Double pentagon prism
A double pentagon prism ( , also (double) -angle prism ) is a simple but important optical instrument in geodesy to stake out right angles or to check right angles, to bend a point onto a straight line (i.e. the perpendicular from the point to this Straight) and to align yourself in a straight line .
Angle prisms are available in different designs. The most common are the double pentagon prism and the cross sight. The double pentagon prism, also double pentagon or double pentaprism, is the easiest to use. Two pentagonal glass prisms stand on top of each other in the double pentagon prism. The angle prisms have a hole or a notch at the lower end of the handle in which a plumb line can be inserted. The location of the angled prism, which is relevant in terms of measurement technology (perpendicular under the angled prism on the ground), can thus be easily determined.
Construction
A prism is a translucent glass body whose (flat) boundary surfaces are ground and sometimes also mirrored. A prism is used to deflect light rays through light reflection or refraction.
The double pentagon prism has two opposing prism mirrors and a window through which all three can be viewed at the same time. If you are roughly on the straight line between two alignment rods , you can observe the left rod in one prism mirror and the right rod in the other prism mirror. When both images are congruent, the angle prism is exactly in the straight connecting line . If a third rod is now seen in the see-through window in such a way that it also coincides with the other two rods, it is exactly perpendicular to the alignment line, the base of which is in the prism.
use
Angling up (of a point):
The point to be angled is sighted through the see-through window and, as shown in the figure for the beam path, is brought into congruence with both observed points of the measurement line by aligning the angle prism. Then the angled prism is aligned with the measurement line and the plumb line you are looking for goes straight through its location: the distance from the location to the angled point runs at right angles to the measurement line.
Set down (at a right angle):
A point to be set is determined by making the angular prism perpendicular to a specified point on the measurement line.
Align (in the measurement line):
Searched points, which should lie on the measurement line, are determined by dropping the angle prism. This procedure is used for alignment when the observer does not have a second employee available.
Today's meaning
The angle prism is mainly used in the orthogonal method. Since this has largely been replaced in practice by the polar method, angle prisms are no longer used as often as they used to be, but are occasionally still in use , for example in cadastral surveys .
Web links
- Excavation dictionary: angle mirror