equal area stereonet with small circles showing consistent size. Small circles Angles are slightly distorted and make the circles appear as ellipses. The x-axis. This is a printable 2 degree equal angle (Wulff) stereonet in PDF format. Equal angle versus Equal area nets. Two projections used in structural geology. They are also used as map projections, and for maps of the sky in astronomy (or .
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The stereographic projection is a methodology used in structural geology and engineering to analyze orientation of lines and planes with respect to each other. The stereonets is a type of standardized mapping system that allows us to represent various angles in 3D dtereonet on a 1D paper. They are used for analysis of various field data such as bedding attitudes, planes, hinge lines and numerous other structures.
This is a very useful tool because it can reduce the workload by avoiding lengthy calculations. In structural geology, we use the bottom half or hemisphere of the spherical projection.
If you are a mineralogist, you will use the top half of the spherical projection for crystallographic analysis. The reasoning behind which hemisphere we used is more conceptual than anything. This will be explained in depth in a different article. What is important to someone who just started using steronets is to recognize that steronets represents half a sphere where the cross section has degrees.
Planes are lines are drawn on steronets as they intersect at the bottom of the sphere Figure 1. There are two widely used types and may be more of stereonets equzl structural geologists.
Stereographic projection for structural analysis
They are equal area stereonet and equal angle stereonets. The choice either should not affect the data analysis. The analysis and interpretation of data achieved through the use of either equal area of equal angle steronets should result in same conclusions.
Equql, the equal area steronets will reduce the area distortion. In other words, it is often used to analyze accuracy of data from several different regions of the same area. It is also useful in structure stereonet contouring. Hence, most educational institutions prefer equal area steronets for their students over the equal angle stereonets.
2. Stereonet — InnStereo 0 documentation
The equal angle stereonets are suitable for kinematic analysis. Esual other words, they provide the best projection for analyzing the direction and the vectors of structural forces. This is because the equal angle stereonets preserves the true relationships between stratigraphic and structural features.
It is the true North which is denoted by the azimuthal angle of degrees on the primitive. All strike angles are measured with respect to the true North. It is the outer most circle is the primitive.
It is at degrees from the center of the stereonet. Primitive circle is also a great circle but, it contains N, E, S and W directions at, and degrees intervals. A circle on the surface of the sphere made by the intersection with the spehere of a plane that passes through the center wtereonet the sphere.
The great circles run North-South longitudinal or up-down and bisect the sphere precisely. The great circle is divided in to degrees like degree protractor because maps are designed based on same azimuthal bearing directional vectors. If you have understand how 3D vectors work, this should be a no-brainer. A circle on the surface of a sphere made by the intersections of a plane that does etereonet pass through the center of the sphere.
Small circles run left-right latitudinal on the stereonets and are perpendicular to the great circles. On the animation above, I drew two vectors out of several which can be used to interpret a normal fault.
The red arrow is the displacement vector which is obtained by the horizontal and vertical displacement. The horizontal displacement is indicated with the brown arrow vertical displacement is NOT shown. The rake of the fault is between the left most edge of the footwall and the displacement vector red. The green arrow represents the rate of drop with respect to the original block.
We use slickensides to interpret the sense of motion in the field. For example, from intersection point 3 upwards towards NW direction of the great circle intersection of plane A. Plane B rake is downwards towards SE direction. The point 1 and 2 are best fit line points for the poles that lies about the center of the diagram.
It is measured on the great circle itself. A detailed diagram… Hand written sample. There are absolutely no differences between the interpretations made using manual drawing and software-based drawing of datasets.
In work environment, we usually use software to generate stereonets. The software often eliminates many user errors, produce much better quality steronets extremely detailed analysis of datasets and make it easier to share with other over electronic devices. For someone who is starting in geology or structural geology, it is highly recommended to use paper and pencil over software.
This will help you learn the fundamentals of stereographic projection. Typically university geology and engineering students are expected create stereonets by hand. Standing on the shoulders of giants.