GSN SunShot Procedures Using Internet Solar Position Calculator Purpose: Determine astronomical (true) north for instrument orientation by observing the Sun s azimuth using a Theodolite and Sun Position Table. Tools Required: - Theodolite w/tripod. - GPS clock and/or handheld GPS receiver. - Straight edge, square, and chalk line marker. - The Sun. - Protractor - Access to an Internet based solar position calculator WARNING: MAY CAUSE BLINDNESS. This procedure, if performed incorrectly, may cause instant and permanent blindness. Always use Sun filter when looking at the Sun through the theodolite telescope. Solar Calculator: The below US Naval Observatory solar calculator (preferred) is at http://aa.usno.navy.mil/data/docs/altaz.php. It puts out data for a whole day in a tabular format in time increments that you choose. It is accurate to 6 minutes or 1/10 th of a degree. Filling in the form is self-explanatory. 1
The advantage of the USNO calculator is that you have a whole days worth of azimuth data for the sun. If you don t have site access to the Internet, you simply use your table that you prepared in advance to pick a time of convenience to take your sun shot. Another advantage using this table and the theodolite is that you are taking the observed azimuth and rotating to true north using the theodolite and marking your line. This is more accurate than marking a shadow and rotating to true north using a protractor Below is a table generated by the USNO solar calculator in ten minute intervals. Another good solar calculator can be found at http://www.srrb.noaa.gov/highlights/sunrise/azel.html 2
The NOAA solar calculator is the most accurate, but it does not generate a table. It does one calculation at a time. You could enter multiple times and generate your own table of sorts if you want to use this calculator. Time Zone: In order for the solar position calculator to work correctly, you must enter the correct time zone on the form. It s not as easy as you would think. Sometimes the daylight savings time thing bites you, or sometimes the obscure pacific atoll get s you. You ll know if it s wrong, if you find that your sun elevation from the theodolite looks to be an hour off from your table. The elevation is the key. Theodolite Set-up: A. We have 3 different models of theodolites. Become familiar with your theodolite s features and controls by using the respective theodolite manual. B. If you have a magnetic compass and know the approximate declination, use it to determine where to set up the tripod. This is just a crude method to place your tripod in a location from which you will be making your true north marking. C. Level up the tripod and theodolite using the instructions that came with the theodolite. Course leveling can be done with the bubble levels and finer adjustments can be made with the optical plumb bob telescope or the internal electronic tilt meter function, if available. The theodolites have a turning procedure to verify that they are level. It will beep at you if you are out of range. Read the manual. Level the theodolite well. If the theodolite is not level you will introduce azimuth errors. D. WARNING: Do not look at the Sun through the theodolite telescope without the filter. Blindness will occur. Install the Sun filter and right angle eyepiece. In the case of the Topcon theodolite, don t use or install the right angled eyepiece. You need tools to install the right angle eyepiece and it s a pain in the butt. E. Alternatively, you can use the theodolite without the filter, provided you use a dark colored piece of paper to project and focus the image of the sun onto it. In this method you are not looking through the telescope, but at the image projected on the paper. This method is useful if the Sun is at a very high angle making it hard to observe through the telescope by eye and with the filter in place (this is especially true of the Topcon theodolite). F. Practice a bit with the horizontal and vertical controls by locking and unlocking the fine movement knobs. Try finding the Sun. Once you have found the Sun, observe that the Sun is traveling across the sky relatively quickly. Hint: If you preset the vertical elevation using your table and just ahead of the current time, you ll find it more quickly by turning the theodolite horizontally only. 3
Procedure: A. Now that you ve set-up and leveled the theodolite, had some practice finding the sun, and are familiar with the Sun table, you now need to be sure you are familiar with your theodolite s Zero Set or just 0 Set feature. When you find the Sun at your appointed time from your table, we will be pressing this button. The 0 set is just as it says. It zeroes out the display to zero degrees. B. A second feature of your theodolite that you need to learn is the R/L feature. This is the Right to Left or Left to Right display reading. This means that in your table, the Sun s azimuth is reported as East of North (clockwise). So, if your theodolite is set to display degrees Right to Left, the display from the 0 set will increment from 0 degrees to 360 counterclockwise. Let s say that your table says that the Sun is at an azimuth of 108.1 degrees at the appointed time. If you use the R/L feature, you would rotate the theodolite 108 degrees, 6 minutes counterclockwise. Without using the R/L feature you would have to manually calculate the counterclockwise turn from 360 degrees, or 360 108.1 = 251.9 or 251 degrees and 54 minutes. I think you get the idea. You don t want read backwards from 360. C. Select a time of day where the sun s elevation is less than 45 degrees up from the horizon. Look at your table. If the sun is lower on the horizon, your azimuth reading will be more accurate and it will be more comfortable to use the telescope. Higher angles to the sun will be less accurate, especially if the theodolite is not leveled well. Avoid the times one hour before and after celestial noon when the sun is nearly overhead. D. Ok, time to find the Sun for real. Look at your table. Pick the time from your table and track the Sun with the crosshairs at the center of the Sun and not the trailing edge. Have a partner give you a countdown to your selected time using the time from a GPS receiver set to UTC. When the appointed time arrives, press your 0 Set button. E. From your 0 set position, make sure you have your R/L ( ) feature on. Now rotate counterclockwise to the number of degrees and 10 th s of a degree as shown on your table. Your view through the telescope is now pointing true north, or conversely, by flipping the telescope, you re pointing true south. Marking the Ground: A. Now that true north is known, it s time to mark the ground. This is best done with a partner. One person looking through the telescope and the other person to receive commands to place the marks in order to draw a straight line. B. Let s say that you are standing above a surface vault or at a place you want to set a reference sensor. Just tilt the telescope up or down at a convenient point on the surface. Direct your partner to that spot and have him mark it. Then tilt the telescope further to mark a second point and mark it. Now all you 4
have to do is connect the dots using a straight edge or chalk line. To make your mark permanent, paint the line using parallel strips of masking tape. Some FE s have marked their points with a cold chisel or drilled reference holes in the concrete in the event the painted line should fade over time. C. Let s try this scenario; What if you are at the mouth of a short tunnel and that the shot into the tunnel is not north? Let s say that in the previous example, we had to turn our theodolite 108.1 degrees to get to true north, but if we are only able to turn the theodolite 89.9 degrees to shoot straight down the tunnel to the pier, what do we do? Simple, we shoot the 89.9 degrees down to the pier and mark it as such. Then from the 89.9 degree mark we have to translate the balance of the 108.1 degrees (18.2) to the pier using a good protractor or you can set up the theodolite above the 89.9 degree mark on the pier and shoot the line that way. Moving and setting up the theodolite again is more work and is not worth the effort if you are only translating one line in a straight tunnel. In this example you will have to translate 108.1 89.9 = 18.2 degrees with the protractor. What do we do in the event the mouth of the tunnel is 120.1 down to the pier? Again, simply take 120.1 108.1 = 12 degrees. Mark your shot as 120.1 and use the protractor to translate the 12 degree difference. Example Next Page 5
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