Lat/Long Format

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kennharriet

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C Dory Year
1999
C Dory Model
22 Cruiser
Vessel Name
Lochsa
Of the three ways to display latitude and longitude, which is proper for marine use? Is it Degree, Minutes, decimal Minutes, ie: 48*36.12'N, 114*08.12'W. Which datum is used for marine use? ie: WGS84. What is displayed on a DSC radio?

Aircraft and aeronautical charts use the above. Does it stand to reason marine use would be the same?
 
The nautical charts I use are marked down to arc minutes, and then a scale is shown with 10 divisions, indicating a decimal part of an arc minutes. I have several navigation programs, and they allow me to select my display as to decimal arc minutes or arc seconds. Remember that 0.01 arc minute is 50" and I'd hate to bet on any chart being that accurate, though the C-Map display usually will beat that.

As to using a consistent WGS-84 datum, good luck. Remember that nautical charts have been developed over the years, and obviously, WGS-84 was adopted in 1984. I just bought new Canadian charts, and they use several different geodetic models. If you think about it, one can use the chart to get in the vicinity and then estimate the offset between the chart and reality. I have a Mexican chart of Baja Calif which hasn't been updated for 100 years. Same for the Bahamas. Wonder why that submarine ran into a seamount over at Hawaii?

Boris
 
Most US and Canadian charts are in degrees, minutes, and decimal minutes. To be sure, count the divisions in one minute along the chart edge. If there are 10, its decimal.

Most current charts are based on the WGS 84 datum. Some Canadian charts are still using NAD 27 datum. The datum should be shown on the chart in the legend. NAD 27 was the datum most in use prior to WGS 84.

A DSC radio displays the GPS coordinates fed to it by the GPS unit. If the GPS is set to WGS 84, thats what you see on your DSC radio. If you are receiving a position report from another DSC radio, the coordinates you see are what the other boats GPS fed to it's DSC radio.
 
I suggest setting your GPS to decimal minutes. The more numbers the more precise!
 
Thanks for the lesson. So, if one was in trouble and calling for help using the radio, they would say: "My position is forty eighty degrees, thirty six, decimal one two minutes North, by one one four degrees, zero eight, decimal one two minutes West". I have an older non-DSC radio so would only have voice communication. Is there a standard format the USCG uses? What format would they give to rescue aircraft?
 
Most GPS read to three decimal points in the minutes. Using the nomenclature of 80 degrees, 37.345 minutes would be proper in communication.

However don't ever assume about the marking of charts or datum. It will be marked on the chart. I checked the nearest chart to my chart pile and it was for Kanoehoe bay, and it is in Degrees, Minutes, seconds and decimal seconds. A nearby Small Craft chart, reads in Degrees and minutes. The datum is NAD 83, considered equivilant to WGS 84, and gives a correction factor for NAD 1927.

Most chart chips read WGS 84, but again, check the label, and double check what the datum is set on for the chart plotter in the setup menu.
 
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