What's a Hundred Feet, Anyway?

I've flown in and out of Washington National Airport (DCA) a number of times, mostly in the 1980s when I lived in D.C. It always felt a bit hairy, since the planes access the runways very low over the Potomac, but I had faith in the systems and people that controlled the space and machines. 

That used to be true, I think. But maybe not.

It's now known that the January 29 mid-air crash that killed 67 people was the result of a military helicopter flying higher than it was supposed to, so that it was in the airspace allocated to commercial planes. 

And why was that? At an National Transportation Safety Board hearing yesterday, a military expert testified that the types of altimeters used in their helicopters are only accurate within plus or minus 100 feet. And the helicopters in that airspace are not supposed to exceed 200 feet.

So.

That seems like a rather wide margin of error for such a low clearance. 

The collision occurred at 278 feet, within the margin of error but also within the airplane's designated space.

Has no one ever realized this before? Has it been only luck that has prevented this from happening before?

Shrug emoji, I guess. Whatever.