WORKING WITH COORDINATES AND UNITS

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Maidenhead

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.

Local Grids

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 Grids

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.

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