The problem with semi-transparent geometry is that, unless you depth-sort it every frame to make sure that the closest faces to the camera are drawn first, then you will get artifacts due to the alpha blending. A cheap trick to take care of these artifacts somewhat is to disable the depth mask when drawing your semi-transparent model:

First off thanks for your all of your help!

Is there another way to depth sort things? I've disabled the depth mask, however the problem I'm getting is that it doesn't seem to depth sort by vertex, but it depth sorts by GLModel's overall position (or something similar). So I'm getting this:

the geography GLModel has a slightly larger radius here to show the sphere (which is inside the geography, consdering the cartesian coordinates of all of the vertices) is blocking everything inside, in front of and behind it. You can then see a ring of unobscured geography lines around the outside. I should be seeing the geography on the back side of the planet, but much more faintly, while the geography on the front should be solid untransparent white.

Is there a way around this problem? The red dots, the geography and the sphere are all GLModels, but the meridian lines are drawn with arc().

Thanks!

Do you want the entire surface of the earth globe to be semi-transparent, or only the area that corresponds to the oceans?

What I want to do is hopefully quite simple. The geography lines are completely opaque, but I want this 'sphere' in the middle to be semi-transparent so that all of the lines on the far side of the earth appear faded out. Would it be better to test the position of each vertex for whether it is on the front or the back of the planet and then change its colour correspondingly, or is there a way to properly depth sort every vertex?

Thanks!