Special data fields are defined to enable the satellite images to be plotted in the MapServer interface including the following: x_pixel_unit (pixel size in the X direction in map units per pixel), y_pixel_unit (pixel size in the Y direction, which is usually negative map units per pixel), x_correction (rotation about the X axis in degrees), y_correction (rotation about the Y axis in degrees), x_coordinate (X coordinate of the center of the upper left pixel in longitude degrees), and y_coordinate (Y coordinate of the center of the upper left pixel in latitude degrees). These values are made accessible to the MapServer by including them as part of the data object. These six data fields correspond to the six lines in the so-called world file.
From Wikipedia (http://en.wikipedia.org/wiki/World_file), "a world file is a plain text computer data file used by geographic information systems (GIS) to georeference raster map images. The file specification was introduced by Esri.
Small-scale rectangular raster image maps can have an associated world file for GIS map software that describes the location, scale and rotation of the map. These world files are six-line files with decimal numbers on each line.
World files do not specify a coordinate system; this information is generally stored somewhere else in the raster file itself or in another companion file, e.g. Esri's .prj file. The generic meaning of world file parameters are:
- Line 1: A: pixel size in the x-direction in map units/pixel
- Line 2: D: rotation about y-axis
- Line 3: B: rotation about x-axis
- Line 4: E: pixel size in the y-direction in map units, almost always negative
- Line 5: C: x-coordinate of the center of the upper left pixel
- Line 6: F: y-coordinate of the center of the upper left pixel
This description is however misleading in that the D and B rotation parameters are not really rotations (in degrees or gradients) and in that as soon as D or B are not zero, the A and E parameters do not correspond to the pixel size anymore."
The data object is served with two columns representing the satellite image. The clickable link in the "time" column links to the satellite image that includes a color bar and a clickable link to the actual digital values of the image. This image undergoes a transformation during the serving to include, among other things, the color bar. The overlay image is another representation of the satellite image but this one is used by the MapServer in order to create the overlay image of the satellite image on the display map. This image also undergoes a transformation (by a different procedure) to enable it to be transparent among other changes.
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