degrees and decimal mins to sexagesimal

[Shearwater]

Looking through my Garmin GPS manual (the unit is on the boat of course) I realise from all the diagrams the lat and long format seems to be Degrees and M.m (ie decimal minutes).

I can find the formula for converting DMS to DM.m so I could input a wpnt from chart to unit ( D=D, M.m = M+(s/60) )but am having problems finding the vice versa. I may want to move that wpnt on the unit's map and then pencil my move onto the chart. And I really want the formula, not a website for obvious reasons.

Perhaps I can change the Garmin to accept sexagesimal....have I missed something? As always many thanks to those more learned than I !

 

[JumbleDuck]

I can find the formula for converting DMS to DM.m so I could input a wpnt from chart to unit ( D=D, M.m = M+(s/60) )but am having problems finding the vice versa.

D=D, M=M, s = m x 60.

For example, 14^o 38.25' = 14^o 38' (0.25 x 60)" = 14^o 38' 15"

 

[AngusMcDoon]

In code, starting with d and m, and ending up with D, M and S...

D=d;
M=int(m);
S=frac(m)*60.0;

 

[VicS]

Looking through my Garmin GPS manual (the unit is on the boat of course) I realise from all the diagrams the lat and long format seems to be Degrees and M.m (ie decimal minutes).

I can find the formula for converting DMS to DM.m so I could input a wpnt from chart to unit ( D=D, M.m = M+(s/60) )but am having problems finding the vice versa. I may want to move that wpnt on the unit's map and then pencil my move onto the chart. And I really want the formula, not a website for obvious reasons.

Perhaps I can change the Garmin to accept sexagesimal....have I missed something? As always many thanks to those more learned than I !

Formula ?? Its just simple arthmetic.


As you have already said divide by 60 to convert seconds into a decimal part of a minute

eg 15 seconds = 15/60 = 0.25 minutes


Threfore multiply the decimal part of a minute by 60 to get seconds !

0.25 minutes = 0.25 x 60 = 15 seconds

 

[Shearwater]

Thank you. I am arithmetically challenged.

 

[Stork_III]

My 12Xl will accept D M s as an option. In the NAV Setup to change format.

 

[JumbleDuck]

My 12Xl will accept D M s as an option. In the NAV Setup to change format.

So will my ancient GPS128.

I have just discovered that 1/60 of a second was a third, and that there were also fours and fifths. Hooray for Babylonian mathematics!

 

[VicS]

So will my ancient GPS128.

I have just discovered that 1/60 of a minute was a third, and that there were also fours and fifths. Hooray for Babylonian mathematics!

you meant to say 1/60 second is a third presumably ? and a 1/60 of that a fourth etc

 

[JumbleDuck]

you meant to say 1/60 second is a third presumably ? and a 1/60 of that a fourth etc

D'oh. Now corrected. Well spotted, that man.

 

[nigelmercier]

you meant to say 1/60 second is a third presumably ...

Third?

 

[VicS]

Third?

yes.

1/60 second is a third or tierse. I believe that the numbers should be in Roman numerals

Obsolete ..... except in Duckland

 

[JumbleDuck]

Obsolete ..... except in Duckland

I did say that I had only just learned about thirds and fourths, you know. I found it interesting because it links "second" the position with "second" the angular measure ... and, I presume, in a Babylonian way, with "second" the time interval.

 

[tillergirl]

Looking through my Garmin GPS manual (the unit is on the boat of course) I realise from all the diagrams the lat and long format seems to be Degrees and M.m (ie decimal minutes).

I can find the formula for converting DMS to DM.m so I could input a wpnt from chart to unit ( D=D, M.m = M+(s/60) )but am having problems finding the vice versa. I may want to move that wpnt on the unit's map and then pencil my move onto the chart. And I really want the formula, not a website for obvious reasons.

Perhaps I can change the Garmin to accept sexagesimal....have I missed something? As always many thanks to those more learned than I !

I'm sure I've missed something here: what chart have you got that shows 'seconds'? UKHO charts (1:50,000) will show minutes and tenths of minutes which is either .1 or 6 seconds so surely you just plot the Garmin DM.m. If your chart is say 1:150,000 the Lat/Long scale of minutes will be divided into 5 i.e. .2, .4, .6 etc or 12, 24, 36 seconds etc. Again stick with the decimal. At the 1:150,000 scale you have to have a really thin pencil to benefit from seconds. And in more detailed large scale charts I'd think I would be wanting to know the survey on which the chart was drawn as you may be expecting too much of modern positional accuracy when plotted on a chart based on very old data: for example round here some parts of the shore were last surveyed 40 years ago which is before the birth of GPS. And even then 1:5000 is still using tenth's of a minute in the scale.

 

[VicS]

I'm sure I've missed something here: what chart have you got that shows 'seconds'?

Perhaps the OP is confused by the way in which minutes and decimal parts of a minute are written

In #2 above JD writes it as 14° 38.25' but it is often/normally written 14° 38'.25. This could perhaps be confused with 14° 38' 25" . ???

 

[JumbleDuck]

In #2 above JD writes it as 14° 38.25' but it is often/normally written 14° 38'.25. This could perhaps be confused with 14° 38' 25" . ???

I've seen that format (12^o34'.56), but I don't know how official it is, and I don't feel like spending CHF138 on ISO 6709, which is the relevant standard. The wikipedia entry suggests that Annex D suggests that the symbol should always come after the value, but it's not very clear. Where's antarcticpilot when we need him?

 

[VicS]

two places I've looked. My ( elderly) Imray chart of the IOW and large scale Admiralty charts on Visit my Harbour . Both use the format DD° MM'.mm

I cannot see it specified in 5011

My thoughts were simply that, bearing in mind what Tillergirl says, the Op might be confused into thinking that something in the form DD° MM'.mm could mean DD° MM' SS"

 

[AntarcticPilot]

In code, starting with d and m, and ending up with D, M and S...

D=d;
M=int(m);
S=frac(m)*60.0;

Requires care - that COULD result in something like 30' 60" if the actual value was 31' 0", because of tiny rounding errors caused by the incremental nature of floating point representations.

My usual procedure is to round (using the round function if available) to the expected degree of accuracy, then compute the degrees, minutes and seconds as suggested.

 

[AngusMcDoon]

Requires care - that COULD result in something like 30' 60" if the actual value was 31' 0", because of tiny rounding errors caused by the incremental nature of floating point representations.

My usual procedure is to round (using the round function if available) to the expected degree of accuracy, then compute the degrees, minutes and seconds as suggested.

It's not real code, just pseudo-code to represent how to do it by hand. In real code of navigation devices I have worked on the internal representation of coordinates is in double precision floating point numbers with a single number in D.d format.

 

[tillergirl]

internal representation of coordinates is in double precision floating point numbers with a single number in D.d format.

I have no idea what that means but am really glad that there are clearly people around who do!

I have always understood the standard annotation is 50(degree sign)10'.8N to represent degrees, minutes and tenths of a minute (that's in the RYA Navigational manual). My GPS output goes to three decimal places (which is pretty daft if you think about it - who can place a cross on a chart to that accuracy) and since the first decimal place equals a tenth so surely that's an end to it.

 

[Shearwater]

I'm sure I've missed something here: what chart have you got that shows 'seconds'? UKHO charts (1:50,000) will show minutes and tenths of minutes which is either .1 or 6 seconds so surely you just plot the Garmin DM.m. If your chart is say 1:150,000 the Lat/Long scale of minutes will be divided into 5 i.e. .2, .4, .6 etc or 12, 24, 36 seconds etc. Again stick with the decimal. At the 1:150,000 scale you have to have a really thin pencil to benefit from seconds. And in more detailed large scale charts I'd think I would be wanting to know the survey on which the chart was drawn as you may be expecting too much of modern positional accuracy when plotted on a chart based on very old data: for example round here some parts of the shore were last surveyed 40 years ago which is before the birth of GPS. And even then 1:5000 is still using tenth's of a minute in the scale.

Tillergirl gets my vote as being the most acute.

After 2 or 3 hours of struggling through the Garmin VHF DSC manual, followed by the new Cobra hand held VHF manual and revision of press cuttings from 20 odd years of PBO
I looked upon the graphics of the Garmin GPS manual and suddenly thought I didn't know how to convert DMS to DM.m and, more to the point, the other way round without stopping to think I didn't have to. Too much reading, too much thinking ....... probably.

So apologies for wasting peoples time, yes I do realise fully that minutes are subdivided decimaly on charts and GPS, that that is a GOOD IDEA.