How to deal with the slack in the system transitioning to rappel from bolted anchors at foot level on the edge?

The situation arises when there are bolted anchors at foot level on the edge of the cliff:

The issue is that if you have set up the anchor off a cordelette (or a sling) as in...

and you clip a dynamic rope to this anchor, and then set up the rappelling device (extended ATC with Prusik knot)

you may very well find yourself hanging from some 4 to 5 feet of slack.

If you can't set up a static rope in the back of the ledge (say, off a tree or boulder) to lower yourself smoothly until the rappel device comes under tension, I only see two solutions to avoid downclimbing on a slack rappel setup with a possible nasty slip:

1. Set up a static rope off one of the bolts (using one of the locking biners already in use for the main anchor), and use a grigri to lower yourself until the Prusik engages the ATC and makes the rappel device taut. This would entail connecting a dynamic system and the static rope to a single biner, which is probably not a good idea.

2. Perhaps using a personal anchor system. And this is the question... Good idea? Can the Petzl device below (with progressive lengthening options) avoid a bad fall with a huge fall factor? How can the PAS be disengaged after placing the rappel system under tension?

Like this...

... using the Petzl personal anchor system:

I've also found that in these circumstances it's important to make sure the descender-and-rope system is in the correct orientation for rappelling once you make the transition from facing OUT at the top of the cliff (above the anchor) to facing IN during rappel.

Rapping off some trees, especially if they are growing on a very steep face without a ledge to stand on below, presents the same issue.

In the pictured situation, I usually sit down, clipped in as "tightly" as is possible/reasonable; a PAS is good because you can tighten it down as you "slither" around to get "in front" of the bolts, then clip into the rap.

If the PAS is clipped into one bolt, or into a separate sling through both bolts, I don't see the issue, unless there is an immediate overhang below. You may need a shorter sling between your harness and the descender, but you should be able to get up high enough to unclip. (Rapping directly off the harness' belay loop, or from it with a very short sling, makes it easier to get higher.) 

If the bolts are at the ledge,  would it be necessary to go above them? Using a personal anchor, to secure a hanging stance, all that work as well as cleaning the anchor and rappelling can be done with bolts at eye level. Otherwise a "sit n spin" would be in order I think. 

Sorry I didn't read your post. I'm to enamored by your art work! That's a friggin masterpiece! What medium is that? Microsoft paint?

Mongoclimb wrote: If the bolts are at the ledge,  would it be necessary to go above them? Using a personal anchor, to secure a hanging stance, all that work as well as cleaning the anchor and rappelling can be done with bolts at eye level. Otherwise a "sit n spin" would be in order I think. 

I think they're asking about hanging a TR from a top-access point, rather than leading up to it.

I would usually just downclimb while pulling on whatever is through the bolts as necessary (anchor, pas or rap setup) - assuming that they are good bolts and that you're not going to load the bolts poorly in the event of a fall.   If you're worried about falling on static rope/slings then either tie in with the rope as your shortest "pas" style connection, or use a more dynamic pas like the Petzl you linked.  I have a buddy that has that one and likes it for whatever that is worth, personally I haven't used it.  Now to answer the more specific questions:

Running a second rap off the same bolts because your first rap is set up extended begs the question why are you extending your first rap.  It's not the end of the world to not extend your rap if it's going to make you uncomfortable.  You're also going to have a lot of things going through your bolts now? A PAS, a static line, the rope, and maybe even your TR anchor too?  In addition this sounds overly complicated, and complicating simple things leaves room for errors.

Yes of course you can use a PAS and a PAS made of dynamic material will be better in the event of a fall.  To transition between the two - pull on the rope and cinch up your atc to load your rap line until there is no weight on the PAS then unclip it.  It's always good practice to load your rappel device before unclipping or untying anyway so this is good to build into your routine.

Did you read this very recent thread?  Not exactly the situation you're asking about, but it has much good info.

https://www.mountainproject.com/forum/topic/117038007/how-to-get-on-rappel-from-tr-anchor

Given that there is no anchor point higher than the bolts, I think what follows is the best you can do if you don't just want to grab the anchor and lower yourself over after doing Step 1 below.

1. Install your rap device below power point and tie it off releasably.  (When you have weighted the device and are on rappel, you want to be hands-free.)

2. Tie a catastrophe knot (overhand on a bight) on the brake strands below the device and clip the loop to your harness belay loop.

3. Put a suitable carabiner on one of the bolts, take the strands below the catastrophe knot and clip them to the carabiner with a munter hitch.

4. Grovelling on your stomach if you feel the need, lower your body over the edge, controlling it with the strands to the Munter hitch, until you have fully weighted the tied-off rappel device.  

5. Remove/untie all the extra stuff, release the rap device tie-off, and rappel.

rgold's method is about the easiest I can think of if you insist on the extended rappel situation, I think you could use your prussic instead of the overhand to hold the end of your lowering strand as well... I'd personally not extend this one just because I think that the extra faffing around is probably more unsafe than just grabbing the quad and lowering myself down... maybe clip into the quad with a locker if you want to shorten the whole thing up and you don't trust your tied off ATC.