This chapter covers the various grids, datums, and other measuring items in a Garmin unit. Much of this information is mathematical but I have attempted to simplify this wherever possible and just provide an overview of the information. This simplification can lead to errors or misunderstandings so please let me know if you find any of these or areas that simply need to be beefed up to make them clearer.
The datums and grid preferences can be reached from the main menu
by selecting the navigation setup options. You can set the navigation
datum, the navigation grid, the user units, and if your unit support
cdi, this can also be set here.
What is a datum? First, what it is not. It is not a singular form of the word data which some of you may have heard. Data is a collective noun and the plural of datum is datums when used in a geophysical sense. For GPS, navigation, and geophysical use a datum represents a reference point from which you can measure things. At one time this was a location on the earth and could have actually been just a single point of reference. In modern usage it defines not only the point for reference, it also defines the model of the earth used to support those measurements. The model is a three dimensional one that describes mathematically the shape of the earth, the location of the center, and the location on the surface that represents the starting point for measuring. Some older datums only defined a horizontal measuring model or, in some cases, only a vertical one. Some datums permit measurements to be made worldwide while others are only defined to support a local grid measuring system. If you were to use a different datum to measure the same place you will typically get different results and is some cases drastically different results. Internally Garmin units always store information and compute using the WGS-84 datum but will display the answers in the datum of your choice.
Typical Garmin handhelds let you define any of over 100 datums for use anywhere in the world but there is no check that you used an appropriate datum for the area you are currently located in. The question is: "Why would you want to define a datum?" Since a gps unit makes all measurements using the WGS-84 datum this might seem enough to just display the answer using that datum. In fact this may be enough for some folks and some gps units only support this one datum. However, surveying and map making have been going on for centuries. If you have a map created before the modern age of gps your map probably was not made using the WGS-84 datum. If you want your position data to agree with the map or survey marker on other source of geophysical location information you will need to ensure that your gps datum agrees with the one used to generate the data. Generally on a map this information is given in the legend of the map itself. In addition some maps use a local grid system (covered later) that requires the use of a specific datum to maintain the accuracy of the grid system.
As mentioned before many datums are only defined for the horizontal plane and thus your gps will continue to use the WGS-84 datum for vertical, altitude, information. This datum defines zero height using a mathematical model of an ellipsoid, which is basically a ellipse spun around its minor axis to form a globe shape. The surface of this ellipsoid is considered to represent zero altitude with points above the ellipsoid representing positive altitude and points within the model negative altitude. This model does not take into consideration the many real life changes caused density differences in the earth and earth motion. What we would prefer is a measure of altitude that represents zero with sea level, or more precisely mean sea level. To get this number your gps uses a table that has been defined for just this purpose. It is called a geoid and can permit the translation of gps computed altitude to its mean sea level equivalent. It should be pointed out that while we seem to have defined altitude very accurately, a handheld gps is not a particularly accurate device in measuring altitude. Because of the geometry of the satellites in the sky and the fact that the earths surface blocks our view of most of the constellation the accuracy of a vertical fix is about 50% worse than the accuracy of a horizontal fix. Unfortunately this is often disconcerting to the user since they generally know their altitude to better accuracy than they know their horizontal location and even may measure it with more precision (feet vs. miles). Do not let this difference cause you to believe the gps itself is not reporting an accurate fix.
Translations among the various datums can be a cause of some errors. Garmin used the the Molodensky transform parameters for those datums and performs transformations as needed. This is a simplified model and can result in errors on the order of 10 meters in some cases. With this transform the WGS-84 and NAD-83 numbers are always the same while a better translation will show a slight difference in these two datums. For most of the US this difference is less than a meter rising to about 2 meters on the west coast. Still less difference than the accuracy of the unit in most cases. Modeling something as complicated as the earth does not lend itself to mathematical transformations so to achieve accurate results some information is contained in tables with model information used to access the entry in the table which is then interpolated to get the answer. It is not known where Garmin depends on models alone and where they supplement with tables.
WGS-84 is a world wide datum and is the master datum for use with a gps. Garmin always stores internal information in this datum. The origin is the center of the earth and then a ellipse is defined using the major and minor axis. Information about ellipse flattening and the gravitational constant is also part of the definition. The latter is used to calculate the geoid height. The model is augmented with stations at precisely defined locations on the earth's surface to pinpoint the accuracy of this system.
As more and more mapping systems have become digital in nature and databases have begun to be the norm for surveying there is a tendency to use the WGS-84 datum more and more. This is exacerbated by a need to share data from around the world that was originally generated on a different datum. Hopefully this trend will continue as it simplifies data sharing and reuse of information.
There are several datums that have been or are still being used in the US. One of the NAD-83 is almost identical to WGS-84 and can be considered for navigation purposes to be the same. The differences are well under a meter most places in the US and are caused by a slight difference in the constants used in the calculations as developed by the different agencies administering the datums.
Many existing maps use the NAD27 datum. The difference in coordinates between NAD27 and NAD83 can vary up to 10's of meters. The 1927 datum was determined from less accurate and fewer observations. Factors such as seismic motion have changed station positions over time and computational capabilities did not exist to average readings. The newer datums are geocentric while this datum was based on a point in Kansas, and gravity data was not used in the network adjustment of the 1927 datum. There are other NAD27 datums with other locations chosen as there reference points for areas not too close to Kansas. Most USGS maps were generated using the NAD27 CONUS (continental US) datum.
There are datums in use all over the world and you need to be careful to use a datum that matches the map you are using. Study the map legend for this information or check with the map manufacturer. Some map grid systems assume a specific datum is used such as the British system used in Great Britain. Very few paper maps use the WGS-84 datum so if you need to relate your position to a paper map you will have to deal with the datum question.
The WGS-84 datum relates to other datum in common use around the world in
in terms of differential X,Y,Z coordinates (DX,DY,DZ) and DA and DF
which are the differential equatorial radius and differential flattening.
This information can be entered into Garmin units that support user
defined datums to provide the datum of your choice.
In addition to defining the origin and model to use for measurement purposes the gps receiver with also have a choice of grids to use to display the horizontal location.
Everyone has probably heard the terms latitude and longitude as referencing to a grid system that covers the earth. This system has been around for hundreds of years but gets a new dressing for gps use. Latitude and Longitude assume that the earth is a big ball and defined as a spheroid. The fact that it rotates around the poles on each end is used to develop a grid system that is based on this angular motion of rotation. Basically the idea of longitude and latitude is to measure the angles represented if you were slice the earth into a circle. If you slice the earth at the equator and then divide it up you would have a longitude line at each degree around the circle. These lines would all meet at the poles. Rotation of the earth would dictate that the sun would travel the same angular longitudinal distance in the same time. Each one of these lines is called a meridian. If you slice through a meridian you have a circle and each degree around the circle is called a latitude. While longitudes all meet at the poles latitudes are all parallel to each other.
For more precise measurements the degree is divided into 60 minutes and the minute is divided into 60 seconds just like time. For calculation purposes we often measure and calculate in decimal fractions of a degree or decimal fractions of a minute. We always divide seconds decimally if we need smaller unit. Garmin gps units can be set to display lat/lon in all three measurement systems. The equator becomes an obvious point for measuring latitude it is defined as 0 degrees while points North are measured as degrees on North latitude until you reach 90 degrees at the north pole. Similarly the southern hemisphere is measured in degrees of south latitude. There is no corresponding obvious point for longitude lines. By international agreement 0 degrees longitude is a meridian line that cuts through England and is called the prime meridian. Longitudinal distances are measures west and east until they come together at the international date line 180 degrees later.
While Garmin can display and convert from the three different systems used to define lat/lon there can be considerable confusion among users looking at the display and comparing it to some external information. This is because some data is not very precise in its use of decimal points and the lack of a degree sign on most keyboards can encourage a substitution of the decimal point. When comparing numbers consider that minutes can only go to 59 and will then roll over. If the data just after the decimal has digits above 5 then it is likely to be a decimal part of a degree and not minutes. Similarly for seconds.
Decimal parts of a degree can be converted to minutes easily by multiplying by 60. A space between degrees and minutes is the preferred separator when a degree sign cannot be used to avoid confusion. Similarly decimal parts of a minute can be converted to seconds by multiplying by 60 thus, if you have ddd.ddddd you can convert to ddd mm.mmm by:
Many folks think the lat/lon measuring system is independent of the datum issue however this is not the case. In the precision and accuracy required for gps use the same problems are found with lat/lon as with any other grid system we might employ. It earlier times measurements weren't precise enough to make this obvious. Even today making angular measurements using devices like sextants is not accurate enough to make the datum usage much of an issue.
Angular measurements, however, are not particularly convenient for measuring distances and, even in its simplest form requires the use of trigonometry. Long distances become even worse since a line between two cities at different longitudes and latitudes on a map does not represent the distance traveled between those two points when actually traversing the distance on the real earth. Instead we have to resort to great circle distances and measurements for these cases. Mariners and aviators use nautical miles to help simplify this calculation. One nautical mile is 1 minute of latitude thus a degree of latitude is 60 nautical miles. When measuring distance in the east-west direction the distance correction varies with latitude. While accurate measures would require us to consider the earth as an ellipse a simple estimation can assume the earth to be a spheroid and thus the east-west distance in nautical miles is given by the formula:
Let's suppose we took and orange and cut the peeling vertically around
the orange in several places and then peeled it off and laid the
pieces side by side. Then we took a hammer and flattened the whole
thing out. We might have something that looked a little
like the image below:
This image is similar to the projection system we get to create the UTM grid system. The UTM, Universal Tansverse Mercator, system does a similar thing with the earth by slicing vertically every 6 degrees thus creating 60 such slices around the world. All the slices touch at the equator and get further apart as we get closer to the poles. Another interesting thing to notice is that the longitude lines at the center of each slice are straight while at the edges of each slice they are curved. The amount of curvature increases as we get closer to the edge. A vertical line drawn near the edge would not point directly north.
Each slice, called a zone in UTM, is given a number from 00 to 59 starting at the international date line and progressing east. While not necessary to the measuring system UTM also divides each zone horizontally as well. These divisions start at the equator and are 8 degrees wide. The first half of the alphabet is used for the southern hemisphere while the second half is used for the northern. Thus a point just above the equator would be the letter N proceeding to the letter X while just below it would be an M and proceeding backwards to the letter C. In addition to the letter assignment UTM also measures the distance from the equator in meters along the central meridian (which is the only straight meridian line in the zone). Since every zone is measured directly from the equator the letter is not necessary to define the location except in some special cases described below. East-west measurements within a zone are always referenced to the central meridian.
The full UTM grid system is now defined. There is one latitude grid line, the equator, and 60 longitude lines in the centers of each of the zones. Further each zone is 3 degrees wide on each side of its central meridian. At this point we quite worrying about the angular measurement system and instead just measure direct distances in meters from these lines. Since measuring from a line implies positive and negative values the designers have tried to simplify this problem by defining the central meridian to arbitrarily always be 500,000 meters. This is called a false easting. This means that distances to the left of the central meridian will be subtracted from 500,000 and those to the right will be added to 500,000. In this way all measurements will always be a positive number of 6 digits. North/South distances are measured from the equator directly and can get as large as 7 digits. For this reason we usually add a leading 0 to the east/west distances to make them 7 digits long also. To avoid negative distances in the North/South direction in the southern hemisphere always add 10,000,000 meters to this measured negative distance.
Since each central meridian is always 500,000 meters there needs to be a way of designating which central meridian is being talked about. This is accomplished with the zone prefix (a number between 00 and 59) thus a full UTM specification consists of a two digit zone number, the zone letter and 14 digits of measurement data to measure down to the level of one meter anywhere on earth. The first half of the measurement data is the east/west number while the last half is the north/south component. If you don't need the full precision of 1 meter you can leave off pairs of digits. You could use 12 digits for 10 meter, 10 digits for 100 meter accuracy, etc. Sometimes you will see an odd number of digits by leaving off the leading 0 in the east/west number.
Since there are so many numbers there needs to be some method of separating them out. In standard usage a comma or a point is used to separate the numbers into groupings. Map makers have chosen to vary the font size. For example instead of 0392000E and 3382000N you would see something like 0392000E and 3382000N. This is particularly important alignment information if you leave off some of the digits.
As can be imagined from the drawing above the tips of the points have very little area associated with them and a great big gap. UTM solved this problem by drawing a line at 84 degrees North Latitude and 80 degrees south latitude. The areas above and below those lines are rearranged into two circles like pieces of a pie. These new zones use a different pie shaped grid system called UPS. The southern pie is a bit larger since it needs to cover Antarctica. Two of the letters on each end of the alphabet are used to reference zones in this area. A and B divide the southern pie into two hemispheres while Y and Z do the same with the northern. The map picture for this area would be a polar projection.
A few of areas below the 84 degree separation also have been modified. This has been down to permit an island to be fully contained within one zone. At these latitudes you won't have any problem overflowing the 50,000 false easting even with the wider zones. Zone 32 has been widened to 9° (at the expense of zone 31) between latitudes 56° and 64° to accommodate southwest Norway. Similarly, between 72° and 84°, zones 33 and 35 have been widened to 12° to accommodate Svalbard. To compensate for these 12° wide zones, zones 31 and 37 are widened to 9° and zones 32, 34, and 36 are eliminated.
One of the biggest problems in using UTM is the handling of the zone splits. Within a zone it is very easy to find coordinates and to measure and compute any distance on the map. When you hit the boundary you are suddenly confronted with a different number with no relationship at all to the one you just left in the east/west direction. When a map has a populated area that crosses a zone boundary this is usually handled by placing a secondary grid in the new area using the old zone grid data. This permits measuring and locating objects on a map based on a projection from a location in the other zone. Thus in areas near a zone boundary you will see both zones extended into the other zone area. These other zones will not be aligned with the edge of the map which makes them easy to spot. Zone boundary lines are usually drawn right on the map to help you with this.
Your Garmin unit automatically knows when you leave one zone and enter another. Further if you project a waypoint into another zone it will be automatically recalculated into the correct value for that zone changing the zone number and letter as needed. In addition you needn't enter the zone letter for any waypoint you need to enter except that, because of the false northing for southern hemisphere you will need to be sure the letter code is in the correct half of the alphabet. In addition you will need to use a UPS zone letter to let the gps know when you want a grid in this zone.
UTM is the most precise measurement system on your gps. It reports your position to 1 meter precision. None of the other grid systems can achieve this level of precision.
Some Garmin gps receivers also support the Military Grid Reference System. This system is just another form of UTM so if your unit doesn't support this it is easy to translate to MGRS from the UTM numbers. MGRS replaces the most significant digits of the UTM coordinate with two letters. If you use the example shown above for UTM 0392000E and 3382000W you would see UQ 9200082000. The two letters UQ are in addition to the normal zone number and letter code. The two letter codes of MGRS are not unique and may repeat at other points in the globe but they are unique enough that for tactical use within a few thousand miles the zone number and letter code are not needed. The two letter codes are clearly indicated on the map and are used as the main reference locator. These two letter local square designations permit rapid orientation and rough position indications.
In the same way that was described for UTM you can use less digits for less accuracy when desired. For rapid orientation to a given area something like UQ 920820 could be useful. For gps use you will need to enter the full digit code down to the 1 meter level.
Maidenhead is an angular grid reference system used by ham radio operators to provide a rough indication of their location. It is a supported grid on some Garmin receivers.
The Maidenhead system is a "read right and up" system starting at the 180 longitude and south pole. Its format is two letters, two decimal digits, two letters, and can be extended for a more exact description of a location. The first letter is 20 degree increments in longitude (A-R), the second 10 degree increments in latitude, and these define a FIELD. The first digit is 2 degree increments in longitude, and the second digit is 1 degree increments in latitude. American hams stop here and call it a SQUARE. The next pair of letters are 2/24 degrees in longitude and 1/24 degree in latitude, and so on. European hams generally use six characters. Garmin receivers display the letters MH for Maidenhead and he 6 characters defined above.
All of the other grids that are defined in your gps are local grids. This means they are only defined for a given part of the world and cannot be used outside of that context. On exception is the Loran grid which can be redefined for each area of Loran coverage but is otherwise similar to a local grid. You use a local grid when the maps you intend to use are using a local grid. Otherwise they have no particular use. You can only create waypoints and talk about locations in the context of the area covered by the grid. Most of the time your gps knows the limit that the grid is defined for and won't even display coordinates for an area outside the grid boundaries. In addition local grids generally require a local datum so you will need to set both.
The question arises as to why are there local grids anyway since the UTM global grid covers the whole world. Usually local grids were defined before the invention of UTM and are thus legacy grids. In addition a local grid usually supports a contiguous set of coordinates in the area defined by the grid. For UTM the coordinates jump every 6 degrees around the world and also at the equator. For this reason it may not be as useful as a local grid depending on where you are located.
Loran (LOngRAngeNavigation) is a system that provides location information for mariners. It has been around for many years and is based on some techniques that are similar to a land based version of gps. Basically there is a master station and 2 to 4 slave stations. Readings are taken on 3 stations and the time delay from the 3 stations are using to triangulate a fix. The stations are usually several hundred miles apart and there are several sets (28) of these stations to cover the entire US. The master station is one of a chain of 28 stations and is identified with a chain number. It may also serve a slave station for another chain. The slave stations are designated V,X,Y,Z or Victor, Xray, Yankee, Zulu. A Loran grid is special in that it is calibrated to a specific Loran master station and a couple of secondary stations. A different grid could be developed using the same master and a different group of secondaries.
When using this grid you will need to specify the Loran Chain Number and two secondary stations by name, as defined above. You can usually get this information directly from the paper map you are trying to match. If you are in the range of a Loran chain you can set up your grid to a Loran grid and the Garmin will generally choose the correct setup for you. However there may be some overlap so you map might be dependent on a different chain or secondary station so you need to check. The chain number and secondary stations are listed on the left side of the Loran display. If you leave an area defined by that set of stations then the location information will show all zeros. Realize, of course, that this is only a translation of gps location and the Garmin actually does not use the Loran data.
Many Garmin units support a user defined grid. This can be used to support grid systems that are not supported directly by the receiver. Unlike the user defined datum however a user defined grid may or may not be successful. For example the map that contains the grid you are trying to emulate may be using a different projection than the one assumed by the user defined grid. In addition the allowed references that are displayed may not match the grid you are trying to emulate. For example you cannot match the typical map grid that has letters running one way on the map and numbers the other since the display always works with numbers in both directions. The user defined grid is really just a modification of the UTM grid and assumes a Mercator projection. And, like the UTM grid convention it does not work well with negative numbers so any grid defined should ensure that the numbers will always be positive.
To define a grid you will need to specify the grid origin as lat/lon and then you can specify a scale factor for the grid and a false Easting and a false Northing in meters. When you first enter the user defined grid settings you will find the values for UTM already entered into the unit. Change these values to match the requirements of the grid you need. If the projection system is different you can still get close approximation for a given local area. You may need to adjust the numbers as you move further away from the origin where you defined them. Don't forget that you may need a different datum, perhaps a user defined one, for your user defined grid. Often though you will be able to use the work someone else has done to define the grid you are interested in and you can just enter the data without having to design it yourself. There are web resources that can be used to define some of the grids you may want.
Suppose you want a local grid of your own that measured distances in feet, here is how to proceed.
Thanks to Jerry Wahl for the idea behind this discussion. Dayton Fraim
wrote me and said that sometimes you can't get to the lower left
corner as a starting point or your origin needs to be a location that
is not at the corner of your grid reference (He lives on an island where
the origin might need to be out in the water.) so you might want a
modification to this procedure. The idea is to use the same trick as
UTM does by adding a real false easting and false northing to the
above procedure. Instead of just adding the negative numbers in the
false easting and northing in the above procedure to make the answer
some out to zero you could add an offset to these numbers first such
as adding 10000 to each negative number. Then save your new
configuration and check your waypoint location which should read
10000, 10000 or whatever offset you chose. This offset will need to
be factored into any grid values you use for your work so a simple
number is best. Using this offset will prevent the negative values
Other Units of Measure
In addition to the units of degrees and meters that are used to define a map datum and grid system there are many other units that are used and defined for your gps system.
Time is used in your gps to calculate position. The time used by your gps is a special gps time that is transmitted as part of the satellite message. In addition the satellite transmits information in the form of leap seconds adjustments to permit your unit to adjust the clock display to agree with standard UTC time. (Currently this difference is 13 seconds.) All Garmin units also support the ability to change the time zone so that you can display local time. The etrex and emap also supports the ability to have automatic daylight savings time adjustment while in the other units you must change the time zone to accomplish this. These settings are on the main menu, system settings.
Many units also store the time and date of waypoint creation in a changeable comment field and and all unit store time inside the tracklog. The etrex and emap do not store the times of waypoint creation since altitude information is stored in this area. The time stored is not based on your local time. It is either gps time on some older units or UTC time on most newer models with the latest software. The difference is usually not significant to most users.
Note that the gps receiver keeps very accurate time internally within nanoseconds to calculate your position but the display of time is not a high priority task for the unit. Thus the time display could be up to a second later (or even more) than the actual time. It is still accurate enough for most people to use to set their clocks. On the emap you will need to go to the main menu and choose setup and then time to see the seconds display.
Garmin gps units support 3 different user selectable units for horizontal linear measure. These are kilometers, statute miles, and nautical miles. Vertical units are set automatically from the user's horizontal setting to meters or feet. When using miles some units will revert to feet when distances are small. The displayed units may be different than the internal calculation units or the units used on the computer interface. For example NMEA generally outputs distance in nautical miles, after all the M in NMEA stands for Marine. Nautical units are particularly convenient to use when navigation since 1 minute of latitude change is approximately equal to 1 nautical mile.
Speed calculations are automatically tied to the selected linear measure for display purposes.
Angular measure can also be specified as a navigation preference on your Garmin receiver. Angular measure is used to specify your current direction and bearings to objects. Most folks use degrees but some Garmin units also let you specify mils. A mil measurement divides a circle into 6400 units.
Mil units are primarily used by the military. It is primarily used
to help compute a new direction based on an error in an old direction
and is used when aiming artillery. The formula is:
In addition to specifying the units you can specify the reference for your angular measurement. All units will let you specify true north or magnetic north. Note that a gps calculates this number based on velocity information or between two locations. Most gps units do not have a compass and cannot show a compass heading when stopped but will hold the last setting received.
Some units also support grid north and even a user defined north. Generally you should use true north, but if you are also working with a compass you may need to set magnetic north so that the setting will agree with the compass settings. Some compasses will compensate for magnetic declination and will support true headings so this will not be necessary. If you are using UTM maps and wish to folow bearing taken from the map you may need to use grid north since, as point out above, no longitude line except the very center one actually is a straight line on one of these maps. If you don't have a grid north setting there may be up to 3 degree error. You gps will automatically adjust grid north and magnetic north based on your current location. This is done using built in tables and projection algorithms. You can also define your own offset for North if you wish on some units. This, however, is a static number in this case and will not change as your location changes.
|Go to "Working with Garmin" Table of Contents|