jtanabodee I don't understand, you say you create the taxiways, turn to a polygon, create the line again and re-draw over it?
What's "create shape"?
If I turn the spline to poly, I'll lost the spline. I turn it in to polygon and select the edge just where I want and make spline again by "create shape".
It time for mini tutorial here.
1. I draw a rectangle to be runway here and another spline to represent taxiway.
2.Then I turn it to be editable spline and attache them together. In the picture I have added the taxiway on the south of this runway too. I do the boolean on the runway and taxiway to joint them together. In this pic, I combined the south taxiway already. But the north one is still another spline.
3. Now I got the runway and taxiway seamlessly together.
4. I turn this into polygon. I can apply asphalt texture (if they are the same material, or else you need to separate them.)
5.Now it's time for making edge. You can select the edge of the polygon (1) here where you need the edge of the runway taxiway(2). Note that I don't select the end of the runway. Then click on Create Shape button (3). The dialog box will show up let you choose between smooth and linear. Please choose linear. Now you got the line for lofting to be the edge of taxiway. If there are two splines you need to detach them to be single line (yellow arrow).
6. Now do the lofting as usual. The template of the lofting is the same as making lines but you need to move the center to the either end of the spline that you draw.
Sometime the edge seem to be inward (protrude into taxiway). To correct this you just assign the last vertex to be the first one by clicking on selected vertex and use "Make First" button. You will get the right oriented edge loft.
7. You can apply all the textures now.
Easy, don't you think. You don't need to draw line twice. You got the edge and taxiway runway at the same time. Moreover they will be seamlessly and fit together, no doubt about it.
I have been thinking about making this quite a long time and I feel confident enough to share this method to the others.
For big airport, it is not easy to make. But I think I got two birds in one shot by this method.
The taxiways and runways in my WMKK project are seamless by this way.
I took me an hour to make this mini tutorial. Hope you get what I tried to say.