I think I'm missing something here. When I try to dimension
an ellipse sketch it stays blue and refuses to be defined. I can
dimension its center to the origin and its width and height but it
stays blue. Thanks for the help.
You must constrain the center AND all 4 points of the
ellipse. There are various ways to do this. A simple way is to
constrain the center. Add a horizontal constraint on the ellipse to
the origin of the sketch. Then, add a vertical constraint on the
ellipse to the origin of the sketch. Add a vertical and horizontal
dimension and you should be good to go.
I'm not sure that Tony's explanation was clear. When you have your
sketch dimensioned the way you think that it should be fully
defined, try dragging one of the four points on the ellipse. The
ellipse should rotate around the center. That's why it isn't fully
defined. You need to either constrain one of the points to be
horizontal or vertical with the center, or you need to dimension
the angle that you want one of the axes to be at.