User-Grids for Garmin GPS

Links to Garmin User-Grid Information

“Tricking” Garmin and Other “UTM-Like” User Grids into Complete
State Plane,Transverse Mercator Conversion

Peter H. Dana - Department of Geography - University of Texas at Austin
July 1, 1999

The Garmin 12 XL User Grid (SW Ver 4.54) and other Transverse Mercator (TM) converters that have only a “UTM-like” user grid with a fixed Equator origin latitude can be "tricked" into emulating a complete TM grid with a non-zero latitude origin by computing the necessary false northing value to compensate for the incorrect latitude origin at 0.0 degrees. This is not too hard and is an exact "fix." This technique is useful when either using a true TM grid (about half of the NAD83 United States State Plane Coordinate Systems – usually those zones whose north-south extent exceeds their east-west extent) or emulating one (to approximate a Lambert Conformal Conic grid - most of the other half of the SPC zones). It is not useful for oblique projections such as the Alaska Zone 1 SPC (an Oblique TM projection) or grids that cover large areas over which the TM approximation of the correct projection will simply have too large an error.

The “normal” TM formulas (Snyder, John P. 1987. Map Projections: A Working Manual. USGS Professional Paper 1395. Washington, DC: US Government Printing Office) compute the northing value from a number of terms including two values of “true distance along the central meridian;” one for the position latitude (M) , and the other for the origin latitude (M0). This is the only place in the ellipsoidal TM formulas where the origin latitude is used. The northing value computed using a zero origin latitude (the Equator) is missing only a value of the TM scale (k0) multiplied by M0. If one knew the correct value for k0 * M0 we could just add that to the northing computed for the zero origin case. This k0*M0 value is a constant everywhere the projection is used.

We can compute this k0*m0 value using the incomplete TM conversion in the Garmin 12XL:
The “normal” TM parameters contain the following values (Here we will use Vermont's single NAD83 SPC zone (4400) as a good example of a TM grid in a great state).
Origin Latitude 42:30:00N
Origin Latitude 72:30:00W
Scale Factor (1:28,000)
or
Scale (1-SF) 0.99996429
False Easting 500,000 meters
False Northing 0.0 meters

To define the same grid in a “UTM-like” Garmin 12XL grid:

Make sure the unit is set to the NAD83 geodetic datum (or the practically equivalent WGS-84)
Set the position format to hddd°mm’ss.s”
MAIN MENU
SETUP MENU
NAVIGATION
NAV SETUP
POSITION FRMT:
hddd°mm’ss.s”

Define a waypoint in latitude and longitude at the desired TM origin latitude and longitude.
MAIN MENU
WAYPOINT
NEW?
NAME (A1 or any other name)
N 42°30’00”
W072°30’00”

Change the position format to User Grid
MAIN MENU
SETUP MENU
NAVIGATION
NAV SETUP
POSITION FRMT:
User Grid
LONGITUDE ORIGIN
W072°30.000 (note the need for decimal minutes here)
SCALE
0.9999643
FALSE E:
00500000.0 mt
FALSE N:
00000000.0 mt*
*Even though an origin has a northing value, place zero here

Return to the WAYPOINT LIST
MAIN MENU
WAYPOINT LIST
Locate the origin waypoint (A1)
Find the Easting and Northing
05000000 Easting
USR 4707008 Northing
Record the northing value (4707008)

Compute the “trick” false northing by negating this origin false northing and add the true false northing (0.0 meters for this VT SPC zone).
-4707008
+ 0.0
_________
-4707008

Edit the User Grid to set the False Northing to this “trick” value.
-4707008.0 mt

Check the procedure by returning to the waypoint list. Note that now the origin of the TM system has its correct value of:
0500000 Easting
USR 0000000 Northing

Now one can navigate or convert waypoints in any other system to User Grid coordinates in the NAD83 VT SPC Zone 1.
Comparing with CORPSCON ( Version 5.11.01) <http://crunch.tec.army.mil/software/corpscon/corpscon.html>

For:
NAD83 latitude 44.0000 degrees north
NAD83 longitude 73.0000 degrees west

CORPSCON:
Easting 459898.31703
Northing 166762.01843

Garmin 12 XL:
Easting 459898
Northing 166762

One can often design a TM system to approximate Lambert Conformal Conic or other non-oblique local grid. Errors accumulate as one moves away from the origin and one might be better off using professional equipment that lets a user correctly and completely define a local grid, or that has State Plane Systems imbedded in it. When using cheap recreational receivers with arbitrary and poorly documented User Grids the “tricks” may not be worth the effort.

CAUTION: While use of SPCS in this "trick" procedure provides fairly accurate coordinates for TM projections, it is not correct for Oblique Mercator (OM) and Lambert Conformal Conic (LCC) projections. This procedure provides errors in the range of 50 meters or more for LCC projections and should not be used for OM projections without a careful selection of
a new Central Meridian.