Driving mask stroke with gradient

Ok, so I think I have this working now. I tried a few of the suggestions but couldn't achieve the desired effect, so I've reverted to my original idea, yesit's slow, but it works.

Auto Trace the rendered RGB matte to get the mask outlines

Convert these masks to nested shapes with redefinery maskstoshapes script and add a thin stroke

Explode to individual shapes with ESL3 script (this takes a while)
copy the scripted trim paths effect to all these shapes, referencing an expanding radial gradient layer
pre-comp these to a 3D layer, add camera and zoom out following the line expansions, I'm going to add layer style stroke and turbulant displace/roughen edges on to these 3D layers as it will be easier to mass-control the effects for each element

Additionall the 3D comp is 8k! as I start very zoomed in. Slow but it works. 

I have often created this kind of write-on effect using an animated gradient as a track matte. If you start the artwork in Illustrator, you can add a gradient to the stroke, set the gradient to start at the first vertex, release everything to layers, import it as a comp, then sequence the layers, add an animated color correction, then use the comp as a track matte for a solid to simulate drawing each shape in order. I don't have time to build an example right now, but this workflow will be faster than trying to sample colors on a hundred layers.

I hope that workflow makes sense to you.

I would do it differently. Apply a uniformely timed stroke to all your shapes, pre-compose and use the Time Displacement effect. That way you can simply control the delay with a greyscale version of your map and some gradients. Using multiple such layers can easily create the illusion of varied timing and the map spreading out from different origin points. It'S still a bit of work, but definitely more controllable and intuiive than trying to use expressions that would slow down things to a crawl. You really have way too many shapes to even dream of that. Even animating every stroke by hand would likely be faster than waiting for 800+ expressions to evaluate for every frame, especially since sampleImage() itself is one of the slower methods to begin with.

Mylenium

You cannot sample the masks directly. You would explicitly have to create Nulls from the anchor points and then possibly mess around with more mask path expressions. That being the case and the whole technique generally sounding fishy and rather unreliable (What happens, if masks self-intersect or circle back to the center for instance?) there's probably a better and simpler way to do it, but without actual info about your project and screenshots nobody can advise on that.

Mylenium