His directions are in part to explain a route for the police to follow, so he uses streets directions. I think "down" is not south, but "he's running down the street" (that eventually winds to the back entrance).

(I doubt that from his car, in the dark and possible rain that GZ saw TM turn south off the cut thru path.)  I think there are pools of light and deep shadow.

You think Zimmerman was in his car when Martin turned south into the dog walk?

I put him at I23, about a hundred feet behind Martin. The time would be 7:12, give or take a few seconds.

I don't know what to think about the lighting. I'm not seeing anything on the aerial photo that suggests street lighting.

Martin being in motion would up his visibility.

If Zimmerman reached the T not knowing which way Martin went, I think he would stop there to look around. But his running seems to take him another fifty feet past that point, in whichever direction.

If he went south after Martin, there's the foot chase. Martin may have answered his phone as late as 7:12:59, so there's plenty of time.

Here's some speculation that's just come together for me. When the dispatcher gave Zimmerman the 'we don't need' line, it may have served to remind him that following a suspect is dangerous. But Martin was in view, a hundred feet in front of him and running away. So he decided to halfway take the dispatcher's advice, by continuing to follow until Martin was in any way lost to sight. Then he would turn back, rather than risk an ambush by going to the place of last sighting.

Martin could have escaped Zimmerman's sight by cutting between any pair of houses, or going on past Brandy Green's building and turning either way.  

returning west, he is able to backlight TM against someone's porch light by virtue of his tracking eye line.

I was never in the camp that thought Martin choosing not to go home was damning in itself. I should have made that clearer in some of our more recent discussion.

If Martin saw Zimmerman go east on the crosswalk, and wanted to avoid him, I think he would take this opportunity to get further away. Or he could at least find a better hiding place between the houses.

Austin Brown, the teenage dog walker, has said 'Only one porch light was on.' In front of that one porch light might be the worst possible hiding place.

going west it's easy to spot the youth, who has not moved  - maybe he's under a patio or a tree to keep himself dry from the rain.

I don't think there was a downpour at this time. Zimmerman is still on the police call, and there's no striking change in the ambient sound to suggest such a thing. The Teacher in her first CNN interview said that when she first heard loud voices 'I had just shut my window because it had just started pouring out rain.' That suggests a lull before that time.

He looked down the  dimly lit sidewalk and didn't see the kid running there, so he continues east to Retreat view and looks N and south.  The nearest option from there is the corner to his north.  Once he's done that, in order to eliminate other possibilities he needs to go back to his truck after that, and drive around.

He gave adequate direction to his vehicle already

Z: And then they go past the mailboxes. They'll see my truck.
D: What address are you parked in front of?
Z: I don't know. It's a cut-through, so I don't know the address.
D: OK, do you live in the area?
Z: Yeah, yeah. [Crosstalk]
D: What's your apartment number?
Z: It's a home. It's one nine five zero - oh, cr*p. I don't want to give it all out. I don't know where this kid is.
D: OK, do you want to just meet with them right near the mailboxes then?
Z: Yeah, that's fine.
D: Alright, George, I'll let them know to meet you [unintelligible].
Z: Actually, could you have them call me and I'll tell them where I'm at?

The dispatcher has so despaired of Zimmerman's directions that he's ready to send the officers to any address 'in the area'. When that doesn't work, it's the dispatcher who suggests the mailboxes as Plan C.

Zimmerman clearly doesn't like the mailbox suggestion, and grudgingly agrees because at first he doesn't have a better idea. Why is he so unhappy? My only explanation is that the mailboxes are well to the west, and Zimmerman may have last seen Martin heading east.

Here's another key point. Why was Zimmerman even giving directions to his truck? The phone talk had to be distracting. When the dispatcher asked him where he wanted to meet with the officers, he could have said then that he wanted a call back.

This was about 7:12:40. He could have wrapped up the call by 7:13:00, over half a minute sooner than he actually did.

Why indeed did he not call in a street address in the minutes after he ended the call?

However, later at the police station he may or may not have recalled all that he said to the operator, and so his alibi seems plausible to him.

Zimmerman seems to be one of those naive souls who trust the system.