Positions is the 3rd Decimal necessary?

[onesea]

Another post commented on the Coastguard giving positions to 3 decimals.
A nautical mile is 1852m
The third decimal is giving a positson to 20m, ok for a buoy position or wreck or similar.

However when it's a vessel it's going to move more than that.

I mentioned to my misses on passage (we're trying to get in habit of writing a log), she's doing her day skippers. 20m that's thinner than the line you draw on the chart.

Yes for exams but it here, If youfeeling the need to use the third decimal. You should be navigating by other means.

Transits buoys etc, a GPS position says very little.

They're will always be exceptions.

 

[Juan Twothree]

Another post commented on the Coastguard giving positions to 3 decimals.
A nautical mile is 1852m
The third decimal is giving a positson to 20m, ok for a buoy position or wreck or similar.

However when it's a vessel it's going to move more than that.

I mentioned to my misses on passage (we're trying to get in habit of writing a log), she's doing her day skippers. 20m that's thinner than the line you draw on the chart.

Yes for exams but it here, If youfeeling the need to use the third decimal. You should be navigating by other means.

Transits buoys etc, a GPS position says very little.

They're will always be exceptions.

That assumes the CG gives positions in degrees and decimals of a degree, whereas they actually do so in degrees, minutes and decimals of a minute.

Each minute of latitude is 1nm (2000 yds), therefore that third place of decimals referred to in the original post is actually 0.001 nm, which is 2 yds.

 

[Momac]

By three decimal places you mean like this? I make the third decimal place about 2metres (just over 6 feet).

 

[lustyd]

Of course it’s necessary. If they didn’t they’d have to do rounding in their head and read a number differently to what they see, all very error prone compared to just say what you see.
It sounds like a smart thing to say, but smarter people have thought of and discounted these ideas and most things are done for a reason, especially in safety critical areas.

 

[jlavery]

It's always important to understand the difference between precision and accuracy.

 

[onesea]

That assumes the CG gives positions in degrees and decimals of a degree, whereas they actually do so in degrees, minutes and decimals of a minute.

Each minute of latitude is 1nm (2000 yds), therefore that third place of decimals referred to in the original post is actually 0.001 nm, which is 2 yds.

Yes my brain fart which only makes my point clearer.

Of course it’s necessary. If they didn’t they’d have to do rounding in their head and read a number differently to what they see, all very error prone compared to just say what you see.
It sounds like a smart thing to say, but smarter people have thought of and discounted these ideas and most things are done for a reason, especially in safety critical areas.

Ok I am stupid you are clever and know best you have made your point.

In the meantime, IMHO in a control room is a better time to refine numbers and reduce the information to be transmitted.

It makes less chance of mistake for those on the other end of radio trying to note it down or remember.

You can of course you always navigate to a 2 meter accuracy.

For me in the channel or out the channel, didn't hit the rock, passed clear of the race and arrived at a destination is enough.

When the accuracy is down to meters or less I think you will find there is a whole new world of pain required. The WGS84 datum is not good enough. All to do with the world not being uniformly round.

 

[Sandy]

Positions is the 3rd Decimal necessary?

No, but it keeps some posters happy.

 

[capnsensible]

Yes my brain fart which only makes my point clearer.


Ok I am stupid you are clever and know best you have made your point.

In the meantime, IMHO in a control room is a better time to refine numbers and reduce the information to be transmitted.

It makes less chance of mistake for those on the other end of radio trying to note it down or remember.

You can of course you always navigate to a 2 meter accuracy.

For me in the channel or out the channel, didn't hit the rock, passed clear of the race and arrived at a destination is enough.

When the accuracy is down to meters or less I think you will find there is a whole new world of pain required. The WGS84 datum is not good enough. All to do with the world not being uniformly round.

I reckon that as with all these things, the protocols are set for the professional seafarers who do this stuff all day every day.

Of course, leisure sailors who do bobbing up and down for a few hours here and there are a minor cog in a big wheel. I would rather be grateful that standards are set high for commercial and military shipping and I'm content to ride on their wake.

 

[Baltika_no_9]

I can appreciate the points made but for me I am happy to ignore the third decimal point. I can't see much (any) downside in ignoring that third place but would be content to have the view changed. I think the OP is referring to a case where a Mayday had been received by CG. The position shown on a boat's GPS depends on where the antenna is positioned. If it is on the stern rail and the boat is 10m long and the vessel was lying say due N-S then the bow would be more than 0.005 minutes of latitude away from the indicated GPS position. A boat is not a dot, move around with a handheld GPS and the position will change in that third decimal place even if the boat is not moving.

 

[jwilson]

When properly offshore I log lat and long from GPS every hour - but only to full degrees/nearest minutes. If GPS or electric/electronics die (and I have had both happen more than once) a position within half a mile is a pretty good starting point for DR, or getting out the sextant and watch and pen and paper.

 

[Frank Holden]

When I hear people giving their position to the third decimal place of a minute I wonder which end of their boat they are speaking about.
Me? A mile is usually close enough ... if I was plotting bommies in a tropical lagoon three decimal places would be a good idea -otherwise the first decimal place would be the most I would ever use.

 

[Momac]

Aside from the accuracy debate what is the issue ,from a practical perspective, with reporting the 3 decimal places ?
In an emergency is it not a simple matter of reading out the figures from the navigation equipment and not over thinking the validity of the last number?

 

[Stemar]

Aside from the accuracy debate what is the issue ,from a practical perspective, with reporting the 3 decimal places ?
In an emergency is it not a simple matter of reading out the figures from the navigation equipment and not over thinking the validity of the last number?

I agree. It does no harm, even if it does no good apart from reducing a source of (minor) error.

 

[Buck Turgidson]

If the person writing down your position is expecting a standard format then do your part and use the standard format. If you are logging position for your own use use whatever you like.

 

[capnsensible]

If the person writing down your position is expecting a standard format then do your part and use the standard format. If you are logging position for your own use use whatever you like.

Spot on.

 

[johnalison]

Should I find myself in the position of calling for help on the radio I would follow my approximate position with lat and long to one decimal place. At a tenth of a mile at sea near a buoy it appears that you are almost touching it in anything but thick fog, and the chance of an error in reading my message is much reduced.

 

[Slowboat35]

Demands for such extreme and unnecessary precision are usually an indication of a system that has lost touch with its practical roots and has been taken over by unthinking bureaucrats/technogeeks rather than practically-minded people who actually understand the job.
Surely no-one would ever describe the CG in such terms?

 

[boomerangben]

The nav system in the SAR helicopters I used to fly only took 2 decimal places and since standard GPS is only accurate to 5m or so anyway the third is irrelevant. If you are looking for something (buoy, harbour entrance, person in water, the right house on a street) you are looking out of the window most of the time, not looking at the 3rd dp.

I agreee that standard formats are very important for VHF coms and passing data between multiple agencies. Who decides on what that standard is more get want to have a word with themselves

 

[LittleSister]

I'm thinking that for the circumstance outlined in the OP in the other thread - i.e. the challenge of taking down and locating a position given by Coastguard in Lat/Long to see if it is of any relevance to the listener - one could ignore the decimals entirely. The minutes of a degree will suffice for that purpose.

In the extremely unlikely event that you find on checking that -
(a) you are close to the thing referred to; AND
(b) you could help or it is a danger to you; AND
(c) you can't actually see it;
you could ask the Coastguard to give the full position again.

 

[Sandy]

Spot on.

How big is this spot?

Can you get cream for it?