First answer is yes, the white lines on the borders define the edge of the track. That's the simple answer.
Now, you could take the south american point of view. Second answer is, well, what happens if some crooked steward decides to repaint subreptitiously the white lines by scurrying along the track in the middle of the night, taking advantage of the new moon, the torrential rain and the bad timing of guard dog patrols and searchlights?
Well, if somebody tries to throw, I don't know, Yuji Ide, your favorite driver, out of boundaries, by repainting the lines arbitrarily, and I were working for that team and I happened to stroll by the track during the rain after the repainting and I noticed the dirty trick, then, after a proper bribe, I would ask stewards to re-repaint the lines using what's called "the reference width".
I've posted insistently over the years, to no avail in view of the disparate comments in this thread, the list of oficial FIA circuits, Class 1 and 1A (that is, "Formula-one-able") specifying that reference width. You can check for the list, is interred in another very "archeology-able" thread, for your delight and entertainement. Of course, nobody read other people posts in this forum, being all of us very occupied writing our own posts.
The smarty pants that abound in the net could say: "Ha! Ciro, how moronic is your answer. Haven't you noticed that the track has not the same witdth along its length? Are you so blind (blind as a moderator! Haha!) or is it the colombian rum you're drinking? Hahaha!"
Well, my skeptical friends, let me assure my vision is far from impaired: on the contrary, old rum has sharpened it to superhuman levels. Yes, I've noticed: the reference width is not mantained through the track. There is a way to calculate what is called the "extra-width" around curves, depending on the angle of the curve, the length of car, and the distance between axles.
Finally, you can always use the plans to the track, that indicate, with millimetric precision, the position of the edge lines. Unfortunately, those plans are interred somewhere in FIA building in Paris, right besides the official FIA/FOTA toilet paper stock, I imagine, and they are not readily available on the Net.
If you have no access to FIA building you could use the lines I've drawn of three tracks only (during the last three years or so). Give me another fifteen years and we're done.
You can check Appendixes N and O to regulations (I think), the ones that describe track homologation, to go into the mesmerizing and dangerous details. Just go into the FIA regulations link in the Home page of this forum (Tomba thinks about everything) and, once in FIA page, then click on Appendixes or Circuit Regulations, I don't remember. There is an entire appendix devoted to how you have to draw the plans, complete from the Autocad Blocks to the line colour to the way to draw the edge lines.
I also posted the list of track regulations somewhere around this site, just in case.
Now, given the coordinates of the edge lines, you could enforce any of your crazy plans about what to do when an infortunate driver goes out of boundaries.
My personal favorite is the crocodrile moat, equipped on the edges with laser beams that blind the driver. That, or the Paul Ricard abbrasive tungsten stripes, I haven't decided yet.
Out of boundaries=Out of tyres - Perfection is everything at Paul Ricard. Those stripes can shred you tyres in no time
The blue ones allow you to limp back to pits. The red ones shred your tyres instantly, throwing you into a wild spin and crushing the car. Nah, just joking.
Rev cuts or tyre shredding are enough? I don't know. If they're not subtle, they're at least equally subtle. The current hot question is what to do: if to use a disintegrator/annihilator raygun (useful in traffic jams, also) or to keep track of GPS location and give a pit penalty to the driver. Which one looks better on TV?
Nothing to do with whether it works or not, just a mental link between a stretched flexible surface and bouncing.