Comment by throw310822

Yes, they have instruments and they ignore their readings when they appear to diverge wildly from their assumptions. (Here the mention of gravity is confusing because that's one thing you don't need an instrument to measure and that directly translates to mass- but I interpret the sentence you quoted as meaning: "our instruments read a powerful gravitational field that doesn't match with the mass reasonable for a 500 metres station, so it's probably a faulty reading").

In any case, the following report clarifies:

"To our surprise we find that the station is far larger than we guessed. [...] This fine vapour obscured the substantial bulk of the station and led us to assume that it was no more than a few hundred metres in diameter."

So the reporter refers to his previous estimate as a guess, not a measurement, and specifies that the assumption was based on what they could see. The following estimates are similarly guesses based on their current understanding and explorations, not the output of some specific instrument.