Geological compass
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There are a number of different specialized magnetic compasses used by
geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...
s to measure orientation of geological structures, as they map in the field, to analyze and document the geometry of
bedding plane In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' ...
s, joints, and/or metamorphic
foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...
s and lineations. In this aspect the most common device used to date is the analogue
compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
.


Classic geological compasses

Classic geological compasses that are of practical use combine two functions, direction finding and navigation (especially in remote areas), and the ability to measure strike and dip of bedding surfaces and/or metamorphic foliation planes. Structural geologists (i.e. those concerned with geometry and the pattern of relative movement) also have a need to measure the plunge and plunge direction of lineations. Compasses in common use include the Brunton compass and the
Silva compass Silva Sweden AB is an outdoors products company, most known for their high-grade compasses and other navigational equipment including GPS tools, mapping software, and altimeters for aircraft. They also offer a marine range. The company's founde ...
.


Modern geological compasses

File:FPM compass with annotation en.svg, Setup of a modern geological compass after Prof. Clar (Freiberger), total view File:FPM compass with annotation (top view) en.svg, top view File:FPM compass with annotation (bottom side) en.svg, bottom side File:Brunton Axis.jpg, Brunton Axis with simultaneous strike and dip and trend and plunge measurements The concept of modern geological compass was developed by Eberhard Clar of the
University of Vienna The University of Vienna (german: Universität Wien) is a public research university located in Vienna, Austria. It was founded by Duke Rudolph IV in 1365 and is the oldest university in the German-speaking world. With its long and rich histor ...
during his work as structural geologist, which he published it in 1954. An advantage of his concept is that strike and dip is measured in one step, using the vertical circle for dip angle and the compass for the strike direction. The first implementation was by the VEB Freiberger Präzisionsmechanik in
Freiberg Freiberg is a university and former mining town in Saxony, Germany. It is a so-called ''Große Kreisstadt'' (large county town) and the administrative centre of Mittelsachsen district. Its historic town centre has been placed under heritage c ...
,
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
. The details of the design were made in a close cooperation with the
Freiberg University of Mining and Technology The Technische Universität Bergakademie Freiberg (abbreviation: TU Bergakademie Freiberg, TUBAF) is a public university of technology with currently 3655 students in the city of Freiberg, Saxony, Germany. Its focus is on exploration, mining & ...
. In 2016 Brunton Inc. introduced the Axis Pocket Transit which, for the first time, offered simultaneous measurements of both strike and dip and trend and plunge in a variety of configurations. It featured an unconventional lid design that swung a full 360 degrees in both directions and two axes that allow precise measurement of vertical and horizontal angles on all configurations of bedding surfaces. Stratum compass-clar hg.jpg, Breithaupt geological compass COCLA with damping of the oscillations of the magnetic needle Brunton.JPG,
Brunton Brunton may refer to: Places *Brunton, Northumberland, England (near Alnwick) *Low Brunton, Northumberland, England (near Hexham) *Brunton, Wiltshire, England *Brunton Memorial Ground, Radlett, Hertfordshire, England *Brunton, Fife, Scotland; a Lis ...
Pocket Transit, it combines the features of classic and modern geological compasses Freiberger kompass.jpg, FPM Gefuegekompass-klar hg.jpg, Chinese geological compass Geological compass ГГК.JPG, Russian geological compass


Usage

Geological compasses are distinctive because of the anti-clockwise direction of the numbers on the compass dial. This is because the compass is used to determine dip and dip-direction of surfaces (foliations), and plunge and plunge-direction of lines (lineations). To use the compass one aligns the lid of the compass with the orientation of the surface to be measured (to obtain dip and dip direction), or the edge of the lid of the compass with the orientation of the line (to obtain plunge and plunge direction). The compass must be twisted so that the base of the compass becomes horizontal, as accomplished using the spirit level incorporated in it. The needle of the compass is then freed by using the side button, and allowed to spin until the damping action slows its movement, and then stabilises. The side button is released and the needle is then firmly held in place, allowing the user thereafter to conveniently read the orientation measured. One first reads the scale that shows the angle subtended by the lid of the compass, and then depending on the colour shown (red or black) the end of the compass needle with the corresponding colour. Data are then recorded as (for example) 25°->333° (dip and dip-direction) or (plunge and plunge-direction). This compass has the most use by structural geologists, measuring
foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...
and lineation in
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
s, or faults and
joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...
s in mining areas.


Digital compasses

With the advent of the smartphone, geological compass programs based on the 3-axis
teslameter A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, on ...
and the 3-axis
accelerometer An accelerometer is a tool that measures proper acceleration. Proper acceleration is the acceleration (the rate of change of velocity) of a body in its own instantaneous rest frame; this is different from coordinate acceleration, which is accele ...
have also begun to appear. These compass programs use vector algebra to compute plane and lineation orientations from the accelerometer and
magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
data, and permit rapid collection of many measurements. However, some problems are potentially present. Measurements made by smartphone geological compasses can potentially be susceptible to noise, mainly due to vibration or rapid hand movement. Users of a smartphone compass should carefully calibrate their devices and run several tests against traditional magnetic compasses in order to understand the limitations of their chosen program. With traditional compasses there is no record of error caused by poor damping or operator movement. This limitation is removed by use of a digital compass, though these may be more error prone because of the sensitivity of the accelerometer, which programs use to determine vertical and horizontal. Therefore, professional use of a digital geological compass requires the recoding of variance in individual measurements. There is no data that suggests digital compasses are subject to any measurable form of magnetic disturbance. Modern remote sensing techniques as LiDAR and photogrammetry allow to obtain accurate and dense 3D point clouds. These point clouds allow the measurement of orientations of planar surfaces. Jordá et al. performed a comparison of the orientations of discontinuities measured by means of classical geological compass and a photogrammetric 3D point cloud demonstrating that remote sensing field discontinuity collection provides a reliable alternative to the use of geological compass.


References

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External links


Breithaupt Precision Instruments Geological Compass

Freiberg Precision Instruments (FPM) Geological Compass
{{Webarchive, url=https://web.archive.org/web/20150214164326/http://www.fpm.de/lang_en/index.php?c=1&s=geocompmirror , date=2015-02-14
Brunton Geological Compass
Geological tools