When measuring the orientation of a geological structure such as a bedding plane we normally want to compare its orientation with both north and the horizontal.
"North" comes in three flavours.
· True North. That’s the direction of the North Pole. For our purposes this is the axis of the earth’s rotation and usually what you would like to use as a reference.
· Magnetic North. This is the direction that your compass points towards. It is not the usually same as true north. Moreover the variation ("magnetic declination") changes over time and with your position. Topographic maps will usually explain how it can be calculated for the area in which you are working. Some compassed will directly correct for this variation (see your compasses manual).From here on we will assume that you will allow for magnetic declination in all readings.
· Grid North. Most topographic maps feature a grid (often 1km squares). The vertical lines do not necessarily run truly north-south. Topographic maps should indicate the amount of this variation. If it is significant you should account for it when plotting data onto maps.
An inclined plane has one line that can be drawn on it that is level. This is called the strike line or (only in text books) the stratum contour. If you adjust your compass clinometer so that it works as a clinometer, moving its long edge round on the plane so that it indicated a horizontal reading will let you find the strike. Lightly mark it on the rock with pencil then take your compass and measure the bearing of this line. Most usefully this will be written down as the angle that this line makes clockwise with north. Since strike lines are not 'directed' a strike of 12 degrees is the same as one of 192 degrees. Next, making you compass into a clinometer again find the maximum dip by measuring the angle of the rock (to the horizontal) at right angles to the strike. There are various formats to record the data in your log book but you must be consistent. We suggest entering the strike bearing, entering the dip bearing (always the strike +/- 90) followed by the amount of the dip.
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Stereographic Projection is a
method of representing and interpreting orientational data (such as the dips of
bedding planes or joints). There are two parts to the concept.
Firstly consider the idea that any direction can be represented as a point on a hemisphere. We will concentrate on 'lower hemisphere projection' which means that we will consider our hemisphere to be convex down like a grapefruit half in a bowl. If you wish to represent the direction of a mine drift (tunnel) that dips down at an angle of 25 degrees towards the east then imagine the tunnel passing through the centre of an enormous grapefruit half. It will pass out through the skin on the east side of the fruit. If we draw the grapefruit (skin) half from above then this point will represent the direction of the direction of the drift. The skin will be a circle and the point will lie between its easterly margin and the centre. If you wish to represent a plane rather than a line then you must consider this passing though the grapefruit centre. The result will be a line on the grapefruit resembling a line of longitude on map of the world. It is indeed referred to as a Great Circle
The second problem is how to transfer lines (conceptually) drawn on a hemisphere or grapefruit onto a flat page or computer screen. There are two methods. Equal Angle projection ensures that, measured along great circles, the size of an angle is proportional to its separation on the plot. Equal area projection ensures that if two regions have the same area plotted on the hemisphere then they will be the same size on the plot.
The choice of projection is
important when working manually. When using Cauldron 2 it is unimportant as the
entire plot can be instantly toggled from on to the other using the projection button
option found on the 'style' tab.
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