially useful when combined with Categories
. An icon for an individual item can also be specified directly in RSS