.rigid_spline files are used by NTW and ETW to determine borders, rivers and roads. On request, I wrote a simple tool that converts these files to and from text files that you can edit a bit more easily. More specifically, a spline in ETW/NTW language is a line from a starting point, to an end point, going through a variable number of intermediate points, with straight connections between each point and the next being drawn.
I'm sorry that I can't provide anything more sophisticated right now but I've got enough things to eat my time and this is far from being a high priority thing for me.
To convert splines, install python 3.0, make a new folder, copy all splines you want to convert into it, extract the archive to the same folder, then run "convert_all_splines_to_text.bat". This will output a text file for each spline which you can edit however you wish.
You can add new points by adding lines. The numLines entry is just an FYI (not used when reconverting). To reconvert, simply rum "convert_all_text_to_splines.bat". This will overwrite any existing files so be careful. Then put them in the game borders folder and see what happens.
Changelog
Usage Example
0. Make sure you have Python 3.x installed. You can get it at http://www.python.org. If unsure, simply download and install the newest version. You also have to add python to your PATH environment variable for this to work correctly. To do this, open the Control Panel, then System, navigate to the Advanced tab and click on Environment Variables. Look for the one called PATH and add your python installation directory to it, separated with a semi-colon ";" from the previous entry (for me it's "C:\Python31")
1. Create a working directory somewhere, paste the border files into it, then extract the attached rar archive into it.
2. Navigate to your NTW/data/campaign_maps/nap_europe/display/borders directory. Select and copy the rigid_spline files you want to edit (for example eur_scotland_1.rigid_spline). Make sure you create a backup of the files you're planning to change in case you screw something up.
3. Run "convert_all_splines_to_text.bat" to convert all .rigid_spline files to text files.
4. Open the file you want to edit first with a text editor, notepad for example, and do any changes you like. The first few lines contain some header entries of which I'm not sure what they do, so don't edit them.
The rest of the entries consists of 3d points: three coordinates for each point of the spline. The first coordinate is the horizontal east-west coordinate (east is positive, west is negative) - I'm not sure in what units. The third coordinate is the north-south coordinate (north is positive). The second coordinate is probably the height above ground: it seems to be the same for all borders - but I don't think it's used at all.
You don't have to update numEntries if you add or remove lines, the converter will recalculate it automatically and it's just there for your information.
5. Once you're done with your changes, save the file. Then run "convert_all_text_to_splines.bat". This will overwrite your old spline file with your changes (you can verify this by the checking the timestamp changes when running the converter).
6. That's it, copy the file back to your borders directory and have a look at the results in your game. I added a point before the first in Scotland's border with some more exotic coordinates and this is the result:
Have fun!