Knowing your location

I was listening to the VHF radio on channel 16 this morning and listening to all the boats in some form of distress (perceived and real.)  For most communications the US Coast Guard will switch folks over to Ch. 22A for the discussion portion as 16 is for hail and distress only.  Anyway, an interesting discussion took place on Ch. 22A this morning when a boater reported a “row boat” that was unmanned partially submerged in the vacinity of the Bay Bridge.  The US Coast Guard ultimately asked the good samaritan to give their position.  The position that they read off of their GPS was in degrees and decimal minutes.  Interestingly my IPhone gives GPS coordinates in several ways which can be a little confusion especially if you are trying to take GPS coordinates and apply them to a paper chart.  I am sure that the US Coast Guard is smart enough to sort out whatever coordinates you give, but if you are trying to take a GPS coordinate and put it on your paper chart you need to understand how to translate.

If you are not familiar with latitude and longitude, here is a crash course in navigation. Lines of latitude and longitude are hypothetical lines on the surface of the Earth. On the Earth, lines of latitude are circles of different size. The longest (largest in diameter) is the equator, whose latitude is zero, while at the poles, at latitudes 90° north and 90° south (or -90°), the circles shrink to a point. On the Earth, lines of constant longitude (meridians) extend from pole to pole, and cross the lines of latitude. Every point on the surface of the Earth has coordinates where a given line of latitude and a give line of longitude intersect (cross). To sum it up, latitude is measured from the equator, with positive values going north (0 to 90) and negative values going south (0 to -90). Longitude is measured from the Prime Meridian (which is the longitude that runs through Greenwich, England), with positive values going east (0 to 180) and negative values going west (0 to -180). So, for example, 65 degrees west longitude, 45 degrees north latitude is -65 degrees longitude, +45 degrees latitude. Now that the designations (and reasons for them) are perfectly clear, here is the set of formulae if you need to do this manually.

Degrees Minutes Seconds to Degrees Minutes.m (GPS)
Degrees = Degrees, Minutes.m = Minutes + (Seconds / 60)

Degrees Minutes.m to Decimal Degrees
.d = M.m / 60, Decimal Degrees = Degrees + .d

There are 60 minutes in a degree and 60 seconds in a minute; 3600 seconds in a degree. There are 360 degrees in a complete circle or sphere but in all longitude and latitude measurements, the total of the degrees is expressed as 2 halves of 180 degrees each.

Below are the three common ways to express the location of J World Annapolis:

Decimal Degrees (WGS84)

Latitude Longitude
38.969936 -76.477496

Degrees, Minutes & Seconds

Latitude Longitude
N38 58 11 W76 28 38

GPS (Degrees and Decimal Degrees)

Latitude Longitude
N 38 58.196 W 76 28.650

How to convert among the three different forms of Latitude and Longitude

This document outlines how to manually convert among degrees minutes seconds, degrees and decimal minutes and decimal degrees:

To convert from degrees minutes and seconds to degrees and decimal minutes

  • Divide the number of seconds by 60 and add that value to the number of minutes – do not change the number of whole degrees
  • Example
    • To convert 40º 05’ 18” N to degrees and decimal minutes do the following
      • We are converting the seconds to a decimal part of a minute
      • Divide the seconds (18) by the number of seconds in a minute (60) this will give the decimal part of a minute that 18 seconds represents
      • 18/60 = 0.3
      • Add the decimal minutes to the minutes and “discard” the seconds
      • 05 + 0.3 = 5.3
      • So the answer is 40º 05.3’ N

To convert from degrees and decimal minutes to decimal degrees

  • Divide the number of minutes by 60 and add that value to the number of degrees – do not change the number of whole degrees
  • Example
    • To convert 40º 05.3’ N to decimal degrees do the following
      • We are converting the minutes to a decimal part of a degree
      • Divide the minutes (5.3) by the number of minutes in a degree (60) this will  give the decimal part of a degree that 5.3 minutes represents
      • 5.3/60 = 0.08833
      • Add the decimal degrees to the degrees and discard the minutes
      • 40 + 0.08833 = 40.08833
      • So the answer is 40.08833º

NOTE: To convert from degrees minutes seconds to decimal degrees do the task in two steps as shown above, first go to degrees and decimal minutes, then convert to decimal degrees

To convert from decimal degrees to degrees and decimal minutes

  • Multiply the decimal part (the numbers to the right of the decimal point) times 60, that will give the number of minutes and add that value to the whole degrees – do not change the number of whole degrees
  • Example
    • To convert from 40.08833º N to degrees and decimal minutes do the following
    • We are converting from decimal degrees to minutes
    • Multiply the decimal part (0.08833) times the number of minutes in a degree (60) and replace the decimal degrees with the minutes
    • 0.08833 x 60 = 5.3
    • So the answer is 40º 05.3’ N

To convert from degrees and decimal minutes to degrees minutes seconds

  • Multiply the decimal part of the minutes( the number to the right of the decimal) times 60, that will give the number of seconds and add that value to the whole minutes
  • Example
    • To convert from 40º 05.3’ N to degrees minutes seconds do the following
    • We are converting the decimal minutes to seconds
    • Multiply the decimal part (0.3) times the number of seconds in a minute (60) and replace the decimal minutes with seconds
    • 0.3 x 60 = 18
    • So the answer is 40º 05’ 18” N

NOTE: To convert from decimal degrees to degrees minutes seconds do the task in two steps as shown above, first go to degrees and decimal minutes, then convert to degrees minutes seconds

Due to the fact that GPS information is so readily available now for daily weather information, automotive use, aviation, marine and personal use, many companies have set their own standards as to how that information is displayed and disseminated. (Get NAD27 and NAD83 conversion info from NOAA here…) Any method is very accurate in the native form but converting between them can create substantial error.

When using a hand held GPS for instance, you may have to use the information and convert it to or from what a GIS map displays. We have created this series of converters to take degrees, minutes and seconds and convert them to decimal degrees or degrees and decimal minutes. In order to establish a point, you MUST have latitude and longitude coordinates in one form or another. If you need to get that information relating to distances between points, use the GPS Latitude and Longitude Distance Calculator. If you need to get that information to a different format, use these converters. (Please keep in mind that this is a conversion utility and NOT an entry for a point. Validity is NOT tested. The minutes and seconds designations are OF ARC and not of time.) The different formats are as follows:

Degrees, minutes and seconds (DDD, MM, SS) is the conventional method of presentation. Remember that in the case of degrees of latitude, the format would be negative or positive DD, while longitude could be negative or positive DDD. There is usually a quadrasphere designation as well, such as N, S, E or W, based on the equator and the prime meridian. For the hemisphere, either North or South, the designation is N or S for latitude. East of the prime meridian is E (positive) and West of the prime meridian is W (negative). Negative numbers (in certain situations, Southern latitude is displayed as negative; if you see a negative latitude, it is South while a negative longitude is West) can also be used to express a quadrasphere designation. In this converter, we do neither since it does not matter; we are ONLY converting numbers. All numbers are assumed to be positive.

Decimal Degrees is displayed as the degrees in normal value, with both minutes and seconds in decimal format, as a degree value. There is ONLY a degree designation. (DDD.DDDD) A maximum of 4 decimal places is adequate.

What is most often known as GPS format (though not all GPS units use it o agree on it…), is displayed as (DDD,MM.MMMM) in which seconds are converted to decimal minutes, as a minute value. Seconds have the value of 0 to 60, with 0 and 60 (usually designated as 0 to 59 and then restarting at 0) being the same value, minutes have the same characteristics and degrees are valid from 0 to 180 and 180 to 0, (0 and 180 do NOT have the same value) both North and South of the equator, and East and West of the prime meridian.

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