When trying to enter a waypoint on a GPS unit, many people run into problems. Either the GPS rejects their input, or they end up with a waypoint in the wrong location which leads to navigational errors.
Let’s look at why this happens.
There are different formats for recording Lat/Long locations and one needs to recognise which format is used.
The three most common, and most confusing formats are:
a) HDDD MM SS (Hemisphere, Degrees, Minutes, Seconds)
Example: S33 45 15 (South 33 degrees, 45 minutes, 15 seconds)
b) HDDD MM.mmm (Hemisphere, Degrees, decimal fraction of a minute)
Example: S33 45.25 (South 33 Degrees, forty-five and a quarter minute)
c) HDD.dddd (Hemisphere, Degrees, decimal fraction of a degree)
Example: S33.75417 (South 33 degrees, plus 0.75417 fraction of a degree).
I deliberately omit here the Longitude portion for simplicity, which works exactly the same.
Are you surprised, that all three these refer to exactly the very same position?
So how is it, that these three are actually the same?
a) S33 45 15
b) S33 45.25
c) S33.75417
The trick is to look at the spaces and the decimal point.
Fifteen seconds is a quarter minute (in time, angle and position), which can be written as 0.25 minutes.
So 45.25 minutes is the same as 45 minutes, 15 seconds.
Don’t proceed until you fully grasp this.
In the same way that 60 minutes of time equals one hour, so too, does 60 minutes of angle equal one degree angle.
To convert seconds to hours or degrees, divide it first by 60 to convert to minutes, then divide by 60 again to determine the hours or degrees… or simply take the shortcut and divide by 3600 (which is 60 times 60).
In our above case, 15 seconds [from (a) above] divided by 60 is 0.25 minutes [shown in (b) above].
Divided by 60 again, gives you 0.0041666 or rounded off to 0.417 [as shown the last part of (c) above].
If the format is not indicated, then look at the spaces and decimal point to determine which format is used. Note that there can not be more than one decimal point in both the Latitude and/or longitude.
If you are given co-ordinates in one format, and your GPS is expecting co-ordinates in a different format, you will run into problems. For example, trying to start a minutes or seconds field with any number greater than 59 cannot be accepted, because 60 seconds would be one minute, just as 60 minutes should be one degree.
So how do you handle this problem? You have two choices:
a) Either convert your co-ordinates to the format the GPS expects, or
b) Change the settings on your GPS to expect the same format that you wish to enter the co-ordinates in. Go to the Position or Location settings, and choose between:
HDDD MM SS or
HDDD MM.mmm or
HDDD.ddddd
Often, co-ordinates are handwritten or typed with mistakes in the spaces and decimal point. If someone gives you this as a co-ordinate: S33.15.25
What would you make of that?
I would assume that to be S33 15 25 but its also possible it should be S33 15.25
The trick is to plot both versions on a map and hopefully you can spot which is the most likely.