|Basic Tips, Hints, Explanations, or Whatnot......|
Compasses and Compass Ring
Date and Time
Elevation or Altitude
EPE or Accuracy
Locations and Points
Mapping GPS Receivers
Navigation, Bearing, Course, Track, etc.
Vibration - Power Shutoff
Waypoint Comment Field
Waypoints, Routes, and Track Logs
|"Global Positioning System". "GPSR": a device that receives
the data from the GPS satellites. A GPS satellite broadcasts its
location and timing information so that the receiver can determine how
far it is from the satellite. By doing this with a few satellites
in different directions, the receiver can determine where it is located.
A GPS receiver can be incorporated into a cell phone or an FRS (Family Radio Service) transceiver to send your location to someone else but otherwise a GPS receiver does not transmit anything.
point or location in our three dimensional physical space
represented by two sets of numbers with the assumption you are on the
of the Earth. Sometimes a third number is added for the elevation
(altitude). A GPS receiver may also add a fourth number for depth
if a depth sounder is connected to the receiver. In other words, depth
and elevation are often listed separately even though they are both the
The two sets of numbers representing your horizontal position may be given in Latitude/Longitude or UTM coordinates or some other system. This is the grid. It is also important to know the datum - what the numbers are relative to.
The GPS receiver manufacturers give different names to points depending on the purpose they serve. There are waypoints, map points, route points, via points, track points and event markers to mention some common ones.
Some are essentially independent such as waypoints and map points. Others are only defined when connected to other points such as route points and track points. They define line segments. Sometimes the independent ones can become connected points as when waypoints are used to form a route.
The GPSR gives you a point. A compass gives you a direction. If you have some reference points stored in the receiver, it can tell you the distance and direction to those points. The GPSR can give you a direction if you move by using the point where you were with the point where you are now. You should be moving about 2 mph or faster to get a good direction reading.
|Waypoints, Routes, and Track (Trail/Road) Logs|
|A waypoint is a location and is given some sort of
A route is a collection of waypoints linked together for navigating and
navigation information. A route probably has a tiny fraction of
number of points that a track log has. A track log is a
trail consisting of lots of points. Track log points are usually
not named in the GPS receiver and not named in some software
Because both a route and a track log are a connected series of points,
some software programs treat them the same but there should be a way to
tell the software which type you want to transfer to the GPS
Some software programs will give track points some sort of name - a
code followed by a sequential number. And some software programs can't
transfer a track to a GPS receiver - some (older) receivers can't
accept a track.
See More on Defining Waypoints, Tracks, and Routes for a more in depth explanation.
See Garmin Screen Examples
for much more detail and some map information.
|This is the height above mean sea level (MSL) .
The GPS receiver can not receive any signals from a satellite "below" you so the geometry doesn't support an elevation value as accurate as it does a horizontal value. By the same token if you are in a canyon, the geometry can support better accuracy along the axis parallel to the canyon than along the axis perpendicular to the canyon.
Some GPSR models also have a barometric altimeter in them. Some brands and/or models will use the GPS signals to calibrate the barometric altimeter over time whereas others don't. Whether the barometric elevation or the GPS elevation is shown, stored in the track log, or stored with a waypoint depends on the brand, the model, and the firmware version.
The GPS elevation tends to have larger short-term errors than a barometric altimeter and is not affected by the weather in the long run. The barometric altimeter tends to be fairly stable in the short-term but has larger long-term errors since it is affected by the weather. Combining the two can be the best overall (early attempts at doing this weren't so good).
|The basic satellite system consists of 24 satellites with
four in each
of six orbital planes. There are additional spares and the actual
number of working satellites varies. The satellite pattern
will vary. They orbit the earth in 12 hours, which produces a
track of 24 hours. In other words you will see the same pattern
24 hours but the satellites produce the same overhead pattern every 12
hours. The difference is because the Earth is also turning
the satellites. They are about 12,600 miles above the Earth in orbits of about 16,500 miles.
Some periods of the day are worse than other periods for getting a position fix. The satellites rise about four minutes earlier each day so the same pattern will repeat the next day only four minutes earlier. If you have buildings, trees, or mountains in the way of the satellites, then expect that there will be times when you can't get a location fix and other times when you can.
The signals from the satellites will go through some fabrics (non-metallic) such as a nylon day pack or clear glass. They will not go through glass with a metallic film that is found on some vehicles or a half-inch of standing water. The signals will "go through" rain. Items that contain water, such as leaves, living beings, etc., will attenuate the signals if thin enough and block them completely if thick enough.
For a look at satellite availability, DOP (dilution of precision), overhead views:
|For many GPSRs take course as a synonym for track defined
So a course in many GPS receivers is the "line" (planned route) from
starting location to the destination or the line from the previous
to the next point which would be a leg of a route. The track in
GPSRs is the direction you are actually traveling over ground. A
bearing is the direction from your current location to some other
The bearing is the direction you would head, if possible, for land
The highway screen is really for air and water navigation. It gives those users a virtual "highway" to follow. Land users have a real "highway" to follow.
From Bowditch, "American Practical Navigator":
Bearing (B, Brg.) is the direction of one terrestrial point from another, expressed as angular distance from 000° (North) clockwise through 360°.
Course (C, Cn) is the horizontal direction in which a vessel is steered or intended to be steered, expressed as angular distance from north clockwise through 360°. Strictly used, the term applies to direction through the water, not the direction intended to be made good over the ground.
Track made good (TMG) is the single resultant direction from the point of departure to point of arrival at any given time. Course of advance (COA) is the direction intended to be made good over the ground, and course over ground (COG) is the direction between a vessel’s last fix and an estimated position (EP). A course line is a line drawn on a chart extending in the direction of a course.
Heading (Hdg., SH) is the direction in which a vessel is pointed, expressed as angular distance from 000° clockwise through 360°. Do not confuse heading and course. Heading constantly changes as a vessel yaws back and forth across the course due to sea, wind, and steering error.
Track (TR) is the intended horizontal direction of travel with respect to the earth. The terms intended track and trackline are used to indicate the path of intended travel. The track consists of one or a series of course lines, from the point of departure to the destination, along which it is intended to proceed.
|Mapping GPS Receivers|
|Most of the common consumer GPS receivers have a "map" screen
which displays the locations of waypoints, routes, and track log
and your current location against a blank background.
A "mapping" receiver displays a background map (topographic, roads, etc.). The others have a "blank" background. Mapping receivers usually come with a built in basemap and can except downloaded maps from, in almost all cases, only the manufacturer's software.
Only Magellan's MapSend™ can download maps to Magellan's receivers. Use Lowrance's MapCreate™ to download maps to Lowrance receivers. "Only" Garmin's MapSource™ can download maps to Garmin's receivers except that some people have reverse engineered Garmin's data structure and created GPS Mapper as one example. Look for "GlobGPS", "MapDekode", and "GPSMapper" to create your own maps (a lot of work).
For GPS Mapper try: http://gps.chrisb.org/gps_mapper.htm and http://www.gpsinformation.net/gpsmapper/gpsmapper.htm
For MapDekode try: http://planeta.terra.com.br/informatica/download1/dekode_contract.htm
Also check the GPS
site for hints on map creation using MapDekode.
Magellan's structure has also been reverse engineered so some tools are also available for Magellan.
|Track Logging or Plot Trails|
|Options to consider include what does the receiver do when
area is full and how does the receiver decide when to log a
A smart algorithm for logging points can make efficient use of a
Most receivers let you turn logging ON and OFF. When the storage area is full, the receiver might give you the option of either overwriting the oldest points that were stored or to stop logging track points. Not all models give you the option to stop, even from the same manufacturer. For instance the Lowrance iFinder and Garmin eMap don't have an option to stop logging when the storage area is full. They will overwrite the oldest points they recorded unless you stop them.
Most receivers support logging by time or distance intervals. Garmin also has an "Auto" setting that logs a track point when you deviate from a straight line by a certain amount or speed drops to zero. The Lowrance iFinder also has an "Auto" setting that logs a track point when you deviate from a straight line by a certain amount. The "Auto" setting will probably give you the best representation of your route while using the fewest number of track points. The Magellan SporTrak series has an "Auto" and an "Auto Detailed" setting.
Old receivers generally only record horizontal coordinates and
elevation (altitude). Newer receivers record three coordinates
y, and z) in both the waypoint mark and the track log. So don't
to get a graph of the altitude if track logging is off or if you have
an old receiver.
|EPE or Accuracy|
|"Estimated Position Error". The "accuracy"
on some receivers and "EPE" on others) value is the estimated error of
the location reading. Calling it "Accuracy" is a bit of a
misnomer. It is a measure of the receiver's confidence in the
location displayed based on what limited information the receiver can
gather about the data it is receiving. Given an EPE of 20 feet,
the location displayed might be within 1 foot or 40 feet (or more) of
the actual location. Given an EPE of 60 feet, the location
displayed might be within 1 foot or 120 feet (or more) of the actual
location. The location displayed with an EPE of 60 feet may be closer
to the actual location than the location displayed with an EPE of 20
feet but it is more likely the
other way. The receiver doesn't
"know" where you are so it can only give you an estimate.
The map accuracy circle shows the possible location in
to the map. It includes inaccuracies of the base map or other
loaded map and any
caused by the zoom levels on Garmin GPS receivers. The accuracy
is usually considerably larger than the accuracy value. The
value might be about a 50% confidence level not a 95% confidence
Different maps have different accuracys.
|Real Compasses and the Compass Ring|
|Some GPS receiver models have a magnetic field sensing compass (a "real" compass) in them. Most others have a "compass ring" that is not the same as the "compass" you are probably familiar with. The compass ring only works when you are moving and the faster you are moving the better it works (a fast walk will be OK).|
|Some GPS receivers have an audible alert and others do not. It can be handy to have a model that supports audible alerts. Then the receiver can alert you by sound that the battery is about to die, that the track log storage area is full, that you are within a certain distance of a dangerous area (proximity alarms), etc.|
|Some GPS receivers have a
comment or message field for waypoints. This field is handy for
full name (or longer reduced name) or additional information about the
location. The Garmin eTrex series don't have this field.
Some Magellan models support 500 waypoints but only 200 of them may
have a comment.
|Date and Time
|The date and time are set from
data received from the satellites when a lock is made. An
internal clock keeps running (as long as power is available) when there
is no lock or the unit is "off". This is necessary for fast
satellite acquisition when the unit is turned on. Otherwise it
won't know which satellites are overhead and will have to start
searching. How long it searches depends on when the GPSR guesses
|Intermittent Power Shut Off
|If you use a GPSR on a
motorcycle, ATV, or bicycle where there is a lot of vibration, the
batteries can bounce around especially when the vibration is close to
the natural harmonic frequencies of the battery-spring
combination. This can create small gaps that the electric current
can jump across (electric arc) and start building up black carbon
residue on the contacts and on the battery terminals. The carbon
deposits are an insulator that will make the power problem even
worse. The use of dielectric grease on the contacts can help
prevent this from happening. Dielectric grease can be found at
places such as Radio Shack and automotive supply stores.
If two batteries go into a battery compartment end-to-end, try putting a long piece of tape (possibly of a type that has some give like a rubber band) on one side (not all the way around) of the batteries to help hold them together. Also try putting pieces of foam (non-conducting) in the battery compartments to help dampen vibrations and change the natural harmonic frequencies. Silicone glue on a battery cover can also work as a flexible "stuffing". Foam behind the metal leaf spring contacts in the eTrex series can help prevent the metal from cracking at the bend. Don't put too much of anything in the compartment - putting stress on the battery cover can lead to cracks or "open" it enough that water can get in. Measure the thickness needed between the batteries you use and the inside cover and keep in mind that batteries vary in diameter.
On a motorcycle your best option is to run off of external power. If you have a dirt bike with an AC power generator for a headlight try looking for an AC to DC power converter with a smooth output voltage and especially one that outputs a constant DC voltage from a wide input range so the GPSR won't shut off when the engine drops to idle. A dirt bike with an alternator and regulator built to charge a battery probably has a "noisy"/crude output voltage that will destroy electronic gear - so beware.
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|Copyright © 2003 - 2006 Dan Anderson. All rights reserved.|
GARMIN, eTrex, AutoLocate, and TracBack are registered trademarks, and MapSource and Personal Navigator are trademarks of GARMIN Corporation or GARMIN Ltd.
GlobalMap is a registered trademark, and iFinder, MapCreate are trademarks of Lowrance Electronics Inc.
Magellan®, SporTrak™, Meridian®, MapSend™, DataSend CD, are registered or common law trademarks of Thales Navigation, Inc.
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