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GPS Coordinate Explanation

There are many different GPS coordinate systems in use. Three different ones are presented here, numbered below.

Coordinate Basics

The easiest way to depict a point on a two dimensional map is with two lines that cross. One line that goes up and down on the map and another that goes side to side. Where they cross is the location of the point you are interested in -- the campsite, the waterfall, or where you are standing at a given point in time. Those working with maps refer to those lines as the coordinate lines. Usually they are given as a set of numbers for one line and another set of numbers for the second line. Placing the two lines on the map indicates a point.

The earth is a sphere, or close enough that any error doesn't matter much to us. Coordinates can be used on the spherical earth, too, but it's more difficult. This is reflected in the maps we use and the different coordinate systems developed to pinpoint locations on those maps. If it were easy, there would be just one "best" coordinate system, but some systems work better for some uses than for others, and this is shown by the kinds of maps in use. Out of the several hundred different coordinate systems, three are explained below. Pick the one that suits you best for the kind of maps you use most and the types of information you require from them.

Distortion

Maps (except for globes), are flat, but the earth is a sphere. Trying to depict a round surface on a flat map distorts the true relationships somewhat, sort of like trying to flatten an orange peel. That means that there is some distortion in almost every map made. It may be almost unmeasurable in the very large scale maps, or enough so as to not matter to most users. GPS units have the capability of being accurate enough that the error might always be measurable. This distortion complicates use of coordinate systems, and is one of the reasons that there are many different ones.

Map Grids

Most maps have a grid of some kind marked on them, lines that crisscross the map in two directions. Probably the most common is the latitude and longitude system, usually marked in degrees. The Earth's equator is the zero line, the base line for latitude, and the numbers increase both to the north and to the south from there to a maximum of 90 degrees, which is a single point at each geographic pole. You cannot get more than 90 degrees away from the equator. Like the equator, the other latitude lines all go east and west, go all the way around the earth, and are referred to as the "parallels," because they do not cross each other. They are the same distance apart all the way around the earth. If you cut the world up along the latitude lines, like most people do to an onion, you'd have a bunch of little disks.

The longitude lines, also known as the meridian lines, go north and south, and are a bit different because they all cross each other at the poles. If you cut the Earth up like most people do to an orange or an apple, you'd get a mess of semicircular wedges. The line number also does not extend past either pole, the extension of a line is known by a different number on each side of the poles. The base line for longitude runs through a little town in England called Greenwich (pronounced there as "GREN-itch"), heads towards each geographic pole (not the magnetic poles), and then around to meet on the opposite side of the Earth from Greenwich. This Greenwich base line is called the prime meridian, and the numbers for each line to the east or west of it increase in those directions until the geographic north and south poles where it continues at a common 180 degree line. This line is basically the international date line and exactly opposite the prime meridian. This is an example of the different names and numbers when a line crosses the poles as mentioned above, (used to be called the anti-meridian).

The longitude lines are numbered east and west from the prime meridian. There are 360 degrees in a circle, but since we are measuring from a base line in both directions, the degrees east and west can only go up to 180 in each direction before meeting the base line on the opposite side of the Earth. You cannot get more than 180 degrees east or west longitude. In fact, zero and 180 are neither east or west, but as soon as you go off of either of them you are either east longitude or west longitude.

Now you have many lines circling the world -- pick one going in each direction. Easy ones would be the equator, the base line for latitude, and the prime meridian, the baseline for longitude, and it's extension, the international date line. These lines cross at two points. Unfortunately, those two points are seldom the ones people are on or headed for, so.....

1. Degrees/Minutes/Seconds

The most popular coordinate system is the one that shows standard Degrees/Minutes/Seconds coordinates. Remember that there are 360 degrees, but you can only go up to 90 degrees north or south latitude, or just short of 180 degrees east or west longitude. At the equator a degree of longitude is over 111km (69 mi) wide, so smaller divisions are required for depicting points accurately. The system in use is best remembered like what is used on a clock. Think of a degree as an hour. Each degree is broken up into 60 minutes, each minute is broken up into 60 seconds. A minute is still over a mile wide (1.6 km) at the equator. A second, at .0192 miles, is getting more manageable, but that is still over 100 feet (30 meters). Longitude lines get closer together until they reach the poles, but latitude lines stay the same distance apart all the way to the pole. The distances between latitude lines are easier, always those same figures given above for the equator longitude data.

For more accuracy,

An example of a DMS coordinate is:
N61° 11' 05.5" W130° 30' 10.0"

Usually presented like this:
7. RAPIDS N61° 11' 05.5" W130 30' 10.0"

The first number (7) is the waypoint number. A waypoint is "GPSese" for a point indicated by coordinates in the GPS receiver unit. The word RAPIDS is the waypoint name. The long string of numbers after it is the actual coordinate set. Last is a description or comment of some kind.

The N61 is the number of degrees of North Latitude (N = north). Remember that the degree numbering starts at zero, the Earth's equator. In this case the N61° line runs through the southern Yukon and Northwest Territories, just above the other northern provincial borders. The 11' is the number of minutes (' = minute) north of that. A minute is 1/60th of a degree. The 05.5" is the number of seconds (" = second) north of 11 minutes. A second, like on a clock, is 1/60th of a minute.

The same system works for the rest of the coordinate, west longitude part. Remember from the basics above that we're working from a meridian baseline running through Greenwich, England. The east-west latitude lines are parallel and don't cross each other, but the longitude lines all cross at both geographic poles, not the magnetic ones your compass points towards. Also remember that the degree, minutes, and seconds lines get closer together the closer to the poles on any map you are working with.

The north latitude part of the coordinate indicates a line that runs around the whole earth at a given distance above (north of) the equator. The west longitude part of the coordinate indicates a line some distance west of the meridian line running north and south through Greenwich, England. Because the latitude lines are parallel we can assign a given number of feet or meters for each degree, or minute, or second of distance. That doesn't work for longitude, though, because the lines keep getting closer together the farther from the equator you get.

2. Degree Decimal Minutes

Another popular system is a decimal form that I understand is the standard many governments have set between themselves for handling coordinate data. It also seems to be used by at least some GPS receiver manufacturers for handling data within their units. When you want to see a different coordinate format, everything is converted from the decimal data in the machine to your requested coordinate system and shown on the GPS screen. There are probably several different names for the decimal system (means I have not found a standard one!). One form of decimal coordinate looks like this:

N61° 11.0924' W130° 30.1660'

The full version will look like:
7. W RAPIDS N61 11.0924 W130 30.1660

You'll notice that the degrees and minutes are there, but the seconds seem to be missing. They're there, just indicated as a decimal fraction of the minutes in each longer than pure D/M/S number. This is figured from the D/M/S system by just taking the seconds part of the coordinate and dividing by 60, which converts it to a decimal form of the minute part of the coordinate. Now just use it to form a complete decimal of the minutes by putting a period after the minute and stringing the division answer out behind it.

For instance, here is the D/M/S north coordinate used above:
N61° 11' 05.5"

Divide the 05.5 by 60, which gives you 0.0916666. Put a period after the 11 seconds, then string the numbers you got from the division of the seconds by 60 after it, to look like this:
N61° 11.0916'.
(Most units only show it to four decimal places as given above.)

To convert a decimal coordinate back to a D/M/S coordinate take the decimal part and multiply by 60. Remember to use the .0916 not just the 916 without the decimal. .0916 times 60 equals 5.496, and just make a D/M/S coordinate out of that:
N61° 11' 05.49"
which is pretty close to the 05.5" we started out with. You'll have to convert both the latitude and the longitude parts of the coordinate to get the full decimal equivalent coordinate, of course, like the sample above. Gardown, one of the shareware GPS software conversion packages (written for Garmin receivers only) uses the above as it's default mode between the computer and the GPS unit (newer versions of Gardown allow you to use other modes like UTM, etc.).

This concept can be taken further by converting the whole latitude or longitude coordinate to a decimal degree. We already have the above N61° 11.0916' Take the 11.0916 and divide that by 60 to convert it to a decimal amount. You'll get 0.18486, so tack that after the N61 which will now read:
N61.18486°

Each half coordinate is one (long) number. It can be shortened in the number of decimal places, depending on the accuracy you require.

To convert back, multiply the decimal part by 60 (don't forget the decimal point) -- .18486 x 60 = 11.0916, which means 11.0916 seconds, or 11.0916'. If desired, continue as above to multiply the .0916 by 60 to give you the whole degree/minute, seconds coordinate two paragraphs above. Again, remember the decimal, and you'll have to convert both the latitude and the longitude portions to get the whole grid point. Sure is easier to let the GPS receiver do all the math, isn't it?

GPS interface software packages other than Gardown (like Waypoint+) may do this differently than above, and may call it something else.

3. Universal Transverse Mercator (UTM)

UTM coordinates are a military grid system used for quickly pin pointing map locations with reasonable accuracy. It is very easy, and great for wilderness trippers passing info from map to map. This form of map coordinate makes it very easy to transfer a point of interest like a campsite or a waterfall marked on my topo map to your own copy of the same map. It can also be done on the 250K series, but of course, not as accurately.There are specific inaccuracies involved in the UTM system, most of which are insignificant enough to not matter to us wilderness tripping types.

UTM coordinates are much quicker and easier to use out in the wilderness than the standard degree/minutes/seconds (also deg/min/sec, or DMS coordinates promoted by GPS users and manufacturers. The UTM grid lines are physically printed in the body of all Canadian topo maps at the present time, whereas the DMS coordinates are only indicated along the map edge. Also, the UTM grid squares are uniform, but the distance between longitude lines changes significantly from the bottom to the top of every map this far north. Degree lines are not straight or square, especially in northern Canada where the "good" wilderness is found, but UTM lines are straight and much easier to figure accurate location from. Because the degree lines are not printed on the map, careful alignment and measuring on the map are required to effectively use DMS coordinates. Special purchased equipment (admittedly inexpensive, but must be carried to be useful) helps considerably with DMS, but all that is required with UTM is a pen or pencil. Most GPS receivers will convert DMS coordinates into UTM coordinates in the blink of an eye, or from UTM to DMS or to many other coordinate systems, saving you the laborious math. A GPS can be used with either DMS or UTM, but is not required with UTM. A GPS can make either coordinate system easier, but not necessarily more accurate until Selective Availability is turned off.

UTM Coordinates are quick and easy to figure because all the help you need to actually use it is printed on all the Canadian topo quadrangles that I have seen. The actual coordinates look very cryptic at first glance, but they are really easy. Basically, you just count squares and estimate parts of one. A coordinate example is:
09V 0419200 6784100
The full version is:
7. RAPIDS 09V 0419200 UTM 6784100
For general use the above coordinate can be shortened to
UTM 192841.