Wrong value in topographic error function?

[SandroMartens]

So a topographic error occurs when the two bmu of a sample are not adjacent. Shouldn't then t = 1? If the bmu are two hops apart in a corner, their euclidean distance is sqrt(2) = 1.4142 . So with distance > 1.42 this doesn't count as an error.
Or am I missing something?