Solar azimuth

from Wikipedia, the free encyclopedia

The sun azimuth is an astro-geodetic determination of the direction using the sun , which is carried out on a survey or polygon point to another measuring point. Such azimuths are used for a quick but sufficiently precise alignment of surveying networks to astronomical north or grid north .

Choice of target point

The target point can be a neighboring polygon point or a target mark , but it is usually a long-distance or high target (church tower, chimney, high mast) or another survey point at a greater distance; Distances of around 1 km are useful. For the approximate orientation of buildings, antennas or astronomical passage instruments , nearby auxiliary points or mires are also used as target points.

The measurement of a solar azimuth is relatively easy for a geodesist , civil engineer or military technician , because apart from a theodolite and the surveying tripod, no other aids are required . The times of the day, measured to around 1 second due to the earth's rotation, can now be easily read (or even stopped more precisely ) on a mobile phone , and the projection method is recommended instead of the filter used for solar observations .

Advantages of the projection method

The sun is projected onto a piece of white paper by adjusting the telescope eyepiece a little extra-focal (about a quarter turn counterclockwise). A convenient projection distance is around 15–20 cm, which makes the image of the sun a few centimeters large. With such a bright picture, the lines of the cross hairs on the paper are also visible (if not, you can shade annoying daylight a bit).

This method has a number of advantages over looking through the eyepiece (which is still dangerous even with a filter !):

  1. It is completely safe, you only have to be careful when looking for the sun - the easiest way is to align the telescope of the theodolite with the help of its shadow and not even look in its direction (the viewfinder can also be dangerous).
  2. It is easier than the direct measurement at a mostly rather steep angle of elevation .
  3. It allows a comfortable measurement of both edges of the sun within a short time, which (after averaging the two directions and times) results in the center of the sun in an almost perfect way.
  4. The coordinates of the center of the sun can be calculated relatively easily with small programs, as they do not have to be more precise than about 0.001 ° (the easily achievable measurement accuracy).
  5. Since otherwise (without such a PC program) an astronomical yearbook and various time corrections would be required, the use of the Nautical Almanac is recommended . This yearbook, designed for navigation , is designed precisely to the accuracy (0.1 ′) that can be achieved with the projection method.

The measuring process itself

  1. Aiming at the other survey point or the Mire (for setting up an antenna, a sundial , etc., it can also be the edge of a house or the like).
  2. Focus on this target and read the horizontal angle
  3. Projection of the sun, aiming at the left edge of the sun
    • When the sun runs exactly into the vertical thread, the helper reads the clock (or cell phone). A good command for this is: "... Aaah ... TOP!" Without a helper you can count 1–2 seconds (twenty-one - twenty-two) and these 1 or 2 seconds from the Subtract clock reading .
    • Reading of the direction of the sun (the telescope can be tilted down to a more comfortable zenith distance )
    • The same is repeated with the right edge of the sun (the order of the directions is left to the observer)
  4. Finally, the terrestrial target again.

With this method, which the geodesist calls a “ half-sentence ” (see below), you hardly need 2 minutes with a little practice. It easily achieves an accuracy of around 0.01 °, which is sufficient for many simple measurements (e.g. of a small piece of land or an antenna).

Accuracy and measuring process II for higher demands

If, on the other hand, you want to achieve up to 0.001 ° (a few arc seconds ), you repeat the above measuring process in the other circular position - i.e. H. you break through the telescope - and measure the above diagram symmetrically, ie "from bottom to top". The angles read on the theodolite must differ by almost exactly 180 ° for the terrestrial target ; for the sun there can be differences of up to a few degrees. Because the star of the day moves westwards by about 15 ° in the sky every hour.

If you want to achieve an accuracy better than about ± 0.005 ° (a few milligons or about ± 30 ") , for example for the precise orientation of a surveying network , for checking a stakeout or for a Laplace azimuth , you have to have 4–5 additional aspects note:

  • exact centering over the measuring point
  • Careful leveling of the theodolite (it is best to check it in a second "circular walk" and during this time shield the dragonfly bubble, which often "drifts away", from too much solar radiation )
  • Check the above-mentioned 180 ° target differences immediately (if necessary, aim at the point again immediately) and
  • Pay attention to the focus (so-called " dance rehearsal " against eye parallax)
  • Inquire about the time correction dUT1 on the Internet and attach it to the measured times (if more than 0.3 s). The perpendicular deviation, however, can i. d. R. remain out of consideration.

The evaluation can take place with navigation or PC programs; a sample can be found on the website below .
"Beginners" achieve accuracies of around 20 "with the II. Measuring scheme (2 half-sentences = 1" sentence "of the angle measurement) and around 10" after averaging a few sentences. If you have already taken the measurement a few times, you will reach 3–5 ".

The most accurate: polaris azimuths

In principle, the same process is also possible with the Polarstern , with which one can reach up to 1 " with a second theodolite or modern tachymeter . With a larger universal instrument one can even advance into the range of 0.1". From about 3 cm lens aperture can Polaris even during the day measure if it can be estimated to some degree his place at twilight or daytime sky.

Such precise polarization azimuths were the basis for the national surveying networks until a few decades ago and are still carried out today - for example for Laplace points and when new networks are set up. The ideal but still transportable theodolites for accuracies below 3 "are the Swiss instruments DKM-3 from Kern and the T3 or T4 from Wild-Heerbrugg , which are still relatively widespread in Europe . For solar azimuths, however, they bring little (apart from a few kilograms payload) a simple building theodolite or a tried and tested second theodolite is sufficient for them.Of course, a somewhat faster measurement is possible with digital tachymeter instruments, but one should not do without the directional controls.

Web links