Is there a way to use a surveyor’s transit to get the azimuth of the sun?
The sun’s usually much too high in the sky to point my transit at it, and I don’t have any filter or other means to reduce the light. But I thought there should be some technique to scatter some sunlight with a perpendicular thread or something.
Or, how else can you easily and accurately measure the sun’s azimuth while surveying a site?
Wouldn’t the sun’s shadow of a vetical post mounted at the center of a compass rose oriented to true north give the azimuth angle?
With telescopes, you use the shadow of the scope on the ground to aim directly at the sun, because you can’t look through the finder safely at the sun. Basically, you move the scope around until it’s shadow is circular, and you know it’s aimed at the sun.
I’m not sure if the same technique would work with a surveyor’s transit.
>Wouldn’t the sun’s shadow of a vetical post mounted at the center of a compass rose oriented to true north give the azimuth angle?
Yes, if you built such a thing and oriented it. But I want to do this so I can find out true north (using a table later). Orienting the rose requires that I already have my problem solved.
>Basically, you move the scope around until it’s shadow is circular, and you know it’s aimed at the sun. I’m not sure if the same technique would work with a surveyor’s transit.
No, they don’t aim high enough. They are for looking horizontally, or at least roughly so, and can’t aim higher than maybe 20 or 30 degrees.
I’m not sure I follow you, here. Any method for determing azimuth, using any instrument at all, will require you to already know the direction of north. You could get relative azimuths easily enough, but you could do that with a shadow stick, too.
You beat me to it. The sun’s azimuth angle is unrelated to true north. That you find with a well compensated magnetic compass and a magnetic variation chart for your location. Or you can do it at night if you know your universal time and have a good star almanac.
>Or you can do it at night if you know your universal time and have a good star almanac.
Or during the day. You only need one star, right?
There are plenty of ways to find out what the sun’s azimuth ought to be for a given location and time, including after the fact. For example the Navy has a site that gives you a table with azimuth and altitude by the minute for your location and date.
I am doing this so I can figure out how my site is oriented. So I would record the sun’s azimuth in my local (as yet arbitrary and unknown) coordinate system, and the time. Later I would find out what the true azimuth of the sun would have been then, which gives me the difference between my local coordinate system and the real world, thus translating my other measurements into real azimuths.
I could also use a sightline to another known object. Last Friday I was using a cell tower and a smokestack on the other side of the valley. But getting access to those things to determine their location precisely enough can be a problem (turned out the cell tower had a fence around its base). And some sites deep in the woods don’t have a sightline to any suitable objects. But the sun is often available, at least at some time. THAT’s what I want to use it for.
Magnetic compasses are another option (I have one), but I am exploring ruins that often have big iron in them, and I don’t know how reliable magnetic north is on such a spot. Besides, I already have the transit, which makes very accurate angular measurements, so it’s natural to try to apply it to this. The compass drifts around a few degrees, not inspiring in the world of mapmaking.
So your real aim is to find true north so that you can make a proper map?
OK, this is pretty much what I thought. There’s still no reason you couldn’t do it with a shadow stick, though. All you’d need is something you know to be vertical and preferably somewhat tall (either of the landmarks you mention, or a plumb line will do). It may seem at first that the transit would be more precise, but in either case, the limiting factor is likely to be the half-degree angular size of the Sun. Were you to do the equivalent experiment at night, you could use a star, which would solve the angular-size problem, and you could pick one near the horizon, so as to be able to use your transit. It’d also make the sighting itself easier, since there’s no risk in looking directly at a star.
1- Buy theodlite
2- Set pin where you can do the most good at the site.
3- Use a GPS unit to get an good aggregate lat-long of point #1
4- Sigh to some open place as far as you can that can be driven to.
5- Set pin in #2 position.
6- During a dark time, send tripod and blinking light with helper to point 2 and aim it towards point #1.
7- Use gun to get a good angle and then establish a decent backsite at the local position.
8- Do math and kill anyone who messes with point #1.
5a- Get good GPS on point #2.