The river has been rising at a rate of about two meters (six feet) a day and is within 26 meters (85 feet) of the top.

Can I replace "within 26 meters of the top" with " 26 meters below/under the top"?

I know I've raised a similar question before but still don't see the difference. Please bear with me.

Thanks in advance!

within 26 metres is not precisely 26 metres from/below/under the top
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Yes, you're right. How about less than 26 meters below/under the top?
Can't see anything wrong with that
Within 26 metres - less than 26 metres from/below/under the top
Thanks, Optilang.
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"Within 26 meters of the top" literally means that the distance is anything from zero to 26 meters. However, in this context (and others like it), "within" by convention means that the distance is approximately 26 meters, and just a little less. (Otherwise it would be kind of pointless... in the same situation you might say "within 500 meters of the top" and theoretically not be wrong, but this doesn't help anyone to know the actual level.)

"Within" also conveys a sense of urgency -- the sense that the level is rising, or has been rising.

"26 meters below the top" means exactly what it says. "26 meters under the top" means the same but is less natural in this context.