Geological compass

The geologists compass is a compass that is used primarily for determining the spatial position of rock structures (especially discontinuities ). In addition, he is but also used for geological mapping and orientation in the field. For this reason, it usually is in conventional geologists compasses a combination of surveyor compass ( with inclinometer ) and marching compass ( with visor and mirrors).

Use

In geology, the spatial position of surfaces ( such as bedding planes, foliation surfaces, fault planes, etc. ) is defined by two values:

• The strike is the imaginary intersection of the area concerned with the horizontal, ie with the map layer.
• The fall is the maximum angle between the surface and horizontal. The felling direction (ie the direction in which one marble would roll on this surface ) is always at right angles to the strike.

These two values ​​allow the correct position to write a geological area in maps, profiles or 3-D models, or digitally to exploit.

Measurement strike

When geologists compass stands in contrast to the marching compass set the compass rose and the needle is pointing directly the direction ( azimuth ) of the strike at. The compass rose is therefore a mirror image to the correct reading.

In small-scale measurements, such as to layer and foliation surfaces, an edge of the compass is applied to the surface to be measured and, adjusted with the help of a dragonfly in the horizontal. The painting can now be read directly. In larger-scale structures, such as the outcrop of rock or Ganges Erzganges in reasonably level ground, the strike can also be estimated " freehand standing " over the structure.

Most geologists compasses have a compass rose with a degree from 360 ° to allow orientation in the field by bearings on the visor. Like a marching compass this, the azimuth is read via the mirror. Since a string line from example 135 ° (southeast ) but at the same time has always, even after 315 ° (NW ), is to measure the strike actually half of the 360 ° scale redundant. Therefore, there are also compasses, where the rose is divided into four quadrants. Here, the North and South are assigned the values ​​0, East and West, however, 90 The strike northwest-southeast is then noted as N 045 W.

For this purpose, a right-angled edge is applied to the previously determined streakline at ordinary geologists compasses. The dip angle is then read from the built-in inclinometer. The inclinometer is usually equipped with a second bubble, if not he just settles by gravity itself. Horizontal surfaces have a fall of 0 °, vertical 90 °. It should be noted that as an area with a northwest-southeast strike can occur either to the northeast or southwest now. In the complete measurement of this must therefore be noted: 135 ° / 50 ° SW or N 045 W / SW 50

Structure compass

A special form of geologists compass is the structure compass with which the spatial position of a surface can be determined in a single measurement. Here not the dip and strike, but the dip and direction of fall is measured. In the structure of the compass lid serves as a protractor. The flap is hereby placed on the layer surface, and after weighing the compass using the level the compass needle is fixed with a locking device. After the dip angle can be read on a scale on the lid hinge (height circle), the direction of fall of the compass rose. Since the direction of fall is read on a 360 ° scale, the value is unique, the notation for the above example would read 225 /50. The painting can now be easily computed by adding or subtracting the case with direction 90 ° (315 or 135 ). The Freiberger precision mechanics provides to the degree scale commonly used nor the division into Gon, which allows a much more precise subdivision. When specifying the height in Gon circuit has ( for reading the angle of incidence ) and the Klinometerteilung ( the division of the pendulum clinometer to the incidence angle determination ) a classification into each 100 Gon instead of 90 °. Also the compass circle in 400 Gon is divided instead of 360 ° for this equipment.

Because of the speed and clarity of the measurement structure compasses are particularly suitable to collect large amounts of structural geology data. Since the scale on the lid hinge is not very accurate, even a second, bob shaped inclinometer ( clinometer ) can be used to more accurately measure the dip angle. An orientation in the field is only possible with a compass structure, since here the compass direction data for rapid determination of the values ​​are reversed. It should be noted that the North and Südnadel are also reversed ( North and South is black is red). However, should this compass used for orientation in terrain, then the targeted direction on the red Südnadel is read.

The assemblage compass, however, not only the calibration of the angle of incidence and the incident direction of geological structures is possible, but also the calibration of lineations on surfaces. Lineations are mineral orientations on geological surfaces such as on armor surfaces, foliation surfaces and the like. In this case, a lateral edge of the landing flap to the lineation directly and aligned the compass body on the basis of dragonfly. Directly the direction of fall and the fall angle of the lineation can then be read here again, even if the case values ​​of the lineation does not match the case values ​​of the separation surface on which the lineation is.

The structure compasses the models of Freiberger precision mechanics and Breithaupt have in the European area mainly enforced. This equipment does not differ much from the basic structure. However, the clinometer is based on the models of the Freiberg precision mechanics on the principle of a free- swinging pendulum (if the compass is directly applied 90 ° upright). When swinging the compass from the upright position to the normal position of the pendulum rests on the floor compass, and therefore can not move and change the value. In the Breithaupt compass, however, is the clinometer indicator of a ball, which runs in a semicircular path. Negative is with this principle that the ball is in after folding back the compass can still move and thus distort the value.

The European market is flooded with many " cheap " imitations of these two models. Compasses Freiberger precision mechanics and Breithaupt are very reasonably intense in the acquisition. However, to justify the high cost of the fact that the structure compasses are milled and assembled by hand and they are very robust.