WORKING WITH COORDINATES AND UNITS

(...continued)

Lat/Lon

Everyone has probably heard the terms latitude and longitude as referencing to a grid system that covers the earth. This system has been around for hundreds of years but gets a new dressing for gps use. Latitude and Longitude assume that the earth is a big ball and defined as a spheroid. The fact that it rotates around the poles on each end is used to develop a grid system that is based on this angular motion of rotation. Basically the idea of longitude and latitude is to measure the angles represented if you were slice the earth into a circle. If you slice the earth at the equator and then divide it up you would have a longitude line at each degree around the circle. These lines would all meet at the poles. Rotation of the earth would dictate that the sun would travel the same angular longitudinal distance in the same time. Each one of these lines is called a meridian. If you slice through a meridian you have a circle and each degree around the circle is called a latitude. While longitudes all meet at the poles latitudes are all parallel to each other.

For more precise measurements the degree is divided into 60 minutes and the minute is divided into 60 seconds just like time. For calculation purposes we often measure and calculate in decimal fractions of a degree or decimal fractions of a minute. We always divide seconds decimally if we need smaller unit. Garmin gps units can be set to display lat/lon in all three measurement systems. The equator becomes an obvious point for measuring latitude it is defined as 0 degrees while points North are measured as degrees on North latitude until you reach 90 degrees at the north pole. Similarly the southern hemisphere is measured in degrees of south latitude. There is no corresponding obvious point for longitude lines. By international agreement 0 degrees longitude is a meridian line that cuts through England and is called the prime meridian. Longitudinal distances are measures west and east until they come together at the international date line 180 degrees later.

While Garmin can display and convert from the three different systems used to define lat/lon there can be considerable confusion among users looking at the display and comparing it to some external information. This is because some data is not very precise in its use of decimal points and the lack of a degree sign on most keyboards can encourage a substitution of the decimal point. When comparing numbers consider that minutes can only go to 59 and will then roll over. If the data just after the decimal has digits above 5 then it is likely to be a decimal part of a degree and not minutes. Similarly for seconds.

Decimal parts of a degree can be converted to minutes easily by multiplying by 60. A space between degrees and minutes is the preferred separator when a degree sign cannot be used to avoid confusion. Similarly decimal parts of a minute can be converted to seconds by multiplying by 60 thus, if you have ddd.ddddd you can convert to ddd mm.mmm by:

Many folks think the lat/lon measuring system is independent of the datum issue however this is not the case. In the precision and accuracy required for gps use the same problems are found with lat/lon as with any other grid system we might employ. It earlier times measurements weren't precise enough to make this obvious. Even today making angular measurements using devices like sextants is not accurate enough to make the datum usage much of an issue.

Angular measurements, however, are not particularly convenient for measuring distances and, even in its simplest form requires the use of trigonometry. Long distances become even worse since a line between two cities at different longitudes and latitudes on a map does not represent the distance traveled between those two points when actually traversing the distance on the real earth. Instead we have to resort to great circle distances and measurements for these cases. Mariners and aviators use nautical miles to help simplify this calculation. One nautical mile is 1 minute of latitude thus a degree of latitude is 60 nautical miles. When measuring distance in the east-west direction the distance correction varies with latitude. While accurate measures would require us to consider the earth as an ellipse a simple estimation can assume the earth to be a spheroid and thus the east-west distance in nautical miles is given by the formula:

Page 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9