WorldFile explained (or at least we tried!)
There is a lot of confusion around about what world file represents therefore we thought it would have been nice to put together a couple of clarifications.
The World File
World File is a de-facto standard from early days of ESRI desktop GIS applications. Originally it was composed by at least two files, a raster file, let’ say a file.tiff, and another file (most part of the times called file.wld) that contains numbers that define the affine transformation between the image and the target spatial reference system. This implicitly means that we are talking about georectified raster data. This wld file is what should be properly called the world file. Other extensions that could be used on behalf of tfw are wld tifw, etc… Moreover some other formats beside tif can be used, like gif, png, jpeg, jpeg2000, etc..and the extensions for the world file become pretty fancy (gifw, pgw, etc..).
The concept behind this couple is as follows. An image (e.g. a tif) plus its world file represent a geospatial georectified raster in some spatial coordinate reference system (CRS). The mapping between the raster space (the intrinsic Image space of rows and cols) and the associated model space is represented by an affine transformation whose coeffficients are contained inside the accompanying world file (see here ). This way, practically any image format can be made spatial-aware just adding a world file containing an affine transform that maps image points onto a crs. Here is a good explanation of what a world file contains.
The Projection File
One would now ask, ok, but what coordinate reference system I am supposed to use. The best answer would be “the one the person who produces the image + the world file was using”! Seriously many people make assumptions on the CRS used for the georeferencing affine transform contained in the world file, but in fact this can lead to all sort of problems. The best way to transmit the information about the spatial coordinate reference system is providing a third file with extension prj which contains the well known text (WKT more info can be found here) representation of the user spatial CRS. The Spatial Reference website can be extremely useful for associating the proper WKT to a certain CRS.
Notice that usually the raster space has x going right and y going down while in model space things DEPEND on the spatial CRS, specifically, axes order and axes directions!
ALWAYS ASK YOUR DATA PROVIDER TO DESCRIBE EXACTLY THE SPATIAL COORDINATE REFERENCE SYSTEM THAT WAS USED TO GEOREFERENCE YOUR DATA.
The GeoSolutions Team,