This makes me uninstall Geocart 3!

[Atarimaster]

The extension is .txt and name ends on metric.txt so I don't see what's wrong.

And there is a dot right before the "metric"?
For example, the file must be named greatcircle.metric.txt, not greatcirclemetric.txt

[Piotr]

Added a dot and it works! Yay!

[Piotr]

The final thing I did after adding the database was to set the great circle to favored color, alpha and thickness, and add 0 and 180 degree meridians to excluded.

[Piotr]

My guess is that Geocart, when it checks the Graticule dialog input values for errors, regards this as a mistake – »Why add a meridian that’s already there?«
Furtherly, I guess that your idea is to use a different color (via Line Styles dialog) for a certain meridian that’s within the defined meridian spacing.

If that is the case, a workaround would be to list the meridian in question to the "excluded meridian" list, and then add a meridian that’s only a little bit off; e.g. put 15°W to the excluded meridians, and 15°0'0.0001"W to the additional meridians.

Let's see what happens.
Additional: 179°59'59.99"W
Excluded: 180°W; 180°E
Ouch! On equatorial Azimuthal Equidistant only the left half of the circle has red written over it as it wraps from top to bottom. Maybe adding another additional meridian will fix it?
Additional: 179°59'59.99"W; 179°59'59.99"E
Excluded: 180°W; 180°E
Ouch! The transparency is incorrect, it should be 170. Maybe for transparency, let's use the color scaled value of 0-(sqrt(1/3)), resulting in 108?

Ouch! THERE IS A ****** S*UP*DITY THAT SAYS "YOU'VE CHOSEN DIFFERENT TRANSPARENCY (ALPHA) VALUES FOR DIFFERENT PARTS OF THE GRATICULE!!! THE ALPHA FROM THE ROOT OF THE GRATICULE WILL BE USED YOU ****** WHO TRIES TO CUSTOMIZE GEOCART!!!!!!!!"

[Atarimaster]

Let's see what happens.
Additional: 179°59'59.99"W
Excluded: 180°W; 180°E
Ouch! On equatorial Azimuthal Equidistant only the left half of the circle has red written over it as it wraps from top to bottom.

This is no »Ouch«, this is Geocart doing what you told it to do!
Well, yes, okay, it IS »Ouch« but only because you forgot to do something.

[a lot of screaming]

Geocart is putting all the lines that a drawn according to the setting of the »Graticule« dialogue to one layer. (Which is, in my opinion, actually a good idea when you post-process the PDFs exported from Geocart in an application like Illustrator, Affinity Designer etc.)
The Alpha is applied to this very layer.
I don’t think it’d be a good idea to apply an alpha value to the lines themselves: A line that has an alpha value of 108 and that is with a layer with Alpha = 170 – well, what will it look like in the finished map?

Just like before, the solution is to use a metric lines database.
They will be put to their own layer, enabling you to select an alpha of your choice.


And by the way… I’m no moderator here, so I can’t tell you how to write your postings.
But, if you want help, then I think it’d be better to try to phrase your questions in a halfway respectful way.

[Piotr]

I don't intend the lines to look ugly — ironically, I want it the other way around — cause 108 and 108 alpha combine to 170, right?