Efficiency of portable GPS in the forest

Can I rely on my GPS in the forest? How accurate is it? What are the best tricks to make the most of it? What should I do if I find myself in a critical situation?

Let's begin by understanding how a GPS actually works. The constellation of satellites emitting GPS signals has been designed to allow reception anywhere on the globe. To display a position, a device requires a signal from a minimum of three satellites. The receivers are designed to capture a signal from all satellites found under the celestial sphere above the position where the user is located. The position is calculated by triangulating the position of the signals received. That is precisely the problem: 99% of our forests are mountainous, and the cover can be very dense, leading to poor reception. A microwave signal cannot pass through a mountain. The same situation occurs inside buildings, vehicles or in urban areas near large buildings. In the forest, satellite signals located at a low position on the horizon or behind a mountain will not be detected.

Garmin devices display high-value information when the user needs to know if they can count on the accuracy of the screen reading. On the page indicating satellite reception, a window displays the letters DOP, which stand for "Dilution of Precision." At this point, your receiver cannot pick up the available satellite signals that would allow a wide angle, necessary to calculate the position, and it is forced to rely on signals originating from the same sector of the celestial sphere, which in turn dilutes its precision. This information should be taken into account when marking a point in the forest.

The next information displayed on the screen is the EPE (Estimated Positioning Error). Even under ideal conditions, the passage of the signal through the upper layers of the atmosphere to reach each of the satellites, added to a tiny margin of error in the calculation of the transmitting satellite's distance, will influence the precision. We have seen in the last column that the desired margin of error of the system has been eliminated, which allows us to have the corresponding precision. In open terrain, the displayed precision is approximately 6 metres. The EPE reading is useful in the forest to assess the accuracy of the location's display.

When moving to a pre-selected target, your device will display information under CDI or XTE for Course Deviation Indicator and Cross (X) Track Error, respectively. This value is an indicator of your deviation from the straight line between your departure point and your destination. Needless to say, it is impossible to move in a straight line in the forest. Although this information is useful for aircraft and nautical navigation, it is useless in the forest. However, your device will always indicate the direction to take to reach your destination, by constantly considering your deviations. The direction to your destination is indicated in compass degrees. It is therefore strongly recommended to configure your device to display these coordinates in relation to the magnetic North. Your device knows its own position, and it will automatically calculate your deviation from the magnetic North. If you prefer to use the graphic navigation mode, the direction arrow is actually an illustration of this critical information.

A few tips for critical situations

If you are seeking assistance using wireless radios or phones: When transmitting a position, be sure to mention your coordinate mode, which is, xx degrees xx minutes, xx seconds (or hundredth of a minute or thousandth of a minute). A reading of 45.36.22 can be interpreted in several ways depending on the configuration of the coordinates' mode of display. Keep in mind that the international mode is in degrees, minutes and seconds and that it is the mode that is most likely to be properly interpreted. Another limitation in establishing a position with high accuracy is the map datum mode used. Two different systems are used in Canada. Maps of several territories are still in NAD27 mode and more recent maps are in NAD83 mode. If you want to be sure that your position will be correctly interpreted, mention the geodetic reference of your coordinates.

If you are forced to walk a long distance in the forest as a result of a vehicle failure or forced aircraft landing: In this case, you will need to spare your device's batteries, and the ideal way to navigate is to progress step by step. This method is known in international navigation language as "dead reckoning." You turn on your device, let it display your position and read the distance and direction of your target. After having read this information, close the device and move in the right direction for a period of time before taking another reading again later. The operating time of your device will be limited to less than two minutes per reading, which will allow you to use your device for a very long time.

I should have mentioned from the start that—in my opinion—three tools are essential to safely move around in a forest. Naturally, the topographic map of the area is a sure-value tool but unfortunately, we cannot always have it on hand. After purchasing a GPS, several people wrongly believe that they no longer need a compass. But they are wrong. A compass is an essential tool. Unless you have the new Garmin e-Trex Summit with an integrated electronic compass, you need to have a compass to identify your starting direction because in the forest, you could have to walk a long time before getting an accurate reading of your direction and progress, and the path correction to be made. The three essential tools are therefore: a GPS, a compass and the third . . . lies between the ears.