Maths Question (from GWX manual)

[ryanwigginton]

Excuse me if I'm being stupid... :88)

I've been reading the manual targeting section of the GWX2.0 manual. Can't work out why I need to time for 3min 15sec as opposed to just 3min.

Isn't there a simpler equation that can be applied generically to any number of minutes target observation time? (ie. in SH4, if I time for 9 minutes I can do... (distance travelled by target)/9 x 60 and this gives me speed, simple)

[Pisces]

1 knot is 0.5144 m/s (=1852m/3600s). Then 100meters take : 100/0.5144=194.4 seconds. That's 3m14.4s, but 3m15s is easier to use quickly.

As for the original 3-minute rule: 1 nautical mile is usually rounded off to 2000 yards (Wiki says it actually is 2025.372 yards (http://en.wikipedia.org/wiki/Nautical_mile)) So 1 knot is 0.5555 yards/second (=2000yds/3600s) Then 100yds take 180 seconds, which is 3 minutes. Since the exact 2025.372 yards case leads to a time of 2m58s for every 100 yards movements you can use 3 minutes as a good approximation.

The 15-odd seconds difference is because a yard is not equal to a meter.

[Uber Gruber]

Excellent reply Pisces :up:

[ryanwigginton]

Yes, excellent reply, thanks. So the simplicity of the metric system is seen off by those pesky imperial knots?

[Pisces]

I'm not stepping in that political debate. I just claim I've been indoctrinated to do metric since childhood. According to this other Wiki page on meters (http://en.wikipedia.org/wiki/Metres) meters were originally based on a 10 millionth of the distance between the poles and the equator. Not some body part of Napoleon or something. This is similar to the origin of the nautical mile. Since that is also based on a division of the length of a meridian (pole-to-pole distance) on the earth globe (one arc minute). It just uses a different number of them. So it's just apples and oranges to me. They both look spherical. :)

[Pablo]

1 knot is 0.5144 m/s (=1852m/3600s). Then 100meters take : 100/0.5144=194.4 seconds. That's 3m14.4s, but 3m15s is easier to use quickly.

As for the original 3-minute rule: 1 nautical mile is usually rounded off to 2000 yards (Wiki says it actually is 2025.372 yards (http://en.wikipedia.org/wiki/Nautical_mile)) So 1 knot is 0.5555 yards/second (=2000yds/3600s) Then 100yds take 180 seconds, which is 3 minutes. Since the exact 2025.372 yards case leads to a time of 2m58s for every 100 yards movements you can use 3 minutes as a good approximation.

The 15-odd seconds difference is because a yard is not equal to a meter.
Hi!

I'll update the manual section with a bit more detail on why we suggested 3 min 15 sec: it's basically for the reason you've given.

Pablo