While a gps would be a great device if it only told you where you
were, in practice it will tell you a whole lot more. This chapter will
focus on its ability to help guide you to your destination. This
capability can be useful and might save your life some day.
Navigation has been traditionally performed using a compass and a map
along with your eyes to help you relate where you are to where your
location is displayed on a map. Off road hikers and pilots often
supplement this information with an altimeter that can aid in finding
your location on a topographic map. In some cases a gps can actually
replace both the map and the compass, and to lesser degree the
altimeter, however it is a good idea to have a compass and a map as
a back up along with the skills necessary to use them.
Before starting to learn about the gps navigation capabilities it
would be helpful to spend some time understanding the basic terms used
to describe navigation. There are three basic kinds of navigation in
use today, land, sea and in the air. All three have their own terms
for certain things so you may find the same information labeled
different ways depending on which form of navigation is being
discussed. Garmin has units specifically designed to help in each of
these operating areas but has divided their product line into
land/marine and aviation. This, however, does not mean you can't use
one gps in another environment but some features are specific to a
particular use. Some folks would like under the sea navigation as
well. Unfortunately a few millimeters of solid water makes a gps
useless so, unless you can float an antenna, a gps will not help
for this type of navigation.
To help in understanding the basics of navigation please refer to
the diagram below as needed:
If you could see your destination from the beginning you probably wouldn't need much navigation help in order to get there. Most of the time, however, you can't see the location but may have located it on a map. Using traditional techniques you might draw a line on the map from your current location to the destination and then use a protractor to figure out the bearing. You would then use a compass to keep your track as close to the bearing as you could. Drifting off course could cause you to get the map out and figure out where you are and compute a new bearing. If a straight line is not possible then you might build a route that took you somewhat out of the way to make the trip itself easier. Then you would have a succession of bearings and DTK's called legs that would, together, allow you to reach your goal. This collection of legs is called a route and the individual points where the turns are done are called waypoints on the route.
If you were in a plane or on a boat and had some cross wind you might drift off of the desired track even though your track as indicated on the compass never changed. The distance you've drifted is called crosstrack error or course error. This may or may not be important depending on your circumstances. For example if you were sailing in a channel drifting off course might cause you to run aground which could ruin your whole day. In this case you would want your navigation equipment to alert you to the fact that you are drifting and provide some guidance back to the optimum course. A gps might indicate this kind of error a number of ways. The bearing and track is updated every second or so and if the bearing begins to differ from the track then you are clearly moving off your course. This relationship between track and bearing is important enough that Garmin considers it to be two of the four main pieces of navigation information. The other two being your current ground speed and the distance to the destination. Garmin places these four pieces of information on the map screen and on all navigation screens.
While this information may be enough for many folks, some will want more details or, as in the case of the guy in the boat, more information about getting back to the original course. (Following the bearing will get you to the destination but may not keep you on the original course.) Some of Garmin gps's provide the course to steer, CTS. This is an actual compass heading to turn your craft toward in order to get back on course. By the way this information can be misleading if you are fairly close to your destination. The computation looks at the the angle of the current track and the desired track and computes the most efficient course to get back on track. This might actually overshoot the waypoint so always check it against the bearing. If you prefer a relative direction then you might like TRN, turn, which provides information on how much you need to turn to get to your destination.
Other interesting data includes ETE, estimated time enroute, and ETA, estimated time of arrival. To compute these numbers you need to use an estimate of your speed. Garmin calculates the speed vector that is actually in the direction of the destination and calls this VMG, velocity made good, based on your current speed and direction. The handhelds covered in this manual (except for the etrex and emap, see below) all use the current VMG as the effective speed for calculating ETE and ETA. VMG is a pretty good indicator of your effective speed if you are moving at a steady speed over the ground and are approximately headed in the right direction. It is also accurate if you are in a sailboat tacking toward an up-wind mark by zig-zagging back and forth. Similarly a car traveling on city streets may have to do the same thing but the destination may not be directly "up wind". Either take the VMG on the closest tack in the direction of the destination or better yet average the two to arrive at the most probable ETA.
The etrex and emap also use VMG to estimate ETE and ETA, however the way it is measured differs on these units. Instead of a current VMG they use a previous sustained velocity to calculate VMG. Changes in speed or direction will not change the VMG until the magnitude and direction settles down to a new constant speed. In addition no change in calculation of VMG is done when you are stopped. Under these condition the old VMG will be used to update the ETE and ETA values. Unfortunately there is no way to display the VMG that the unit is currently using, however you can force it to compute a new one. To do this temporarily stop navigation from the local menu and then start it again.
Garmin generally provides two navigation screens for use in guiding
the user to the destination. These are called the Compass (or pointer)
screen and the highway screen. These are illustrated below for the G-12
Each screen supplies basic graphic and textual data. By default non-mapping receivers also show bearing, distance, speed and track while mapping receivers seem to have replaced track with ETE. While mapping receiver provide more customization of the data displayed they will only display 4 things while non-mapping receivers will often display more.
The emap does not have a dedicated
navigation screen but instead it changes the top area of the map
screen for navigation use. Instead of a compass display it just shows
an arrow pointing in the direction to the next waypoint. It also
shows your current speed, the distance to go to arrive at the
waypoint, and the estimated time to arrive. If you are stopped the
pointer arrow is replaced with a compass bearing. This can be
displayed as a number in degrees or as text using the 8 points of a
compass. The destination name is not displayed on this screen but you
can reach it easily by pressing and holding the find button. It will
switch to the waypoint page if you are doing a "goto" or to the active
route page if you are navigating a route with the current destination
indicated. If you use automatic zoom then the destination will be
shown on the map itself.
Working with the Compass Screen
The Compass screen, G-III version shown above, is called that because
of the large circular area that dominates the screen. It looks like a
compass but it is not. Garmin units do not have a compass built in
but simulate a compass using doppler position information to deduce
your direction of travel. In an attempt to avoid this confusion the
latest Garmin offering, the etrex, calls this a pointer page focusing
on the ability of this screen to provide a pointer to the destination
waypoint. If you visit this page when you are not actively navigating
to a destination the pointer in the center of the screen will be
missing but the circles with still rotate to give you a simulated
compass with your current direction at the top of the screen. In
addition your current speed and track will be shown (or other data you
have chosen on the G-III family). These two entries will be changed
to underscores if you ever lose your satellite lock.
The easiest way to use this screen is to use it with the goto key* and an existing waypoint. Press goto, select a waypoint, hit enter and the navigation screen will show you the waypoint name at the top of the screen, the direction to the waypoint, and will begin guiding you to this destination. The gps will compute both the bearing and the great circle distance to the waypoint. For many folks this is all of the navigation they will ever need. Many units also provide for an emergency man overboard capability. Press the goto key twice, hit enter and you will generate a waypoint at your current location called MOB and start the navigation screen with information to guide you to this point you just left to help you recover the person who just fell overboard. You may be able to think up many other uses for this capability.
* Note: the etrex and emap work differently here. There is no goto key so you first select the waypoint you want from the waypoint list and then one of the review choices is "goto". Selecting goto from this list will initiate the goto as described above. The goto can be canceled from the local menu available on either the pointer screen or the map screen.
Due to the fact that track is a computed direction based on movement it can't be depended on when you are stopped. The gps trys to pin this value to the last computed value while moving but the gps to become confused about movement and basically makes this number meaningless. However bearing works even when you are stopped. You may need a compass to actually use the bearing data but at least the value is correct.
Once begun the gps will continue to navigate toward the destination waypoint until canceled. The cancel goto command is one of the menu selection items reached from the goto key itself. The easiest way to reach it may be to go 'up' from the menu which will wrap around and select the entry on the bottom. Or simply continue to press the down arrow key until the cancel command is reached. On some machines pressing the right arrow key will skip all of the waypoint names and jump directly to the cancel goto command.
On the emap and etrex you can use the local menu item, stop navigation. This command has the added benefit that if remembers the previous navigation setting so you can re-enable it using the start navigation item if you wish. This can be useful to remove the navigation goto line temporarily from the map screen. Turning off the unit with navigation stopped will end the navigation session completely.
Some older gps units and the G-III pilot also have a graphic course deviation indicator, CDI on this screen. This is an indicator for crosstrack error. However, most units reserve this information for the highway screen. Its use will be covered in the description of that screen. If you want to view the crosstrack error and you have a selectable field you can select XTE for a numeric readout of this error. Because the etrex is intended for land navigation it does not display or compute XTE. The map screen on the etrex has a setting for the goto graphic line to show bearing to the destination or just a static line that shows the desired track from the start of the goto. This can be used visually as a crude cdi indicator.
In addition to the fields and information described above your unit may offer to display other optional entries (older multiplex receivers do not offer any options but always display track, speed, bearing and distance). On the etrex you can always view the ETE and distance and can press the up/down keys to select bearing, track, speed, and some other entries not directly related to navigating. The III family can customize the display to whatever degree you wish using the local menu to select among navigation data and other entries as well. Other units always display speed, track, bearing, and distance (the golden four) and you can highlight the bottom entry and press enter to select between ETE, ETA, XTE, VMG, CTS, and TRN.
Some folks like to leave the unit on the compass page whether or not they are currently navigating. This is because it has the largest text display of any of the screens and can be seen from a distance. On the III family this can be enhanced by selecting the small compass from the local menu. When the small compass is displayed there are two very large customizable entries to view. The etrex and the III family can display information that is useful even when you are not navigating. The etrex, which has no position screen, will also display essential position data using the up/down keys such as altitude, latitude/longitude, max speed, average speed, trip distance, trip time, and even sunrise and sunset times. Max speed and trip information may be reset from the local menu. The III family can display any of 29 or more different things on these pages. You can basically configure any of the III screen to display any data with just a few exceptions.
Some units can provide vertical navigation. These include the III Pilot and the etrex Legend, Venture, and Vista.
The III Pilot can also provide vertical navigation using the HSI display. You must first go to the main menu and define a vertical profile for your glide descent and the final target altitude. Once this is done the unit will automatically generate a message when you approach the vertical navigation profile. You can then turn vertical navigation on from the local menu on the HSI screen. A horizontal bar will appear on top of the display and indicate your glide slope. It will guide your descent and will disappear 500 feet before your target altitude. Remember that satellite position errors can cause variations in the gps calculated altitude so this data should be treated as information only and not used to provide a precision approach. GPS receivers normally optimize satellites for horizontal position accuracy.
Vertical navigation on the Vista is provided with vertical speed
data and glide slope. Glide slope provides a ratio of the amount of
fall in altitude vs. the distance traveled. It is computed only when
navigation is in progress. There are two selectable fields, one shows
the glide slope to the next waypoint in a route while the second shows
the glide slope to the final destination. It would be nice to use
this to identify the time to start a descent but unfortunately it will
not report a glide slope when traveling level so you must actually
start a descent before it will compute the glide slope. It cannot be
used to compute an ascent. Of course, the destination waypoint must
have altitude stored in the waypoint for this function to work
properly. Otherwise if assumes sea level for the calculation.
Working with the Highway Screen
The highway screen is really oriented around following a prescribed
course. Pressing the Goto key as described in the above compass
discussion not only set a destination but also set a beginning point
as your current location the second you pressed the button. The line
drawn between these two points is your desired course to reach the
waypoint in the minimum time and distance. Use this screen when it is
important to maintain a track that matches the "desired track" or
"course". If you need to reset the course you can hit the goto command
again. It will highlight the same waypoint you used last time and
pressing enter will reset the starting point to the new present
location. Note that the etrex and emap do not have a highway screen.
While all of the highway screens in the various units look similar (the G-III version is shown above) there is a wide variation in how they actually perform in use. The dominate feature is, of course, the highway itself. This shows a three dimensional view of a virtual highway that points from your current location to the destination waypoint. In the case of a route the waypoint is the current intermediate destination. Perspective is used to give the 3D effect and if you ever head the wrong direction you will find yourself at the narrow end of the road! If the highway is centered in the display then you are heading for the waypoint and are on the centerline of the course established when the goto, or route, was initiated. If you drift off the course then the highway will drift left or right to indicate how you should steer to get back to the road. If you head off in the wrong direction the road will turn to indicate that you need to turn to get back to the road. This basic visual information becomes quite intuitive with use and is the same on all of the highway screens.
Somewhat surprisingly the older single channel units provide the most data on this screen as shown above. This coupled with the good performance a single channel unit can support under the open sky that usually accompanies water has led to a long life for these units among boaters. These older units show the four main pieces of navigation information, bearing, distance, speed and track, the name of the destination waypoint, a cdi scale, the ETE, the VMG and an arrow pointing to the destination in addition to the road drawing. Newer non-mapping units show the same thing except that there is no bearing arrow and you must choose ETE, VMG, XTE, TRN, CTS, or ETA. You get a wider choice but you can only choose one to display. This can be a different one than you chose for the compass screen. The inclusion of the cdi scale shows a strong tie to marine navigation.
The mapping units show their tie to land navigation by leaving off the cdi scale and making the road screen a snazzy display with good visual impact but less definitive information. Note that the III pilot uses the HSI screen for cdi so it doesn't need this information here. Mapping receivers can use the zoom keys to zoom the highway and are able to show route destination beyond the current target waypoint. This can be used to predict the direction of the follow-on waypoints. You can even use a local menu item to turn on signposts on the highway screen that indicate the names of the route waypoints. The III pilot will even show airport waypoints that may be close to the "road". Similar to the compass screen you can customize the 4 numeric display fields. You can show different things than are shown on the compass screen and even select different things between the horizontal display and the vertical display if your unit has this feature. An edge view of a compass display is also shown to indicate your track.
If you unit provides maps built in then the map page may provide even more visual navigation information. If you have a unit that can upload maps you can upload topo maps and even include altitude information you read from the elevation lines on the map as part of your navigation decision. For the III pilot the map page can provide essential Jeppesen data marking VOR boundaries and other essential navigation aids. See the database chapter for specific details on what navigation aids are available in the various databases.
Mapping receivers also have text fields that can show ETE at the destination waypoint and ETA at the destination waypoint for a route. Other units cannot display this directly on a navigation screen but can get at the data by going to the active route screen and scrolling to the last waypoint in the route. This information along with the desired track and cumulative distance is available for every waypoint on the route. Even the older units have this entry as a customizable field on the route page.
The active route page should not be overlooked as a source of navigation information. On the emap this screen is the only place that you can see the ETA values. This information is valuable on any of the handhelds. If no waypoint is selected or the last waypoint is selected you will see the ETA at the destination while selecting a waypoint will give ETA to that point. You can quickly reach the active route screen on the emap by pressing and holding the find key whenever a route is active.
All of the units can indicate navigation warnings. Units with sound capability can couple the visual message with a sound if desired. Messages include upcoming waypoints in a route, warnings about hazards, and even warnings that you the upcoming turn is too tight if you are traveling fast enough to be assumed to be an airplane. Turn smoothing starts to appear at 75 mph. The upcoming waypoints message can be automatically generated based on your estimated time of arrival assuming you maintain your current speed or in some units it can be set at a specific distance. The automatic arrival alarm is fixed at one minute on all units except the etrex and emap. On the etrex is set to 15 seconds and the emap has a variable time based on your speed. In some cases you have control over these warning messages and in some units you do not. If you can change them there will be an alarm entry on the main menu page perhaps as a submenu under system.
Some of the units intended for marine use have proximity alarms. These
can be set to alert you if you get to close to a hazard. Each alarm can
be set to a different distance. If you have proximity alarms you will find
them on the main menu.
Tips and Tricks
Here are a few tips to help with gps navigation:
Most of all, have fun using your gps for navigation. Just be careful that you don't trust it too much. Like any instrument you need to know the limits.
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