General Information and FAQs
Using a GPS and Coordinate Systems
Table of Contents
Think of it. billions of dollars in military hardware, just so we can go play in the woods!
The GPS system consists of a constellation of a minimum of 24 satellites, each orbiting at roughly 12,500 miles above the earth. Each satellite weighs a bit over 1800 pounds, and is about the size of an SUV. The satellites contain four atomic clocks (timing is everything with GPS signals!), and NiCad batteries to provide power during eclipse periods. The satellites follow precise 12 hour orbits, and the positions of the satellites in the sky can be determined by your GPSr at any particular period of time or place.
How It Works
To triangulate, a GPS receiver measures distance using the travel time of radio signals. Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite. To make the measurement we assume that both the satellite and our receiver are generating the same codes at exactly the same time (remember those atomic clocks?). By comparing how late the satellite's code appears compared to our receiver's code, we determine how long it took to reach us. Multiply that travel time by the speed of light to get distance.
So: 186,000mps * signal variation in seconds = distance in miles
Satellites move and weather conditions change. If you are having a bad day, give it a minute and try again!
Navigating Under Tree Cover
eTrex models, with their small size patch-type antenna, have slightly less antenna sensitivity and may not perform quite as well as others under heavy tree cover. They will perform as well as any other unit in the open however.
Helix antennas, like those found in the Garmin 60 series, will have lower sensitivity if the antenna is placed close to a metal surface. This will be the case if it is stored in your pack beside metal objects. Patch type antennas work better if you want your GPSr to receive with the unit in your pocket. No GPSr will perform well unless its antenna has a clear view of the satellites. In many cases, sensitivity can be improved with the use of an external antenna.
Therefore, since you can’t cut down the trees:
We can define a point on a flat map using two lines that cross. One line that runs North/South and another that goes East/West. Where they cross is the location of the point you are interested in, a waypoint. Waypoints are presented as coordinates, a set of numbers for one line and another set of numbers for the second line.
The maps we use are flat, but the earth is a sphere. Trying to depict a round surface on a flat map distorts the true relationships a bit. This distortion complicates use of coordinate systems, and is one of the reasons that there are many, to meet the needs of the users. Out of these systems, we will focus on three that are most commonly used in GPS navigation.
The most common system is one that shows coordinate as Degrees/Minutes/Seconds. You can only go up to 90 degrees north or south latitude, and just short of 180 degrees east or west longitude. At the equator a degree of latitude is over 69 miles wide , so smaller divisions are required for depicting waypoints accurately.
The system 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. Longitude lines get closer together until they reach the poles, but latitude lines stay the same distance apart all the way to the pole.
A DMS coordinate looks like this -
The N37 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 N37° line runs through the San Francisco Bay Area. The 19' is the number of minutes (' = minute) north of that. A minute is 1/60th of a degree. The 28.44" is the number of seconds (" = second) north of 19 minutes. A second, like on a clock, is 1/60th of a minute.
Degree Decimal Minutes
The system used most by geocachers is a decimal form of the system above. Each of these systems can be selected within the GPSr preferences, and the GPSr will convert data to display correctly.
A decimal coordinate looks like this -
You'll notice the same degrees and minute, but the seconds are now presented in decimal form. This is figured from the DMS system by taking the seconds part of the coordinate and dividing by 60. For example, here is the DMS north coordinate used above: N37° 19' 28.44". Divide the 28.44 by 60, which gives you 0.474. So you get this: N37° 19.474’. This has been rounded to 3 decimal places as you will see in most applications.
To convert a decimal coordinate back to a DMS coordinate take the decimal part and multiply by 60. So .474 times 60 equals 28.44, and just make a DMS coordinate out of that: N37° 19' 28.44". This concept can be taken further by converting the whole latitude or longitude coordinate to a decimal degree. We already have the above N37° 19.474. Take the 19.474 and divide that by 60 to convert it to a decimal amount. You'll get 0.32457, so tack that after the N37 which will now read: N37.32457°. Each half coordinate is one long number, which can be shortened by the number of decimal places, depending on the accuracy you require.
Universal Transverse Mercator (UTM)
UTM coordinates are a military grid system used for quickly pin pointing map locations with reasonable accuracy.
The DMS coordinates above converted to UTM look like this:
The first number, 10N is the zone. Next comes the east/west number (the ‘easting’), and finally the north/south number (the ‘northing’).
The UTM coordinate system offers several benefits:
Most GPS receivers will convert DMS coordinates into UTM coordinates, or from UTM to DMS or to many other coordinate systems. A Garmin or Magellan GPSr can be preset to display either DMS or UTM. Geocaching coordinates are typically presented as Degree Decimal Minutes.
Conversion between all the systems can also be done using a simple online tool at Coordinate Converter and Mapper
Selecting A GPSr
Features that make a GPSr good for outdoor orienteering or Geocaching may not be the same thing you would look for in a car-based system:
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