Different numbers for "Seconds" in Lats and longs?

[exMaggieDrum]

I was on a couple of land-based search and rescue teams (Colorado) and we used degrees/minutes/decimal minutes. Everyone learned both since some guy lost out there with a cell phone might have radio/phone in in seconds. But to coordinate searches with stuck with our "standard".

I prefer (personal preference) to use seconds instead of decimal minutes on water. I crewed on a boat with a first time skipper who had never been on radio nets before and he had his GPS set to decimal minutes but called it out as seconds (like the examples above - e.g. 85.5 seconds) and this caused some confusion and embarrassment.

[StuM]

I prefer decimal minutes on water since 0.1' is 1/10 of a NM or 1 cable or just under 200 meters and .01' is about 20 meters.

I can visualise those distances easily when I compare two references.

[Stu Jackson]

Originally Posted by valhalla360

I didn't say the math was difficult. I said it was a nuisance. If you have to convert back and forth a lot, it's not something you can do quickly in your head.

I made a simple spreadsheet to do this eons ago.

[jackdale]

Originally Posted by StuM

I prefer decimal minutes on water since 0.1' is 1/10 of a NM or 1 cable or just under 200 meters and .01' is about 20 meters.

I can visualise those distances easily when I compare two references.

+1.

[exMaggieDrum]

Originally Posted by StuM

I prefer decimal minutes on water since 0.1' is 1/10 of a NM or 1 cable or just under 200 meters and .01' is about 20 meters.

I can visualise those distances easily when I compare two references.

Just like everything to do with boats, every body has preferences. I prefer minutes and seconds. One minute is 1nm. 30 sec's is 1/2nm. 10 sec's 1/6nm. I've never had a reason to really navigate down that far though as I use other instruments (eyes, radar, chartplotter, etc.) for the close in stuff. And I don't mean that in a sarcastic way as I'm sure everyone does that. Although I guess if you are trying to find a lifeboat on the ocean it might make a difference.

[OldFrog75]

Originally Posted by StuM

I prefer decimal minutes on water since 0.1' is 1/10 of a NM or 1 cable or just under 200 meters and .01' is about 20 meters.

I can visualise those distances easily when I compare two references.

Me too. And decimal minutes is more accurate than whole seconds.

[DotDun]

Originally Posted by exMaggieDrum

Just like everything to do with boats, every body has preferences. I prefer minutes and seconds. One minute is 1nm. 30 sec's is 1/2nm. 10 sec's 1/6nm. I've never had a reason to really navigate down that far though as I use other instruments (eyes, radar, chartplotter, etc.) for the close in stuff. And I don't mean that in a sarcastic way as I'm sure everyone does that. Although I guess if you are trying to find a lifeboat on the ocean it might make a difference.

For longitude, that's true only at the equator. For latitude, that's consistent everywhere.

[exMaggieDrum]

To help me decide if I am thinking about this the wrong way, can someone give some examples where would be better off with using decimal minutes over seconds. One second (in latitude) equals 192 feet of a nautical mile. If I am withing 200 feet of something like a reef or buoy or whatnot, I am relying more on visuals and radar than I am on trying to plot that on a paper chart. If I have a chartplotter, it already calculates down to the gnat's eyebrow in accuracy regardless of what display value you have. If I am trying to set a waypoint down that far, again I wouldn't trust a decimal down to that range any more than I would a second.

Don't get me wrong, I'm okay with using decimal minutes and can use them as easily as seconds. I'm struggling as why one is so much better than the other. Thanks.

[Stu Jackson]

Originally Posted by OldFrog75

Me too. And decimal minutes is more accurate than whole seconds.

Could you please explain this one?

[OldFrog75]

Originally Posted by Stu Jackson

Could you please explain this one?

Sure. 0.025 minutes is more accurate that 1 or 2 seconds.

[jackdale]

Originally Posted by OldFrog75

Sure. 0.025 minutes is more accurate that 1 or 2 seconds.

I would suggest you are discussing precision, not accuracy.

Accuracy refers to the closeness of a measured value to a standard or known value. For example, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate. In this case, your measurement is not close to the known value.

Precision refers to the closeness of two or more measurements to each other. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy. You can be very precise but inaccurate, as described above. You can also be accurate but imprecise.

For example, if on average, your measurements for a given substance are close to the known value, but the measurements are far from each other, then you have accuracy without precision.

A good analogy for understanding accuracy and precision is to imagine a basketball player shooting baskets. If the player shoots with accuracy, his aim will always take the ball close to or into the basket. If the player shoots with precision, his aim will always take the ball to the same location which may or may not be close to the basket. A good player will be both accurate and precise by shooting the ball the same way each time and each time making it in the basket.

A precise, large scale chart based on horizontal datum that differs from your chart plotter presents problems in accuracy.

[OldFrog75]

Originally Posted by exMaggieDrum

To help me decide if I am thinking about this the wrong way, can someone give some examples where would be better off with using decimal minutes over seconds. One second (in latitude) equals 192 feet of a nautical mile. If I am withing 200 feet of something like a reef or buoy or whatnot, I am relying more on visuals and radar than I am on trying to plot that on a paper chart. If I have a chartplotter, it already calculates down to the gnat's eyebrow in accuracy regardless of what display value you have. If I am trying to set a waypoint down that far, again I wouldn't trust a decimal down to that range any more than I would a second.

Don't get me wrong, I'm okay with using decimal minutes and can use them as easily as seconds. I'm struggling as why one is so much better than the other. Thanks.

First off I think you need to double check your arithmetic since 192' is closer to 2 seconds than 1 second.

I've never seen lat and lon displayed as minutes and seconds but I assume it's whole seconds not fractional seconds (could be wrong about this) and is therefore less accurate, or in deference to Jackdale's comment above, less precise.

I guess it's a matter of preference since no matter which you use there will be a conversion in your head if you want to determine feet, as in how many feet is 16/60ths of a nm vs 0.267 nms, although I will concede that reading 16 seconds as an approximation for 1600' is a lot easier than converting 0.26 minutes to 1600'.

When racing if someone asked me how far to the next mark and I said, "2 miles and 16 seconds," or "2 miles and 16/60ths of a nm," they would be more confused than if I said "2.27 nms" or "2 and a quarter nms."

Might be wrong but I think most Americans are more comfortable using a base 10 decimal system than a base 60 (or whatever it is) system. Probably why rules on our charts are delineated in 10ths of a minute instead of 12ths or 60th's. Same could be said for feet vs. fathoms. Probably depends to a large extent on how one was trained.

[jackdale]

Originally Posted by OldFrog75

Sure. 0.025 minutes is more accurate that 1 or 2 seconds.

Are you discussing precision or accuracy?

Accuracy refers to the closeness of a measured value to a standard or known value. For example, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate. In this case, your measurement is not close to the known value.

Precision refers to the closeness of two or more measurements to each other. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy. You can be very precise but inaccurate, as described above. You can also be accurate but imprecise.

For example, if on average, your measurements for a given substance are close to the known value, but the measurements are far from each other, then you have accuracy without precision.

A good analogy for understanding accuracy and precision is to imagine a basketball player shooting baskets. If the player shoots with accuracy, his aim will always take the ball close to or into the basket. If the player shoots with precision, his aim will always take the ball to the same location which may or may not be close to the basket. A good player will be both accurate and precise by shooting the ball the same way each time and each time making it in the basket.

A precise, large scale chart based on horizontal datum that differs from your chart plotter presents problems in accuracy.

As well, if you give me a lat and long from your chartplotter/GPS that uses a different horizontal datum than mine, accuracy suffers.

[StuM]

Originally Posted by Stu Jackson

Could you please explain this one?

Compare 17°17'17" to 17°17.17'

1 Minutes is 1852 meters
A second is 1/60th of a minute.
So seconds are accurate to the nearest 31 meters
Two decimal places is 1/100th of a minute.
Therefore accurate to the nearest 18.52 meters.

But that's not why I prefer it.

I just find it easier to visualise decimal second distances.
0.17 is ~340m (double the number and add ten)
And I can easily relate 340m to multiples of football fields, athletic tracks or whatever.

I have to put a lot more thought into "roughly how far away is 17/60 Nm?"

[StuM]

Originally Posted by jackdale

I would suggest you are discussing precision, not accuracy.

Absolutely.

Whilst precise, OF75's statement was not accurate.

I should have made the same distinction in my previous post.
( Hey, I'm the pedant around here, you trying to usurp my throne?)