Using UTM Convergence

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This option enables the use of a UTM convergence angle. Setting this rotates the cave so it will be aligned to true north even when plotted on top a map that aligned to the UTM Grid. This is done by subtracting the UTM Convergence angle from the azimuth of each shot.

 

The feature should only be used when you are plotting on top of maps whose coordinates are aligned to the UTM grid. On such a map, the a line dawn straight up the page will be aligned to UTM north, but not to the actual North Pole. This drawing helps illustrate the issue:

 

In the image to the right, the black lines show the grid for a map whose coordinates are aligned to the UTM Grid. The UTM grid is  derived by projecting a flat, square grid onto a spherical earth. Since the earth's surface is curved, the direction to True North is  also curved away from the vertical lines on the map.

 

If you are plotting your cave data on top of a map that is aligned to the UTM grid, a line drawn straight up the page will point to true north from the caves perspective, but to UTM north from the grid's perspective. The result will be that features in the cave will not correspond to features on you the UTM map. For this reason, it is useful to apply the convergence correction to the cave data.

 

Remember, convergence is only useful if you want to plot the cave map on top of a map that is aligned to the UTM grid. Some  topographic maps are aligned to true north, so in that case this option should not be used.

 

Also, remember that if you are using Convergence, north will no longer be straight up the page. Likewise, the Compass north arrow option will point at UTM North, not true north.