WORKING WITH COORDINATES AND UNITS

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User Defined Grid

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.

An example

Suppose you want a local grid of your own that measured distances in feet, here is how to proceed.

1. Pick an origin for the grid. Note that this must be in the lower left corner (SW) of the area the grid is to cover. You can't have any negative numbers in your grid.
2. Go to the origin point and note the lat/lon numbers.
3. Enter the longitude for the origin.
4. Enter a scale of 3.280839895 (This converts metric to the international foot at sea level. For metric you would enter 1.0)
5. Enter a false easting of 0
6. Enter a false northing of 0
7. Save this configuration.
8. Go look at your coordinates and write down the numbers.
9. Now return to the user defined grid and enter the numbers you just wrote down as negative numbers in the false easting and false northing.
10. Save the new configuration.
11. Check your waypoint location which should now read 0,0. Remember negative numbers will not work so if it drifted a bit you may have to move.
12. Use your new system.

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 problem.

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