Parsing value

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The par value or the specification of a dragonfly is the angle of inclination in arc seconds that allows the dragonfly bubble to move on by exactly 1 graduation of the glass scale. The unit of this line scale is usually 2 mm (with older dragonflies 2.256 mm, a Parisian line ) and is also called pars . The smaller the pars value, the more sensitive and precise the dragonfly is.

In the foot of theodolites built and total stations Alhidadenlibelle have Parswerte to 20 ", with simple Bautheodoliten about 30". As can be read to about 0.2 pars, that succeeds Leveling a theodolite to about ± 5 ". The rest remains as a so-called standing axis error , but is only noticeable in the case of steep visions .

For geodetic circular vials , the pars values ​​are between 2 ' and about 10' per 2 mm. So they are only suitable for a rough leveling, as z. B. with an automatic leveling instrument or a gravimeter is sufficient.
In the case of panorama and photo tripods , the pars value can be up to a few degrees , but because of the ring-shaped mark, it can be adjusted to around 1 tenth of this.

The dragonflies are most sensitive for astrogeodetic and astronomical measurements. They are designed as hanging or rider vials and are attached directly to the tilt axis . The par values ​​are between 0.5 ″ and 5–10 ″, depending on the measuring instrument.

The pars values ​​are determined with a level tester . It is a heavy, elongated metal frame, one foot of which is designed as a measuring spindle and a reading circle. The dragonfly or the whole theodolite is placed on the frame and the spindle is turned by constant amounts (for alidaden dragonflies, for example, by 30 ″) until the dragonfly bubble has moved from far left to right. It is best to enter the read pars in a diagram and draw a compensating middle straight line through the diagonally distributed points by eye .

If the glass body of a tubular vial is not curved quite evenly, the pars value of its left and right edge can be slightly different. This is why it is important for precision measurements to keep the game point of the dragonfly bubble close to the center. The temperature of the vitreous can also play a role.