Using Navigation on your Garmin ForeRunner

Your Garmin ForeRunner is a great training aid. The pace, distance, time and lap functions are easy and fun to use. But you also have a powerful navigation tool on your wrist that can help you find your way home if you get lost.

Terminology

We'll use a few concepts. First, a WAYPOINT is a saved location that you can save in your GPS memory. A waypoint has a position (latitude and longitude), and a name. You can create a waypoint any time to record your current position. You can also create a waypoint by entering the latitude and longitude manually.

POSITION corresponds to a given spot on earth. Position is referenced as latitude and longitude.

a ROUTE is list of waypoints that correspond to a course of travel. A route will typically have a small number of waypoints, each waypoint being a trail or road intersection, source of water, top of a hill, or some other significant point.

As you navigate a route you need to know the HEADING and DISTANCE to the next waypoint. This is just like google driving directions. Go north on main street for 5 miles, Turn left and go west on first ave for 3 miles.

GPS units calculate heading or direction by comparing your position over time. Most units (including Garmin ForeRunner) do not include a compass. You need to be moving to get accurate heading on a GPS unit.

Accuracy of Estimated Position

Your GPS uses signals broadcast from satellites to estimate your current position. The accuracy of the GPS's estimation is usually quite good. It can be within 10m or about 30 ft under ideal conditions. The positional accuracy depends on unobstructed signal from multiple satellites. If you're surrounded by tall buildings, heavy cloud cover, or dense forest your GPS will not be able to get a good position fix. It also takes a little bit of time for the GPS to lock into satellites whn you first turn it on.

Your Garmin ForeRunner locks into the satellites automatically and for the most part you don't need to pay attention to the satellites. But it's important to know the positional accuracy before you use yourt GPS to mark your location. If you want to get back to some point, it's important to have the correct lattitude and longitude saved in the waypoint. To see the GPS accuracy hit the mode button until you see the menu, select Navigation, then scroll down to the Satellite menu and hit Enter. You'll see a display with some vertical bars that indicate satellite signal strength. At the top of the screen you'll see Accuracy. You want the number to be small and you want to see the vertical bars with solid black. Watch this display when the device is powering up indoors so that you can learn the difference between good satellite lock and bad satellite lock.

Creating a Waypoint

First, make sure you've got good satellite accuracy. Then hit the Mode button until you see the menu, select Navgation, then select Navigation and then Mark Location. If you select Enter the waypoint will be saved with a default name. It's a lot easier to use waypoints that have sensical names. So navigate to the name field and create a useful name for your waypoint. You can also select an icon which will display on the map. This can be very useful.

If you're venturing out into unfamiliar territory, it's a great idea to mark the start of your route. You can then use the navigation capability of the watch to find your way back.

Navigating to a Waypoint

Your GPS can tell you how to get from your current position to a saved waypoint. To enter navigation mode hit mode until you see the menu, select Navigation, then select Find. From there you can select List All or List Nearest. List Nearest is usually the best. If you save multiple waypoints, at this point you will understand why you should create sensical names for waypoints. Highlight the waypoint you want to go to, then hit Enter. Select GOTO (the default) and then hit Enter.

Your GPS will then enter the GOTO mode. There is now a new display screen which shows distance to the waypoint, time to the waypoint, and an arrow which tells you the heading to the destination. Remember that a GPS needs to be moving to have accurate heading, so run in a straight line for a few seconds to get the correct heading. The arrow on the heading display tells you which way you need to go to reach the waypoint.

The distance and heading are as the crow flies. You can rarely travel as the crow flies. You need to follow streets, trails, and navigate around obstacles. When you're trying to find your way to a waypoint your GPS will constantly update the distance, heading and time to the marked spot, even though your path will probably not follow the as the crow files path.

Back to Start

Your ForeRunner makes it easy for you to get back to your starting point. You can do this by selecting the Back to Start option on the Navigation menu. This works even if you didn't store a waypoint at the start of the activity.

Using the Map

Most people only use the data displays on their ForeRunners. The Map display is very useful. You might need to enable the map display by selecting Show Map on the Settings, General, Map display. The map will display your activity track, waypoints, and while in GOTO mode the as the crow files line from where you started GOTO mode to the waypopint you're navigating to.

Courses and Routes

You can create a course from a saved activity. When you follow a course the GPS will alert you when you deviate form the saved course. A Route is a sequence of waypoints. You can create a route by adding saved waypoints. When you follow a route the GPS will use GOTO mode as you travel from waypoint, automatically switching to the next waypoint as you arrive at the current.


Practice and Have Fun

Like any skill, you need to practice in order to become proficient with GPS navigation. Before you set out on your next adventure into unfamiliar territory spend some time practicing your skills around your neighborhood. You can also try geocaching to set a waypoint for a destination by entering the latitude and longitude, then using the GOTO mode to help you find hidden treasure.

GPS Elevation Data Error and Correction

If you have every looked at the elevation data recorded on a GPS you probably noticed that the data have a lot of errors. For many athletes this is frustrating. We like to keep track of distance, pace, heart rate and use these data to improve our performance. Keep reading to find out how to correct the elevation data so that you can have an accurate measure of ascent and descent to use for analyzing your performance.

GPS Elevation Error

Take a look at the chart below. This shows the elevation recorded using a Garmin ForeRunner 305. The sawtooth pattern makes it look like I was on a roller coaster when I recorded this activity.


The course is a 4.5 mile loop around my neighborhood on sidewalks. There is a steady incline on the first half and a steady decline on the second half. The lowest point is at 0.8 miles at 387 ft. The high point is approximately mile 2.8 at 735 ft. The total elevation gain reported by Garmin Connect is 615 ft.

Doing the Math

Common sense tells us that the elevation gain is equal to the difference between the highest point and the lowest point as long as the path is always increasing. Since this route is a steady climb, the actual elevation gain can be determined using this formula. The difference between the high and low points is 348 ft. The raw GPS data over counted the elevation gain by 267 ft, a 76% over-age.

Why Does this Happen?

The GPS system has inherent errors. The accuracy of the latitude and longitude is superior to the accuracy of the elevation data due to technical limitations of the GPS system. The GPS receiver can also filter out erroneous data points when you are moving along a path. The unfortunate consensus is that this is a limitation of the GPS system as a whole.

The software applications that calculate the total climb for an activity can't look at each successive trackpoint. If the current point's elevation is higher than the prior point's elevation, they take the difference and add that to the total climb. Each tooth in the sawtooth pattern of the data adds additional error to the total.

Improving the Data

There are two ways to improve elevation data. The first approach is smoothing, or averaging. The idea is to average the recently recorded elevation readings and then use the average. The second way is to cross-reference the latitude and longitude for each point in the GPS activity log against a known good elevation database. This latter approach to elevation correction is ideal because there very good data sets available for free from NASA. The elevation data were recorded using very precise lasers from the orbiting space shuttle.

Automating the Process

I'm aware of three ways to use elevation correction. Garmin's MotionBased Gravity system, MapMyRun, and the SportTracks Elevation Corection plug-in.

The image below shows the same GPS activity in SportTracks after the elevation correction plug-in cleaned the data.



Notice that the graph has a very smooth line and none of the saw-tooth pattern found in the un-corrected GPS data in the earlier image. Notice also that the total climb reported is 341 ft.

The Bottom Line

The bottom line is that GPS devices are very good at recording distance, speed, pace and other data, but have a lot of error in elevation data. Correcting the data is easy with the right tools. Once you have clean data, you can use it to analyze your performance using elevation gain as one of the parameters.