Although simple, the choosing the correct north reference setting sometimes confuses GPS users.
The difference between true north and magnetic north is called “variance” on marine charts and “declination” on topographical maps. These terms are to describe the difference between true north and magnetic north.
The difference between the two is constantly changing as the magnetic pole shifts. All official navigation courses teach that all courses should be laid out in true degrees because the magnetic pole is constantly moving.
That means that the variance today at a specific location will not be the same at a later date. If you plotted a course today using magnetic values and attempted to use those same course headings several years from now, the directions would not be the same.
And, to complicate the issue further, the difference changes as you move from one location to another.
The variance for the area south of Lake Calcasieu in 2005 was about 3 degrees east, and was decreasing at a rate of 7 feet per year.
And the variance changes as you move from one location to another.
The 3-degree east variance for the Calcasieu area would not apply if you were off the mouth of the Mississippi River, as the variance there is presently 0 degrees.
So, if you were negotiating a course from one position to another with your GPS set for TRUE readings, it would NOT be the same course as defined by your magnetic compass. The GPS unit would still steer you to the intended position using the true settings, but you should not change from the GPS headings to the magnetic compass headings.
By setting the configuration to Magnetic or Auto Magnetic in some units, the GPS automatically displays the courses, bearings, headings, tracks, etc., in the corrected variance settings. The library date in the GPS knows what the settings for whatever the year should be and what the variance for a particular area should be.
So I recommend that GPS users set their units to the magnetic north or auto magnetic configuration, as it will then display courses and directions in the same language as your magnetic compass. Let the GPS worry about the variance; it makes it easier for you. That way your magnetic compass, and the GPS headings and courses will be in the same format.
Another concern is the Distance settings. Usually most GPS units offer the user a choice of having the speed and distances described in statute miles and miles per hour, nautical miles and knots, and kilometers and kilometers per hour.
The choice is totally yours, but remember that a nautical mile is 1,852 meters (1.852 kilometers). In the English measurement system, a nautical mile is 1.1508 miles (6,076 feet). If that setting is used, the speed units will be in knots.
A statute mile or land mile is a unit of distance on land in English-speaking countries equal to 5,280 feet (1,760 yards) or 1.609 kilometers.
A selection of kilometers for a unit of length, means that the measure of distances equal to 1,000 meters (3280.8 feet or 0.621 mile).
A kilometer selection will display the speed values as kilometers per hour. For instance, the speeds of 30 mph, 26.06 knots and 48.28 kilometer per hour are the same exact speed; they are simply expressed in different terms.
I use the statute mile settings, but use the one you are most familiar with for your unit.
TIME OFFSETS settings
In Menu System Setup you will note that you have several choices to express time. You can select a 24-hour clock (military time) or a 12-hour clock (a.m. and p.m.). You will also note that there is a time Offset icon. You should select –5:00 hours for Central Daylight Saving Time. The unit should then display the correct time of the day in the clock format that you selected.
This sounds very scary, especially when the “warning” comes when you are underway. Relax: The GPS is simply telling you that it lost satellite communication and needs to “find” itself. This will normally occur if the unit has been off for longer than 30 days or has been moved for a distance of more than 200 miles while in an off mode.
Sometimes the warning will also appear if the unit is blocked from receiving the necessary satellites.
Simply by leaving the unit on for a period of time will allow it to receive the necessary satellites and display the position. Some brands have a method of entering a latitude-longitude position or a local nearby city to give the unit software a shortcut to finding the available satellites. That generally speeds up the overall progress.
So double-check your GPS unit. Check the configuration settings in your unit to ascertain if you are getting the accuracy and correct data that will allow you to get the most from the Global Positioning System.