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WORKING WITH COORDINATES AND UNITS
(...continued)
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
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.
Linear measure
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
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:
mils = lateral distance * (1000
/ distance to target)
For example: a 12 mil change would
result in a 12 meter movement per 1000 meters distance. The
fine division of a circle into 6400 units may have other uses
as well.
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.
revision
00/2/10 initial release
00/5/8 added menu discussion
00/6/11 added more loran, utm, and time discussion
00/12/20 added some lat/lon conversion information
01/8/10 added more on lat/lon measurements, added some user
grid info, and corrected some typos.
02/6/25 added data on special UTM zones 31 to 37 at north
latitudes
02/10/10 added an offset discussion to user grid.
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