The Good News about GPS
If you are anything like I am, the last thing you do is... read the users manual. That’s just fine with me, but in the case of a GPS, you’ll be better off reading it sooner or later. It’s perfectly OK if you don’t understand many of the terms, or what some of the options really do. With time, you may find a need for most of them, but for now let’s concentrate on the basic ones.
Put the batteries in and let’s push some buttons. If you have the unit turned on for a while, especially outdoors, a message “POSITION ACQUIRED” (or similar) will appear. Most new units will automatically lock onto satellites and acquire a position within 5 minutes. If it takes longer than that, you might have to enter the initial position manually. The booklet that came with the GPS receiver explains how to do that. Throughout this guide, you might have to refer to the manual, in order to find out which particular buttons to push, in order to get to a particular function. A reminder; the functions described are based on a Lowrance GlobalMap100 GPS receiver. Most other brands of receivers have similar or identical functions, therefore the same principles apply.
Basic setup – the two things you have to do if you plan to use a GPS with maps.
There are only two options you have to set, in order for the receiver to work properly with a printed map. The first option is DATUM. Ron Wilson’s article explains it in more detail, but let me tell you that if the receiver is set to display a position in a different datum than the one indicated by your map, the indicated position might be wrong by hundreds of meters (yards). For topographic maps of North America, the most commonly used datum is NAD27. Some newer maps use WGS84 or the nearly identical NAD83.
The second option is the POSITION FORMAT. It determines the way in which the position is displayed by the GPS receiver. By far, the easiest to use is the UTM. Again, all current topographical maps for North America show the UTM grid. If you use a map, which does not have the UTM grid, you might have to set the unit to Degrees.Minutes or Degrees.Minutes.Seconds. In some countries, the map grid is different than the two mentioned. If that is the case, make sure that the chosen receiver, in fact, can use that particular format, before you spend the money
Basic setup – the convenience options.
There are a number of options, which affect the way GPS works. They are mainly a matter of personal preference, or they may be simply dictated by a particular situation. To make following the guide easier, you might want to set up your receiver the way I do. It certainly is not the only way to do so, but I found it to work really well for bushwhacking. In no particular order there are:
- in the system setup, under “UNITS” I set BEARING to “MAG” (magnetic). This will cause the unit to display course / bearing information in relation to the Magnetic North and match magnetic compass readings. It is not the same as True North! One has to be careful with this setting. There are a number of areas where the earth's magnetic field is locally disturbed, sometimes very significantly. Your GPS may not account for it.
- again, under “UNITS” I set “METRIC”. Understandably, for many of you “STATUTE” may be a more familiar option, but for working with UTM, I STRONGLY recommend getting familiar with the metric system. It corresponds directly with the UTM grids on the topo maps. For the unfamiliar with the metric system, every time I mention “meter” think “yard”, and you will not be that far off.
- POWER SAVE – I leave it OFF. Setting it to ON may conserve batteries, but there is a tradeoff in the sensitivity and possibly in accuracy, especially under a tree canopy. If you use your receiver in wide open terrain, you can set that option to ON without adverse effects.
- POSITION PINNING (an option unique to Lowrance / Eagle) – I set it to ON. It will somewhat stabilize the apparent drift caused by SA, but remember that it does not improve the actual accuracy of the readings
- Plotter orientation; usually I have plotter screen #1 and #3 oriented “North Up” and screen #3 oriented “Track Up”. It allows you to easily switch between the modes, depending on the need. If your unit does not allow presetting different plotter orientation modes, stick to “North Up”, which is the same as the orientation of a typical topo map. Due to SA, the “Track Up” setting may cause the display to sway around at walking speeds or when stationary.
- ALARMS – I have the ARRIVAL alarm set to ON, with the distance set to 200 meters), if navigating a route or to a waypoint. CDI (course deviation) alarm set ON to 500 meters. The CDI alarm alerts me if I stray too far to the side of the intended route. All other alarms I have usually set to OFF.
- SOUND – usually I have it set to OFF. Since I hardly ever use alarms when bushwhacking, there is no need to scare the wildlife with beeping and buzzing noises.
The basic purpose of a GPS is to indicate the position. The best news about it is, that even with the effects of SA in place, it does a very good job. For an outdoorsman it is usually sufficient to know the position within 50 – 100 meters. It is sufficient to put an X on the map. After all, the X covers most of the 50 meters anyway. The ability to read your geographic position from a map or to plot it back onto one is the key to GPS usefulness. For an outdoor enthusiast, it can never be overstressed.
Storing the position information – waypoints.
The most common form of position recording by GPS is the use of Waypoints. A typical receiver will store for each waypoint the following information:
- Geographic position.
- Date and time of creation.
- Unique number (usually automatically assigned by receiver).
- User editable name.
- User selectable graphic icon that will be displayed on the plotter screen.
Depending on the particular receiver, it might be possible to store between 100 and 1000 positions in that form. Some receivers also allow storing additional position points in the form of “Events” or “Icons”. Other than user selectable graphical symbol, there is no stored information that could be accessed directly through the receiver. Events are displayed on the plotter screen in their proper positions in the form of icons. They serve mainly as a visual reference point on the screen.
The most common ways to enter waypoints are:
- Entering coordinates of a waypoint.
- Creating a waypoint at the present position.
- By positioning cursor on the plotter screen and creating a waypoint at that position.
- By uploading waypoints from the computer.
Before entering any unfamiliar territory, I always study the area maps. It allows me to get a general idea of the lay of the land. I never go into a place I don’t know how to get out of. Studying the map allows me to preset a general direction for getting out of trouble. In Andreland it’s simple. If the GPS fails, the map gets lost, and the compass breaks, it’s enough to remember to go in a general Southwest direction to get cornered between the road and the river. It may be a long way out, but it does lead out! When I’m getting familiar with the map, I can create waypoints for the most significant terrain features. Later on, just looking on the plotter screen and comparing it to the map gives a fairly good idea of position, without having to plot actual coordinates on the map. Figure 1 represents a map of Andreland with various waypoints marked.
NOTE: Blue line grid is a UTM grid spaced 1 kilometer = 1000 meters
Plotter screen with preset waypoints.
Let's assume that you are going to stay in the area for a little while. Once you pick the place to set the camp, you want to mark in on the map and record it in GPS. Unfortunately, Andreland has very few terrain features that can be easily recognized on the map. If you happen to camp just North of the straight trail section, the only way to accurately place the X on the map is to use a GPS. You could simply press WPT key twice and have a waypoint recorded. The trouble is that a waypoint recorded this way may be as far off as 100 meters. The circle around the camp icon shown in Fig. 3 indicates the approximate range of the possible error. The actual position may be ANYWHERE within that circle.
The solution is to average that waypoint position over time. From my experience, benefits of position averaging start to be noticeable if done over period of time of 20 minutes or more. 2 hours of averaging usually gives better accuracy than an average outdoorsmen ever needs.
Most GPS receivers have a function (POSITION AVERAGING) which allows them to automatically average the position. If yours is so equipped, just use it. If your receiver does not have such a function, you will have to use the most powerful computer of all – the human brain. Luckily it’s easy, so don’t worry about popping a fuse. Simply leave your receiver on for some time (20 min – 2 hrs) and analyze the plotter screen. You’ll notice a weird track drawn by SA error. Point the cursor to the approximate middle of the blob and create a waypoint right there. See Fig. 4 and 5 below.
Navigating to a place – following a route.
With the camp set up and marked, it’s time to go for a walk in the wild. Our target is SWAMPL (at the top left map corner). While going there, it may be a good idea to go around POND, SWAMPS, and any creeks or rivers. A GPS used with a map and compass makes it easy. Using map coordinates or the cursor on the plotter, we can place waypoints (lets call them TURN1 and TURN2) in the approximate position of anticipated direction change. The arrival alarm can be set to 200 or 250 meters. Fig. 6 shows the waypoints with the 200 meter (radius) circles around them. There is a reason for setting the alarm to 200 meters or more. When using a GPS-indicated compass bearing, due to SA, at distances smaller than 200 meters, the bearing is totally unreliable.
Navigating to the SWAMPL we can take advantage of another typical function of a GPS receiver – we can create a “Route” which will lead from CAMP to TURN1 to TURN2 and finally to SWAMPL. The route with the entered waypoints is shown in Fig7
All that’s left is to start navigating the route. Along the way, two pieces of the navigation information are of the particular interest.
- Bearing – it’s the compass direction we have to follow in order to get to the target.
- Distance – it answers the eternal question: “are we there yet?”
Personally, I like to see my progress on the plotter screen with the above two bits of information displayed. Most GPS receivers will allow you to display that information in various combinations / layouts. Fig. 8 shows a few typical screen displays.
Along the route – a few GPS screens.
When navigating the route, once we get within the range of 200 meters (or whatever the Arrival alarm was set to), the screen shows a message indicating the “arrival”. The alarm will sound if the audio is set to ON.
Regardless of what the receiver indicates at that point, because of the presence of the SA, we might be still as far as 300 meters away from the target waypoint. We can also be a 100 meters to the side of the course. Instead of guessing, it is best to follow, without even looking at the GPS, the originally indicated compass bearing for another 200 meters or so. Then change the direction to the next waypoint bearing as indicated by the GPS.
Using this technique, we can navigate from one waypoint to the next and end up at the edge of SWAMPL.
Since there should be a reason for everything, let’s assume that our reason for visiting SWAMPL is hunting with a camera. Like any type of hunting, it requires the participant to conceal his position. Now, if done right, the hiding place really blends in with the surrounding landscape… It may be hardly visible not only to the animals, but also to humans. That might make it difficult to come back to. With 100 (200 cumulative) meters of possible GPS error, navigating back to the same spot is not very easy… or is it?
Finding the same place again.
In order to find a place with the GPS, we need to know where it is in the first place. A number of orienteering techniques can be used to do so. One of the easiest is to check the surroundings for a very distinct and visible feature. It may be a rock, it may be a tree, just make sure you can get to it in a relatively straight line and easily recognize it among the others. From your position, take the compass bearing to that feature and write it down along with the approximate distance. Make sure to be as accurate as possible – 2 deg. of error in reading the bearing translates into roughly 5 meters at a distance of 100 meters. If your receiver has the function, create a waypoint at that feature position by “Projecting” the averaged waypoint of your current position. Check your manual to see if your unit has this feature. When you want to come back, first navigate to the feature you selected, than forget about the GPS and simply walk the compass course (direction) exactly opposite to the recorded bearing. To do this just add/subtract 180 degrees from the original compass bearing.
Since I am a GPS Nut, I like to use my receiver for the task. A way to do it is to use averaging (as described earlier). In that case over 2 hours is a must! It will typically give better than 15 meters accuracy. Again, you can use a GPS function for it as shown in Fig. 9, or use the “blob” method.
So, two rolls of film later, we have a good position (call it BLIND) recorded in the GPS. We can now pack up and go back to camp. Next day we can follow the same route to the edge of the swamp. This time, the last point in the route would be the BLIND. We know the accurate coordinates of it, but our present position is known with possibly 100 meters error. Not exactly the best starting point for precise navigation. Well, not everything is lost. Once on the last leg of the route, stop, take a break, relax, smoke a pipe and let the receiver average your present position. Create the waypoint at that averaged position. Let’s call it HERE and assume their positions as shown in the Fig. 10.
Now you know where you are and you know where you want to be. The last thing to find out is the bearing and distance between the two places. Some receivers have a function just for that. If your receiver doesn’t, simply create a 2 waypoint route from HERE to BLIND and let the receiver do the required bearing / distance calculations.
Since you don’t want the bearing from the presently indicated position, but from the coordinates of the waypoint HERE, do not “start” the route, rather simply read the bearing (342 deg.), turn off the GPS and follow that bearing with the compass. If your patience allowed you to average the waypoints for long enough, you will not have any problems finding the blind – you’ll trip over it!
Finding a place – fail-safe.
Frequently, when taking a shortcut, one finds himself on a trail, not exactly sure whether to turn left or right to get to the final destination. Navigating back to the CAMP, we might end up on the trail 100 meters to the left or 100 meters to the right. The way around the problem is not to navigate to the CAMP. Instead, create a waypoint (TRAIL), positioned on the trail, roughly, just over 100 meters to one side of the CAMP.
Once you navigate to it and find yourself on the trail, you'll know which side to turn to in order to get to the destination. It may be not the fastest way, but it surely beats the hell out of the disappointment caused by taking the wrong turn.
Words of caution and advice.
The techniques described above work for me. I do not guarantee that they will always keep you out of trouble. In fact, if applied indiscriminately, they can put you in trouble. Some situations may require completely different methods, some will allow you to adopt the described ones. GPS is a tool, not a miracle solution to getting lost.
- Always carry spare batteries for your unit. In the wild, don’t throw out the old ones. They not only are a toxic waste, they might be able to provide enough power for one more position fix, when your back-up battery set dies unexpectedly.
- Nothing beats the real experience. Before leaving for less civilized areas, get a topo map of your neighborhood. Practice navigation in a local park or conservation area. After all, any excuse to be outdoors is good.