What is the issue?GPS transformed the art of navigation forever, but there are subtleties of expressing a position that need to be understood. There are three different GPS conventions for describing a waypoint and it is important to know which convention you are using and the format of information you are being provided with from external sources.
Why address this?Accuracy of position is an important discipline when it comes to staying off rocks.
How to address this?Be aware of the expressions of position, standardise on the maritime approach and have a feel for the distances involved.
Here are the three conventions:
- • degrees, minutes and seconds (d m s)
- • degrees, minutes and decimal minutes (d m.m)
- • degrees, decimal degrees (d.d)
The maritime convention for expressing a position is predominantly degrees, minutes and decimal minutes (d m.m). You should select this as your standard.
Most GPS devices offer a choice of Lat/Lon expressions, and you should check which one your GPS is set up for and select your preference. When navigating check what standard is being presented when utilising external data sources. It should be clear in the way it is expressed.
Be particularly careful when working with older charts as there can be some variance and they often use degrees, minutes and seconds (d m s). So if taking positions from charts you will need to convert it.
To convert seconds to decimal minutes, divide the seconds by 60. For instance, if you have 52 deg. 38 min 17 sec. N converts to a GPS expression of 52 38.283 (divide the 17 by 60 to get 0.283 minutes). Most GPSs express degrees of latitude in only two digits as the highest latitude you can have is 90 degrees. Incidentally, an easy way to remember that the horizontal lines on a nautical chart are parallels of latitude is to remember this 'Lat is Flat’.
To convert decimal minutes into seconds simply multiply the fraction of a minute by 60. 52 38.283 then becomes 52 deg. 38 min 17 sec again (well not perfectly so if you multiply .283 by 60 you actually get 16.98, this slight difference is a result of having only three digits to work with so some of the detail is lost in the first conversion, it rounds up to 17 seconds).
To convert degrees, minutes and seconds to decimal degrees, you divide minutes by 60, seconds by 3600, add to the degrees (degrees + minutes/60 + seconds/3600). Latitudes run from -90.00 degrees at the South pole to 90.0 degrees at the North pole. Longitudes run from -180.00 degrees to 180.00 degrees. West longitudes (locations West of the Greenwich meridian) are positive while East longitudes (locations East of the Greenwich meridian) are negative. Decimal degrees are not used very often in a maritime context; it is mostly for land-based positions or in a computing context.
Finally, GPS accuracy can be very precise. Most people use degrees, minutes and three decimals of a minute which is very detailed. It is worthwhile to make these figures real so you can have a feel for that level of detail.
If we leave aside longitude, as the further from the equator you travel the shorter they become, and use latitude, the most a minute can be is a nautical mile. There are 6,076 feet in a nautical mile, so:
- • The first digit, after the decimal is a tenth representing = 185 metres (607 feet).
- • The next digit is one hundredth = 18.29 metres (60 feet) and,
- • The third digit a thousand = 1.83 metres (6 feet).
So the three-digit level of precision is extreme.
With thanks to:Michael Harpur, Yacht Obsession.
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