I had never in my life heard that unit of measure before, interesting!
The chain (abbreviated ch) is a unit of length equal to 66 feet (22 yards), used in both the US customary and Imperial unit systems. It is subdivided into 100 links. There are 10 chains in a furlong, and 80 chains in one statute mile. In metric terms, it is 20.1168 m long.
So yeah 250 feet, not a worry with the hydros disengaged!
Chalk that one up to jargon shift/crossover. I partner with a bunch of agencies for prescribed fire in the winter time through work. It's the base measure used for describing distances on controlled burns:
"Walk in a half chain and drop dots of fire."
"Mop up all burning material up to a chain in from the containment line"
I know "chain" as a unit of measure comes from surveying and I strongly suspect it's use in prescribed/wildland fire is in itself a jargon crossover from federal forestry and surveying practices way back in the day. Coincidentally, my dad worked as a surveyor briefly before college (1960's?) and I remember him telling me stories about having to learn quickly how to correctly coil a surveying chain so as not to pi$$ off the lead surveyor.
Other phrases from that aspect of my life that have now entered everyday conversation include:
"Let me tie in with you about xxxx"
"Let's circle back to that (issue)"
"Are we wrapped?" - As in are all sides of the burn now lit? More generally are things wrapped up?
"Can we go face to face?" - Meaning let's talk in person instead of over the radio/phone
"That's a solid/good copy" - 10-4... There's a strong preference in fire for not using 10 codes or other phrasing that due to differing backgrounds/experience levels may not be readily understood by everyone.
The last two make me feel/sound like a real dork every time I recognize that I've used them outside of the context of fire, lol...

