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Using Overlays with WXtrack and SatSignal
WXtrack can generate, and SatSignal can use overlay files, which are bitmaps where the colours (more specifically, the palette indices) represent land, sea, country or state boundaries. You could use any program to generate the overlays for SatSignal, or use the overlays generated by WXtrack with any other program you like to create the composite image.
Generating the Master Overlay
WXtrack checks for the master overlay file when it is first run. The master overlay file is a large bitmap which will occupy some 30MB of disk space. You can download the master overlay file from WXtrack Web page (look under Satellite Tools, WXtrack, Optional items, Pre-compiled overlay bitmap, or you can download the tool to create the master overlay yourself from the Mapping tools Web page. (look under Mapping Tools)
There is nothing to stop you editing the master overlay, putting a mark where you live, for example, but please do not alter the palette or SatSignal may not overlay the data correctly.
Generating an overlay for a pass
In WXtrack, simply generate a ground path as normal, and ensure the Save Pass Details option is checked. A number of files will be created in the program's results folder, named according to the usual conventions - e.g. 10252215ovl.png. You have a choice of Windows bitmap (BMP) or the much more compact Portable Network Graphics (PNG) forat for the overlay image. You can choose where the results are saved on the Setup tab. You will also get a file named 10252215pass.dat, which has the satellite azimuth and elevation for each scan line of the pass. Both of these files are transient, and can be deleted when you have done with them. Note that the overlay image will appear as if the pass has been generated in the southbound direction, i.e. northbound passes will appear upside down in the overlay image. This is normal.
Because of the way the data is generated, near the poles the grid lines may appear rather thin, and then peter out. This is normal.
Removing the borders
Les Hamilton notes: To remove the borders, edit the map from Wxtrack in Paintshop Pro to define the border colour and the grid colours to the same blue as the sea. Then save the palette. For subsequent maps, just load this palette, and the borders disappear instantly.
Overlay Text
WXtrack can generate a text file containing a summary of the pass details. Which elements are stored is controlled by the Options, Text overlay... menu. The file name will be 10252215ovl.txt, in the example quoted above, and is created in the results folder.
Using the overlay data
Within SatSignal, using the overlay data is automatic. The overlay filename must match the pass filename being processed, so if you had started recording the pass mentioned above, but a minute late, then your image file would be called 10252216.png (or whatever), so that you would need to rename the overlay file to 10252216ovl.png. Do not move the file from WXtrack's executable folder - SatSignal knows to look for the file in that location. If you have all the pass, there is no need to rename files. (It will be worth setting the minimum elevation in WXtrack to match your system, or your system clock, if you are continually getting mismatched filenames).
WXtrack requirements
Overlays cannot be generated without the master overlay bitmap, and that cannot be generated until the separate Countries.dat is downloaded.
SatSignal Requirements
If you use another program to generate overlays, please note that only 256-colour paletted images, 1040 pixels wide, are accepted.
Presently only NOAA satellites are supported.
The registered version of SatSignal is required.
Program versions
WXtrack 2.0.0 or higher is required for overlay generation.
SatSignal 3.1.0 or higher is required for overlay support.