Wrong reading

2 posts in this topic

Downloaded my way points to easy GPS and all the numbers were different than the garmin 72H read, although when  I clicked on a reading it showed me the correct location on the map but the GPS numbers don't match with the true location.

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Most problems involving GPS and map coordinate mis-matches are due to
choosing the incorrect coordinate format and datum at some point in
the process.  If you are entering GPS coordinates by hand into
ExpertGPS from a book or a Web page, you need to ensure that you've
selected the same coordinate format and datum that the source's author
used, or position errors will occur.

There are four main types of coordinate formats used throughout the
world today: lat/lon, UTM, US State Plane, and National Grids.

Lat/lon coordinates:
There are three different ways to express latitude and longitude:
1. decimal degrees. (42.12345°N, -71.23456°W)  Note that there are no
spaces in these coordinates.

2. degrees and minutes (42° 23.456'N, -71° 43.632'W) Marine, aviation,
and geocaching coordinates are usually given in deg min.min format.
Note that there is a space between the degrees (°) and the minutes (')
part of the coordinates.

3. degrees, minutes, and seconds.  (42° 34' 54.234"N, -71° 24'
14.234"W)  Civil survey and some marine waypoints are given in this
format.  Note that there are three parts to each coordinate, separated
by spaces, and that three symbols are used to show the degrees,
minutes, and seconds (° ' ").

UTM coordinates
Universal Transverse Mercator (UTM) coordinates are
used worldwide. Unlike lat/lon, which is a spherical coordinate system
based on angles, UTM, state plane, and national grid coordinates are
rectangular grid systems where coordinates are expressed as Northings
and Eastings from a fixed reference point.  Rectangular grids make it
much easier to calculate distances and to plot locations on a
rectangular map.  UTM coordinates look like this:  2346212N,
12343523E, 18T.  The first value is the Northing.  The second is the
Easting.  The third value (always a number between 1 and 60 followed
by a single letter) is the UTM zone identifier.  If your data is in
UTM coordinates, it is very likely in NAD27 datum or WGS84 datum (see
below).  The vast majority of  UTM data is expressed in meters.  If
you are using UTM data collected prior to 1983 by a US state entity,
there's a chance that it uses US Survey Feet rather than meters as the
base unit.  ExpertGPS Pro provides an option for UTM coordinates in

US State Plane Coordinates
Almost all of the data available from US state government Web sites is
expressed in the US State Plane Coordinate System. US SPCS coordinates
have two values, a Northing and an Easting. Some US states have only a
single SPCS zone. Other states have a handful, broken down by county.
You'll need to know which zone is in use, and select the correct one
in ExpertGPS Pro. The ExpertGPS Web site lists the counties in every
US state, and the appropriate SPCS zone to use:

National Grids
Outside of the United States, many countries have their own National
Grid coordinate systems.  Many of these use the same Northing, Easting
format described above.  ExpertGPS lists the national grids for each
country in the Add Coordinate Format dialog.

To add a coordinate format in ExpertGPS, click Preferences on the Edit
menu, and click on the My Coordinate Formats tab.  Click Add.  The Add
Coordinate Format dialog will appear.  On the left is an expandable
list of all of the continents, countries, and US states.  As you
expand the location tree, ExpertGPS will show the coordinate formats
used in that region on the right side of the dialog.  Drill-down in
the location side as far as you need to go to see the coordinate
format for your location.  Select the coordinate format, and then
choose the appropriate datum from the list below.

In the United States, you have two main datum choices: NAD27, and
NAD83/WGS84.  NAD27 was used from 1927 up until 1983, when it was
replaced by NAD83 and WGS84.  You can consider NAD83 and WGS84 to be
identical in the 50 United States.

For other areas of the world, ExpertGPS will display the datums used
in that region.  Select the correct datum from the list.

You can add many different coordinate formats to ExpertGPS, and switch
between them at any time to enter data or convert data to another
format.  ExpertGPS Pro users should keep in mind that the
currently-active coordinate format is used whenever you import and
export data.

For more information on datums and coordinate formats in ExpertGPS,
visit these Web pages:


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