Hi again
I've just been through the source code to work out the file format. It actually turned out to be rather simple - just a series of segments made up of 2 byte numbers. Each starts with the number of points in the segment followed by the lons then the lats for that segment. I've generated high res binary files based on the coastline files from the ascii CIA files. If anyone else would like the files then let me know.
Andrew - I think that although the data may have come from the CIA files, I don't think plPlot uses the original format. The ascii files have a much higher resolution than the two byte plPlot files which results in some rather unpleasant aliasing. I've put an example at http://homepages.see.leeds.ac.uk/~earpros/plplottest/trackplot.png showing the SE of the UK. It is still much better than the previous results, but maybe if the devs would consider allowing a 3 or 4 byte per point format the results would be much improved.

From: phil rosenberg <philip_rosenberg@yahoo.com>
To: Andrew Roach <aroach.geo@yahoo.com>
Sent: Thursday, 13 September 2012, 16:04
Subject: Re: [Plplot-devel] map resolution

Unfortunately after a significant amount of searching i've only been able to find ascii versions of the CIA maps (which were on the page you suggested Andrew). The maps with plPlot are binary so these don't work.
If anyone has copies of more detailed maps then I'd appreciate it if you'd let me know. At the moment I'm going to have to read in and plot the ascii maps as a line plot, but I won't end up with the useful features of the plmaps routine.

From: Andrew Roach <aroach.geo@yahoo.com>
To: phil rosenberg <philip_rosenberg@yahoo.com>
Sent: Thursday, 13 September 2012, 12:13
Subject: Re: [Plplot-devel] map resolution

At 03:53 AM 13/09/2012 -0700, you wrote:

> Hi all
> I was just wondering what the format of the plPlot map files is? The maps included in plPlot are rather low resolution so wondered if it was possible for me to download more detailed ones form somewhere?
> Phil

It was the CIA World DataBank map format. Many years ago I did use higher res versions for something or other - they are out there if you look. http://www.evl.uic.edu/pape/data/WDB/ might be some from what I can see.