Why Chains/Hour???

[Dalmatian190]

So...being the fire geek I am...was reading the IAP for the Georgia fire last night.

And saw the term ch/hr for fire rate-of-spread.

Which took a few minutes to figure out is Chains per hour.

Does anyone know why on earth it's measured in Chains???

(A Chain btw is 66' for those about to google it out there :D )

[mtnfireguy]

"Chain" is a standard surveying term and measurement, so without doing in-depth research, I suspect that is why it is used.

[ramseycl]

Mtnfireguys right it is a surveying term for distance, and it is easy to figure out. When you are monitoring a fire, if it is moving about a foot a minute it equals about a chain an hour. It is all relative, but easy to estimate.

[fireman4949]

I've just learned something new! :)

Imagine that!:eek:

[Dalmatian190]

I did too 4949...should've seen me last night when I was reading the IAP (obviously not something I'm versed on...) and I'm like "What the frack is a ch/hr???"

Still, since the wind isn't usually purely N/E/S/W it's not like measuring in chains is a shortcut to figuring out when it'll cross into the next survey township -- you still gotta do trigonometery to figure that out.

[SPFDRum]

Well what ever they do-chains, trigonometry, PFM, it's still safe enough for jr's to do.....;)
I'm in 49's boat, did not know a "chain" was a unit of measurement.

[the1141man]

*LOL* I saw the thread title and figured they meant chains for chainsaws...and I was thinking if they're talking about how many chains they go through in an hour.... :eek: DAYUM!!! :eek: *LOL*

Now I'm wondering where the surveying terminology came from....off to Google... ;)

[Gnufsh]

Doesn't it come from dragging chains to measure distances?

[kd7fds]

When I was in high school, I worked part time as a surveyors assistant.

Back in the day, before lasers and gps, surveyors chains were the most accurate way for surveyors to measure distance. We used one occassionally for short measurements.

They were 100 links. Calibrated for precise measurement and could hold their accuracy while being dragged around the countryside.

80 chains in a mile made the math easy to work with.

BTW, 1 chain = 4 pole lengths. Pole lengths are an even older survey tool.

By law, in the 19th Century, all US Surveys were measured in chains. You can see the result in many things.

Roads were laid out in chain widths. Two lane roadways were 1 chain wide, each lane was 1/2 chain. Property lots were measured in chains. 1 x 2 chains is common in our area.

Check out this wikipedia (http://en.wikipedia.org/wiki/Chain_(length)) link

[Dalmatian190]

I've also since learned -- from my state's own website explaining the Fire Danger system...

When you hear the Fire Danger report and it says "Spread" the number given for spread is the predicted speed in Chains/Hour of the fire!

You know, for 20 years I've heard on the radio during fire seasons that "Spread is _____, Build up is _______, the Fire Danger for today is ______." and never thought more about what "spread" was other than some number they calculated up in a formula somewhere.

[kd7fds]

I've also since learned -- from my state's own website explaining the Fire Danger system...

When you hear the Fire Danger report and it says "Spread" the number given for spread is the predicted speed in Chains/Hour of the fire!

You know, for 20 years I've heard on the radio during fire seasons that "Spread is _____, Build up is _______, the Fire Danger for today is ______." and never thought more about what "spread" was other than some number they calculated up in a formula somewhere.

I love to find out weird stuff like this.

I found out another reason why chains are still used. One definition of an acre is a rectangular area that measures 1 chain wide by 10 chains long.

[fireman4949]

I love to find out weird stuff like this.

I found out another reason why chains are still used. One definition of an acre is a rectangular area that measures 1 chain wide by 10 chains long.


...And here, all this time, I thought an acre was was a rectangle that measured 2 chains wide by 5 chains long.

Silly me!:o :o :o ;)