MORE ON SETTING UP USER GRID

The method we showed students in Sarawak is fine for places which are close to - and north of - the equator. Thanks to Zac Twidale for pointing out that it doesn't work in Australia! The problems which can arise are:

1. The USR read-out is blank. Grid refs are always positive, and the 12XL can't handle negative eastings or northings. In this case the USR display will be blank. You'll have to try non-zero values for the false easting and false northing. It's also limited to 7 digits, so can't show 10,000km or more (assuming we are working in metres), so the full grid ref is not displayed. 

2. You've got one point (maybe the SW corner of the map) spot on, but the others are way off. In particular, as you go north the error in the easting gets worse, and as you go east the error in the northing gets worse. The USR grid is twisted around compared with the grid printed on the map.

When the User Grid is useful

Let's begin by recapping why we might need to use the User Grid. 

A. You have a map with a grid drawn on it, but the grid is not one of Garmin's built-in grids; that's our situation in Sarawak, the Netherlands East Indies Grid is not built in. Maybe you have no idea what grid it is supposed to be.

B. You have a map with no grid, but it does have a north arrow and a scale, so you can draw your own grid on the map. Drawing your own grid is useful if there is not even an indication of latitude and longitude. Click here for tips on drawing your own grid.

In both cases you will need the lat and long for a least one point on the map, either from the indications of lat and long on the map, or by taking your GPS to an identifiable point on the map and recording a waypoint.

The 'false origin'

What's the meaning of 'false easting' and 'false northing': why 'false'? 

Because we're trying to fit a flat grid onto a curved earth, the grid lines and lat-long lines can only be made to coincide at one spot on the planet (more on this below). This is the origin of the grid. The mis-alignment gets worse as you get further from the origin, so it makes sense to stick it in the middle of the area you are mapping (eg in the middle of England).

But if eastings and northings were measured from the origin, half of them would be negative; that would make arithmetic complicated and lead to errors. So we simply give an arbitrary easting and northing to the origin, big enough to make sure that all other places on the map have positive eastings and northings.

As you've probably guessed, the (arbitrary) easting and northing for the (true) origin are the 'false easting' and the 'false northing'. And the 'false origin' is the point where easting and northing = zero: it will be somewhere off the bottom left corner of the map.

Getting a USR read-out

The Garmin user grid positions its origin at the 'Longitude of Origin' you specify and latitude zero (ie the equator). If you're south of the equator, you'll have to put in a false northing greater than your distance from the equator, to make sure the northing you're trying to get out of the GPS receiver is positive. But don't make it too big, or the northings will be too big to show on the USR display and you'll get blanks: you don't need more than 10 000 000m - that's the false northing used for the Australian Map Grid. 

When playing with a map of the north of England I hit a similar problem - no USR readout with false northing of zero - and it went away when I put in a biggish false northing. I don't understand that!

The procedure is much the same as before, except that you have to add the value you guessed for the false northing (eg 5 000 000m) to the correction before entering the values into the User Grid screen -

SUBTRACT the values on the machine from the values from the map, for both easting and northing; this tells us what the 'false' easting and northing should be: Easting Northing From map: 3 955 500 1 037 825 From GPS: 0 055 640 0 138 212 Subtracting 3 899 860 0 899 613 Old false northing: 5 000 000 Add 3 899 860 5 899 613

Note that the GPS readout may be greater than the map northing, in which case the value in the 'subtract' row will be negative.

If the Longitude of Origin is to the right of the area you are working in (we'll see where it should be in a moment), you will have to do the same thing with the false easting - put in a non-zero value to get a USR readout, then add in the value you chose to the correction given in the 'subtract' row.

Untwisting the user grid

The grid lines on your map always stay the same distance apart (usually 1km), while meridians (lines of longitude) get closer together and finally meet at the poles. So the 'north-south' grid lines only line up with one meridian, the Longitude of Origin. 

If you're drawing your own grid onto the map, you choose the Longitude of Origin to make life very simple: click here for tips on this. 

But if you want USR to match a grid printed on the map, you'll need to get the right Longitude of Origin : two grids lined up with different meridians will not coincide (except near the equator, where the meridians are almost parallel, so the choice of Longitude of Origin doesn't matter much). 

Getting the Longitude of Origin right is mainly a matter of trial and error: try a couple of values and see how big the error is for each, then home in on the value which gives zero or negligible errors. Click here for an example.

Incidentally, the example also shows how to adjust the scale for the user grid.

Why can't I make it fit exactly?

You may be lucky and get your user grid to fit exactly with the grid printed on the map. But in many cases you will be able to get one point exactly right but all others are a little bit out, no matter how you juggle with the Longitude of Origin, the scale and the false easting and northing. Why is this?

The earth is approximately spherical (but only approximately!) and maps are flat. Producing a flat map from a bit of the spherical-ish earth is a complicated business which involves a series of arbitrary decisions, in particular about the datum to use, the type of projection and the parameters for the projection. That means that many different maps can be made of the same area, all very similar but not quite identical - and grids drawn on the different maps will not quite line up. If the grid drawn on your map is based on the same datum, projection and parameters as Garmin's user grid, you can make them line up exactly - if not, you can't. But you should be able to get a good enough fit over a small area to be useful.