ATC-20 placarding vs satellite damage classification: what each one tells you
If you've run a post-earthquake operation, you already know ATC-20 isn't a nice-to-have. It's the standard your safety assessment evaluators use to post a red, yellow, or green placard on a structure after they've walked it, looked at the cracking pattern, checked the foundation, and made a call a building official can stand behind. It's rigorous, it's defensible, and it's slow by design, because the person signing off on "safe to occupy" needs to have been inside, in person, not working from a photo.
Satellite-based damage classification is a different animal doing a different job. It doesn't post placards. It doesn't replace a trained evaluator standing in front of a structure. What it does is give you a dated, roof-level read of an entire affected area from vertical VHR imagery, usually within hours of the event, so you know which blocks to send your ATC-20 teams to first instead of working the map in whatever order the calls come in.
Rapid visual screening vs remote sensing
ATC-20's rapid visual screening phase was built for a world where the fastest information you had was a team member's eyes on the building. That's still true for confirming what's inside a structure. But it assumes you already know roughly where to look, and after a wide-area event, that's the hard part. Which neighborhoods took the worst hit? Which blocks have obvious collapse versus ones that look fine from the street but have a cracked column you can't see from outside?
Remote sensing answers the "where" question before a single evaluator has driven out. Pull imagery over the full affected footprint, grade every structure in frame against a simple damage scale, and you've got a prioritized list before your field teams even hit the staging area. It points your evaluators at which block to walk through first, so the walk-through itself starts somewhere useful.
Red/yellow/green tag vs damage grade
A red tag is a legal determination. It comes with liability, with a building official's name on it, with a process for appeal. A damage grade from imagery carries none of that. It's a structure-by-structure estimate, collapsed, heavily damaged, or apparently intact, built off what the roofline and footprint look like from directly overhead right after the event.
Treat a damage grade as ground truth and you'll eventually get burned by a structure that looks fine from above but has a compromised interior stair. Treat it as a prioritization layer instead, the thing that tells incident command where to send the first wave of ATC-20 teams and where shelter demand is likely to spike, and it does exactly what it's for.
Placard vs imagery-based grading: how the two line up
In practice the two run in sequence, not in competition. Imagery-based grading runs first, right after the event, because it doesn't need roads to be passable or teams to be mobilized. It gives you a structure-by-structure read of the whole footprint instead of waiting on a street-by-street ground survey to slowly fill in. ATC-20 placarding runs on the ground after that, confirming or overriding the remote call based on what an evaluator actually finds.
If you're staging search-and-rescue assets or deciding where to open shelter sites in the first 24 to 48 hours, you don't have time to wait for full ATC-20 coverage of a county-sized footprint. You need a way to point your people at the worst-hit blocks first. That's the gap Building Damage Classification was built to close: per-structure damage grades from satellite or aerial imagery, delivered on an event-driven basis so the read is current when your teams need it, not a week-old snapshot.
What it won't tell you
Worth saying plainly: imagery grading won't tell you about gas leaks, won't catch a damaged interior stair in a building whose roof reads fine, and won't give you a legally postable placard. It's RGB imagery at a resolution fine enough to separate "intact" from "collapsed," not a replacement for someone standing in the doorway.
If your first 72 hours are about deciding where ATC-20 teams and shelter planners go before anywhere else, that's the specific problem Building Damage Classification is built around.