Overlay Header
Absolute Coordinates
If the overlay is defined in absolute coordinates, all points in the overlays are given in terms of a longitude, latitude coordinate pair. This sets an absolute position for each point.
The following header commands must be included at the top of the overlay file:
| Command | Description |
|---|---|
| PROJECTION latlon | Informs system overlay is in absolute coordinates. |
| DRAW_CENTER lon lat |
Tells Overlay where to center the image. Generally this is the either the coordinates of the radar site, or a nearby special area of interest such as an airport. |
| FILL_CENTER lon lat | Specifies center of fill area. Typically this is the center of the array of data points you have generated. |
| FILL_RANGE range |
Sets the maximum range that the underlay covers, in km. Overlay draws a circle at this distance from the fill center before filling the underlay. |
Relative Coordinates
If the overlay is defined in relative coordinates, all points are defined in terms of map units. Map units are any arbitrary unit of measure that is E/W and N/S of an origin (0,0), which can be anywhere on the map.
Often millimeters from graph paper are used to designate the points on a map. The map must be in azimuthal equidistant projection, with the specified reference point. A scale factor specifies the conversion between these map units and distance on the surface of the Earth.
The following header commands must be included at the top of the overlay file:
| Command | Description |
|---|---|
| PROJECTION aed | Informs system overlay is in relative coordinates. |
| SCALE scale |
Sets the scale factor in km/map unit. The SCALE n command must be used to specify how many kilometers each map unit specifies. For example, if a RADAR 100 200 and a SCALE 0.5 command are used, then if later a point is specified as POINT 110 200, this point is exactly 5 km due east of the reference position. |
| RADAR x y | Sets the projection point in map units and specifies the location of the reference point in the map units. |
| REFERENCE lon lat | Sets the projection point in absolute coordinates. |
| DRAW_CENTER lon lat | Same as latlon projection. |
| FILL_CENTER lon lat | Same as latlon projection. |
| FILL_RANGE range | Same as latlon projection. |
For backwards compatibility, the old MIDDLE command still has the meaning of setting both the draw center and the fill center.