GPS question for <span class="highlight">Doug</span> <span class="highlight">T</span> ( and others)

Thanks for the quick reply. With Geo caching I can get within one metre of the find but if I am looking for road junction in the desert it should be close enough with a zero as the last digit?

yep sure will Mal

One minute of latitude/longitude equals one nautical mile

One nautical mile equals 2000 yards or 1852 metres

Therefore 00.001 minutes of latitude/longitude equals 2 yards or 1.852 metres

IMO it is easier to use the DDD MM.MMM format.

But then again I grew up with a digital watch ;)

ahhh I dont have a preference and I had a digital watch too

Sorry Mal & Di,

Correction to my last

one minute of latitude equals one nautical mile

depending on your latitude, one minute of longitude has variable distance conversions due to lines of longitude converging at the north and south poles.

If however if you've been entering co-ordinates from one format to another with out making the appropriate adjustments you may find yourself a bit off track.

If you recieve the co-ordinate 27 44' 30.0"S and you enter it into your GPS as 27 44.300'S you will be 400 yards NORTH from where you should be if you enterd it correctly as 27 44.500'S.

As Bonz has stated, 00.500' is equivalent to 00' 30.0" .

Hope I haven't confused things too much.

and that lattitude rule is only accurate at the equator

Parralells of latitude run parralell to the equator and one minute of latitude equals one nautical mile constantly over the earth. As the lines of longitude converge on the north and south poles they do not remain paralell to each other and therefore have differing values of distance.

No so Heefer, Parallels of lattitude shorten as they get nearer the poles. The nautical mile (and hence the knot) was based on the circumference of the earth at the equator. As a knot is a nautical mile per hour and based on the length of a nautical mile at the equator, as lattitudes reduce so to do the length of a nautical mile as it relates to one degree of lattitude.

Hey Heefers, just been looking further. Wikipedia history says the knot is measured on the meridians of longitude, different to what I read elsewhere. This is what they say:

The nautical mile was historically defined as a minute of arc along a meridian of the Earth, making a meridian exactly 180×60 = 10,800 historical nautical miles. It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart. The originally intended definition of the metre as 10-7 of a half-meridian makes the mean historical nautical mile exactly (2×107)/10,800 = 1,851.851851… historical metres. Based on the current IUGG meridian of 20,003,931.4585 (standard) metres the mean historical nautical mile is 1,852.216 m.

The historical definition differs from the length-based standard in that a minute of arc, and hence a nautical mile, is not a constant length at the surface of the Earth but gradually lengthens with increasing distance from the equator, as a corollary of the Earth's oblateness, whence the need for "mean" in the preceding sentence. This length equals about 1,861 metres at the poles and 1,843 metres at the Equator, a variation of one percent.[6]

Other nations had different definitions of the nautical mile. This variety in combination with the complexity of angular measure described above along with the intrinsic uncertainty of geodetically derived units mitigated against the extant definitions in favor of a simple unit of pure length. International agreement was achieved in 1929 when the International Extraordinary Hydrographic Conference held in Monaco adopted a definition of one (1) international nautical mile as being equal to 1,852 metres exactly, in excellent agreement (for an integer) with both the above-mentioned values of 1,851.851 historical metres and 1,852.216 standard metres.

Since the 1929 agreement, all nations have now adopted the international definition. The United States, formerly using a value of 1,853.248 m (6,080.2 ft), did not however adopt this definition until July 1, 1954.

We are probably both misled and both right (or wrong) hahahaha funny what we are taught is what we believe to be intrinsically right.

Bonz

If parallels of lattitude are as by definition parallel to each other would not the distance between any two be the same regardless of where they were on the surface of the globe? As opposed to longitude where the distance measured between any two gets shorter as you near the poles. Also wouldnt a nautical mile be the same length no matter where it was measured? I will stand corrected if I have misinterpreted

Cheers Pop

I agree that technically Bonz you are correct. However the difference in nautical miles between 1min of latitude at the equator and 1min of latitude at any other parallel of latitude is so insignificant as to be ignored by the worlds shipping and navies when conducting navigation. Lines of longitude are a totally different matter as they converge on each other at the poles and therefore have a value of 1min = one nautical mile at the equator and theoretically no distance value at the north or south pole.

Bonz, sorry mate was typing my follow up whilst you were posting yours. Pretty sure that we've both just said the same thing in our last two posts.

Pop, the earth flattens out in a small way towards the poles and therefore the distance over ground of a minute of latitude increases slightly. The difference in distance over ground between a minute of latitude at the equator and at the poles is eighteen metres (thank you Bonz, I honestly didn't know that). I guess that's why we just use the middle ground between those two as our standard nautical mile. Eighteen metres over a quarter of the earths latitudinal circumference is certainly nothing to worry about.

Pop, the discussion is whether nautical miles are a measurement of lattitude or longitude. I always thought and checked just before that it was originally measured from a boat travelling along the equator (so its lattitude) and they dropped an apparatus over the side of the boat and measured how many "knots" in the rope were fed out over a certain time, that gave rise to the calculation of the speed of the boat on knots, or nautical miles per hour, so the distance we are talking about is the distance between lines of longitude (or meridians). At the equator its longer than say at 60 degrees south.

The parallels of lattitude are all equidistant from each other (well at the poles they get closer) so 1 min of lattitude (up and down the earth ) is close enough to the same but as knots were originally measured across the earth, not up and down, the measurement of a naut varies as does the speed of a knot.

no worries Heefer, we're both right, one source measures knots in degrees of lattitude and the other I read measures it in degrees of longitude.

Bonz,

You were doing alright until your last follow up. The definition of a nautical mile has nothing to do with 'boats' travelling along the equator or in any direction. Knots are simply a measure of speed (Nautical Miles per hour). You said in your Wiki reference that a nautical mile was originally based on one minute of arc subtended on a meridian (a great circle which passes through the geographical pole). Hence it has nothing to do with longitude, it relates to latitude. After some pfaffing around, the International Nautical Mile has been agreed (as you point out) as 1,852 metres.

It is now fixed and accepted (by nearly all countries) so the value of a nautical mile (and hence a knot) does not vary.

Cheers,

Matt.

Matt it is NOW fixed it wasnt then...

The device that sailors used to make their speed measurement was called the "chip log." Chip as in chip of wood, and log as in to record in a log. The chip was a wedge of wood about 18" in size; it was tied to one end of a rope on a large spool. The rope had knots tied into it about every 47'3" (more about how that was calibrated below).

The wooden chip was thrown overboard at the ship's stern (back end). Because of its wedge shape, it would "grab" the water and start pulling out rope as the ship moved forward at some yet unknown speed. One man would hold the spool of rope as it played out; another man would start a sandglass filled with 30 seconds of sand; and a third man would count the knots as they passed over the stern board. When the 30 seconds of sand expired, the time keeper would call out and the counting of knots would stop.

The faster the ship was sailing, more knots and a longer length of rope were played out. The number of knots in the rope that were counted in 30 seconds, then, was equal to the speed of the ship in nautical miles per hour. A "knot", therefore, is not a nautical mile, it is a nautical mile per hour. Thus 1 knot was equivalent to 1 nautical mile per hour; 5 knots were equivalent to 5 nautical miles per hour; etc. The similar sound of "knot" and "naut" is entirely coincidental.

Finally, what about the actual values of 47'3" between knots on the rope and the 30 seconds that were used with the chip log? The length was based on converting [1 nautical mile per hour] to [feet per second(fps)], and then multiplying [fps] by 30 seconds (which was a practical time to spend counting knots with a sandglass). The result was the calibrated length in feet at which to tie the knots for a 30-second run of the chip log.

Happy with all that Bonz, but it nothing to do with how the definition of a Nautical Mile came about.

You seem to suggest in your earlier follow up that the method of reading the log was HOW a definition of a nautical was arrived at. This is putting the cart before the horse.You can't measure speed without a definition of distance. The distance of 47'3" was based on the definition of a nautical mile at the time. Change the definition and you simply change the distance between the knots.

The definition of a nautical mile was not determined by a group of Jack Tars running the log at the end of the afternoon watch. Given the came conditions, the ship will cover the same distance in a set time whether it is steering along the equator, heading north or south, somewhere in the middle latitudes or close to the poles.

If you imagine two ships, one sailing along the equator (hence heading due east or west) and one heading south somewhere of Sydney. Now if these two ships were sailing under identical weather conditions, rig, loads, etc, the number of 'knots' reeled out over a set time will be precisely the same. So what is the significance of a ship sailing along the equator?

Matt.

A ship on the equator was the nautical plimsoll line Matt.

If both your ship started at say zero deg N, one at the equator and one at say 60 deg sth, travelling at the same speed in a easterly direction, then the one at 60 deg sth would get back to its starting point first as it has the shorter lattitude to traverse around the world, land masses being taken as sailable

Bonz,.

I agree, although what loading lines have to do with it escapes me.

Yes the southerly vessel will get back first, but (under identical conditions) over the same period of time, they will both cover the same distance (ie, travel at the same rate of knots).

Let me put it this way. A knot is a measure of speed (nautical miles per hour). It is a relationship between distance covered and time elapsed. So in order to measure knots, you must START with a definition of distance (nautical miles) and a definition of time (hours). Not the other way round.

Tell me how the first ever log was designed? How did the designers know how far apart to put the knots? The answer of course is that they could not have without a pre-existing definition of a nautical mile before anyone ever wet a log off the stern of a ship.

Matt.

ahhh sorry i was off on a tangent. What i was saying is...If a knot is a nautical mile per hr, and a nautical mile is a second of a degree (at the equator), then a knot is faster at different lattitudes

Its all sorted now but back then it wasnt necessarily back then, and as I delved deeper some sources said the naut mile was measured in lattitudes and some said longitudes which would make more sense as longitudes are constant

Yep. Think you might have latitude and longitude swapped around in your last sentence though.

Cheers,

Matt.

nah, longitudes are a constant length and the degrees are constant distances except near the poles wherfe the earth flattens out a little.

Crikey Bonz, you're giving me a hard time tonight :-)

Line of longitude are a constant length (near enough) from pole to pole, but the distance between them changes significantly (greatest at the equator down to zero at the poles) as you pointed out earlier.

The flattening of the earth (more accurately a bulge at the equator) is what affests the distance between lines of latitude. But only by a very small amount (as you also pointed out).

So when you said that lines of longitude are constant, only in terms of the angle between them, not in terms of the arc they subtend on the earth's surface though. That is eaxactly why using degrees of latitude for measuring distance is problematic. Not so much for lines of latitude.

One more response and you win ;-)

Cheers,

Matt.

hahahah one more you say?

you said..
So when you said that lines of longitude are constant, only in terms of the angle between them, not in terms of the arc they subtend on the earth's surface though. That is eaxactly why using degrees of latitude for measuring distance is problematic. Not so much for lines of latitude.

I think you meant longitude for measuring distance as being less problematic as they are constant all around the world not so the distance between the parallels.

You do realise that the miscontinuity in the meridians as the lattitudes increase is also reflected in the Magellan meridian units hehehe thats why I use a Garmin

Dammit Bonz, this is defintately my last one (maybe).

You said, "I think you meant longitude for measuring distance as being less problematic as they are constant all around the world not so the distance between the parallels." Granted the distance between lines of latitude varies, but insignificantly for most purposes. But the distance between lines of longitude varies much more significantly (remember they curve in an touch at the poles).

1 degree of latitude at the equator = 59.9 Nm
1 degree of latitude at the poles = 58.3 Nm
Difference = 1.6 Nm

1 degree of longitude at at the eqautor = 59.9 Nm
1 degree of longitude at the poles = 0 Nm
Difference = 59.9 Nm

Which varies the most?

Matt.

P.S. You definately win this time.

heheh I can go one for ages on this, l actually I am learning lots more from what I already knew, and I obviously know much more then you do because I keep coming up with different things (not really I am just kidding about)

I think we are both saying the same thing. Up above in reply 22 you said "That is eaxactly why using degrees of latitude for measuring distance is problematic. Not so much for lines of latitude." (sic)

Just re-reading that are you trying to differentiate between DEGREES of lattitude and LINES of lattitude........ or degrees of LATTITUDE and degrees of LONGITUDE?

I agree that distance between parallels of latt vary much less across the globe, than do the distances between meridians of longitude hence the assertion somewhere up there that measuring knots as degrees ( or minutes or seconds ) in longitude, i.e. AROUND the world, is much more problematic than using knots as degrees of lattitude i.e up and down the world.

QED I think

Deja Vous Bonz, I think I spent last night doing this. I have thought about it though and I think that I know where we (?) are confused.

When we talk about using lines of latitude as a measure (and a whole degree is just another line of latutude), we are not talking about the length of the line (ie east/west), but the distance between the lines (north/south). The distance BETWEEN lines of latitude is (nearly) constant, but the distance BETWEEN lines of longitude varies significantly. That is why when you measure distance off a chart, you use the latitude scale (north/south or X-axis), because they occur roughly every 60 Nm regardless of where you are.

I agree that the LENGTH of lines of latitude varies, but you are measuring the distance between them (at right angle to) to measure distance. That is the value that is constant.

Reply 22 a typo, I meant to say "That is eaxactly why using degrees of LONGITUDE for measuring distance is problematic. Not so much for lines of latitude." Whoops.

Anyhow, I am only going to reply to this thread another 15 or 20 times and then I will run out of puff. Monday night footy tonight, I have other priorities.

Cheers,

Matt.

QED NOW methinks ;-)

Exactly what I reckon Matt

Mal & Di,

I think that I am responsible for the wayward direction that this thread has taken and for that I apologise.

If you aren't doing this already, then when you are entering co-ordinates from the format DDD MM SS.S to your GPS in DDD MM.MMM format then it is a simple matter of taking the original SS.S and dividing by 60 to give you the .MMM reading that you need to enter into your GPS.

For example:-

DDD MM SS.S
36 44' 47.9

The 36 degrees 44 mins stay the same, now take the 47.9 seconds and divide it by 60 to give you .798 therefore giving you:-

DDD MM.MMM
36 44.798

Another example

DDD MM SS.S
106 52 21.6

The 106 degrees 52 mins stay the same, take the 21.6 seconds and divide by 60 to give you .36 therefore giving you:-

DDD MM.MMM
106 52.360

Simple and effective to within 2 yards.

Cheers Heefers

or just set your gps to take the dd mm ss.s coords

You just can't jump to the answer that easy now Bonz...

Andrew

hahaha I jumped to that answer up above

AnswerID: 296890 Submitted: Saturday, Apr 05, 2008 at 23:08
Bonz (Vic) replied:
Mal you can set it to display dd mm ss.s

press menu menu and navigate to setup and hit enter

navigate to the "units" section and hit enter

"Position format" is at the top and you can select "hddd mm ss.s" in there. Thats degrees minutes and seconds to one decimal place (ss.s)

you can also select hddd mm.mmm which is degrees and minutes to 3 decimal places.

whats the difference? take the waypoint for in hddd mm ss.s for innamincka its
S 27 44' 30.0"
E 140 44' 08.4"

in hddd mm.mmm its
S 27 44.500'
E 140 44.140'

so you can see that half a degree (.500') is equal to 30.0 seconds in ss.s speak

doesn't seem to make it easier hmmm hope you understand it

And then someone took me to task hehehehe

LOL.....i might debate your answer.

On second thoughts............

Andrew

I am up for it hahhahah

Andrew,

DON'T go there!

Trust me.

Matt.

LOL.......just a quick question:

Is a nautical mile the distance obtain by multipling 1.852m x 1000 at a latitude of 127.5° E?

Andrew :-)

Andrew your simple answer is NO.... as 127.5° E is a LONGITUDE not a Lattitude

Wow, you ARE good ;-)

Andrew

Cute too I have been told