Degrees, Minutes, Seconds vs Decimal Degrees

[kas_1611]

It all depends on what datum you are using too.

WGS84 isn't any more accurate than any other, it's just become the universal standard because GPS uses it. But if you're in the North Sea you might use ED50 for instance which gives a slightly different Lat and Long to your GPS.

So why not use 10 digit OS Grid References or UTM distances?

Just use whatever you like. I'm pretty sure whomever is on the end of the radio in a distress situation will be able to figure out where you are regardless if you use DMS, DM.M or OS Grid. And realistically within a kilometer is probably close enough for the SAR Helicopter to see you from a thousand feet up

[requiem]

One of the wonderful things about standards is that there are so many to choose from! Lessons learned from response to hurricanes and other natural disasters in the US is that "everyone uses whatever they like" is not a good approach.

My own opinion is that at sea the "decimal minutes" format is most appropriate. If you're talking with a large ship or your local coast guard, that's almost certainly what they'll be using. If you dredge up standards, that's what you'll tend to find. E.g. both US national SAR standards and DoD policy call for decimal minutes or MGRS (on land), all using WGS84. The IAMSAR Manual* similarly recommends decimal minutes in WGS84.

Decimal degrees are probably best left for those developing software, and DDMMSS is now akin to roman numerals: something to use for aesthetics rather than functionality.

* International Aeronautical and Maritime Search and Rescue Manual, jointly published by the IMO and ICAO.

[Time Theory]

Originally Posted by Mike OReilly

OK... so are you asking about DMS vs DM.xx or are you asking for straight D.xxxxx?

Most modern Mercator charts, and marine GPSs , use DM.xx, but there are still plenty of DMS charts out there, and my GPS can be set to DMS. Pure digital degrees (D.xxxxx) has come into being due to mapping tools like Google Maps. It's not something I've seen used in practice in the nautical world.

Being a lake sailor I do not get the opportunity to do any real world chart navigation. So I decided to practice my chart skills by working some example problems. I was solving for set and drift using an old Long Island Sound Mercator paper chart.

While working the problem using DMS, I starting thinking that if I had to radio my position, would using DMS be the proper method to do so? It may be me not understanding, but it seems the norm now is to use degrees and decimal minutes. Am I correct in my thinking?

[El Pinguino]

Originally Posted by Time Theory

Being a lake sailor I do not get the opportunity to do any real world chart navigation. So I decided to practice my chart skills by working some example problems. I was solving for set and drift using an old Long Island Sound Mercator paper chart.

While working the problem using DMS, I starting thinking that if I had to radio my position, would using DMS be the proper method to do so? It may be me not understanding, but it seems the norm now is to use degrees and decimal minutes. Am I correct in my thinking?

Yes, you are correct.

[RaymondR]

Not sure about other parts of the world but in Australia marine areas use degrees, minutes and seconds, which is the graticular system, and land areas Universal Transverse Mercator which is a grid system. There is no exact conversion between the two and the coincidence tends to depend on the size of the areas involved.

This has thrown up some messy outcomes one of which was the zoning of the Great Barrier Reef Marine park. Being more familiar with the UTM system the bureaucrats defined the zones using it and then converted the UTM coordinates into latitude and longitude. As a result the short term bureaucratic convenience has resulted in a long term public nuisance with users obliged to three decimal places of a minute in marine park areas.

However it did prove handy to an offender who was charged with fishing in a closed area and was able to successfully argue that no exact conversion of positions was possible and therefor the boundary position was anomalous. As identified in other posts we are only talking about small differences of a few metres but the magistrate considered the difference significant enough to dismiss the case.

Australian marine charts use degrees, minutes and decimal degrees which convenience and avoidance of error introducing conversions would suggest be always used.

[Snore]

Use ddd MM.MMM only! Why?

Listen to the USCG on 16 or 22, that is what they use. If you do a MayDay in DDD MM SS, they will need to convert the seconds. Hopefully the person doesn’t mess that up.

[chrisr]

Originally Posted by StuM

I minute of latitude is 1852 metres at 45N or 45S so 0.1' is 185, not 170.

A location to 0.1' covers an area of over 34,000 sq m.
(That's 8.5 acres / 3.45 hectares /6.5 US football fields)
That's a lot of bottom if looking for a bommie or small wreck.

Around here we generally plot reef dive locations to at least two decimal minutes (a bit under 350 sq m).

FIFY

believe the length of a minute of latitude varies slightly...more towards the poles...less towards the equator, because the earth is not a perfect sphere.

1852m / 6080ft is a handy average

cheers,

[Tomodore]

I check in to 2 ND radio nets most mornings and the standard for location is degrees and minutes. From time to time someone checks in with degrees and decimal of a degree. For many that got into boating after chart plotters became common they haven't had the training on navigation. I am not throwing stones here I don't know how tow tar manila line it just doesn't come up. The difference between 22° 50' and 22.50° is roughly 20 miles, if you compound that with another error on longitude than it is pretty huge. I realize the folks reading this thread understand but it is the other mariners I worry about if they get in trouble and give decimal degrees when people listening think degrees and minutes.

[SeanPatrick]

I've done calculations with degrees and decimal minutes for so long that I can convert to decimal degrees and back in my head. It's easy to do:


Let's say you want to convert say 37.3 minutes to decimal form. 6 · 6 = 36, and 6 · 7 = 42, so we know we'll start with 0.6°. The remainder is 1.3'. 6 · 2 = 12, so 6 · 0.2 = 1.2. Now we have 0.62° and the remainder is 0.1'. 6 · 2 = 12, so 6 · 0.02 = 0.12. 0.622° = 37'19.2" or 37.32'. Close enough.


Of course, you could keep going ...



But I prefer to use degrees and decimal minutes.

[ranger58sb]

Originally Posted by RaymondR

Not sure about other parts of the world but in Australia marine areas use degrees, minutes and seconds...

Australian marine charts use degrees, minutes and decimal degrees...

???

-Chris

[StuM]

Originally Posted by chrisr

FIFY

believe the length of a minute of latitude varies slightly...more towards the poles...less towards the equator, because the earth is not a perfect sphere.

1852m / 6080ft is a handy average

cheers,

I stand corrected.
It actually varies between 1843m and 1862m (to the nearest integer value).


Note that the convention that 1 nautical mile = 1' of latitude is also inaccurate since 1nm is defined by SI as exactly 1852m regardless of location.

[HJ51]

Can some one please post in narrative form the difference between how degrees and decimal minutes and decimal degrees would be spoken into a VHF radio for reporting your position? Thanks.

[StuM]

Originally Posted by HJ51

Can some one please post in narrative form the difference between how degrees and decimal minutes and decimal degrees would be spoken into a VHF radio for reporting your position? Thanks.

Degrees and decimal minutes:
Nine degrees two seven point nine minutes south, one four seven degrees nine point two minutes east.


Decimal degrees:
Nine point four six five degrees south, one four seven point one five three degrees east.

[Rob_P]

I ran into this last night. Lesson 7 in ASA 103 was on the table (literally, the chart table). As a scout I learned the old method of reporting minutes and seconds. The new way is much easier.

Modern US NOAA charts use decimal seconds as the legend scale for both Lat and Long so there's no need to convert from one to the other or calculate anything, it's all right there in front of you.

The hard part is adding or subtracting whiskey from true virgins. First, you have to find willing virgins. Then you need to convince them to make a beer run. Then you wait and hope they come back so you can find out where you are.

Currently, I'm going broke and getting dehydrated.

[Martkimwat]

Originally Posted by Orin

Nobody cares that we had a perfectly well thought out system that had survived thousands of years of refinement when they suggested that we divide a longitudinal line up into 2 million pieces and call one a meter just to make everything base 10... cuz heck whoever came up with this base 6,12,24,60 system.... whatever it is

Traditional mariners and GPS programmers will be the last people on earth who know what a foot is.

Not to be foul to land lubbers, but do you have any idea how many people I know who don’t even know what tides are or how the moon, sun, and other planets affect them?

Regardless of being right or easy, I think decimal degrees are probably already more common because of the whole Google and land based GPS. I am willing to bet geocaching is more common than celestial navigation.

I don’t want to sound bent out of shape, but I’ve endured countless peers saying “why do we use these old systems anyway, dividing by 100 is so much easier” as they proceed to do just that on a calculator.... Most of the people I meet are disconnected from reality. Subject for a different time tho.

And yet in America you adopted a decimal currency straight away! I miss the old pound shillings and pence in UK and I still use the imperial measurement system for woodwork as it is generally to fiddly to use metric. Imperial and £SD calculations, done mentally back before calculators, kept your mental faculties sharp. As you say, it is ridiculous that people use calculators to do simple decimal arithmetic, are they not taught how to move decimal points in school anymore? At 73 I can often tot up my shopping and have the right change ready before the youngster on the till has entered the prices into the 'cash register' and come up with the price, have several time found an error with the figures entered by the cashier that way.