A few other things to note. By basing the transitions on an ellipse, you'll need to be very careful and consistent with the placement of the edge pixels, or you'll end up with a bunch of tiles that only work together as, well, an ellipse. I think the smart way to go right now would be to keep the transition edges at the exact center of each tile, whether vertically or horizontally (or both, in the case of corner/diagonals), and nail the four diagonals (eight, if you count dirt-to-grass) and four cardinals. What I did with my own transition tiles was to place the edge transition pixels, and then just freeform it from there, instead of sticking to straight lines. As long as the edges line up, the center portions can be as messy as you like.
On a related note, right now you're trying a little too hard to stick to the elliptical guideline. Even if you go with the ellipse, I would advise against sticking so closely, since right now it looks a little unnatural and landscaped. Allow for some variation, and more importantly, allow for some transition space, so you don't go from grass straight to dirt. If you look observe grassy areas, or look at photos of such, you'll never see such a hard transition. Thin out the grass as you approach the dirt, or you'll end up with ground that looks heavily landscaped.