When using GNSS equipment (base & rover), True North is obvious.?ÿ But how do you align and set a total station (i.e. old Leica TCRA1103 Plus) to True North?
I can use a cell phone and "roughly" align the total station using that using either True North or Magnetic North, but what is the proper way to align the station exactly to True North best as possible please?
Thanks guys!
Humm, that is a somewhat broad question. Can you explain in a little more detail what you are attempting to accomplish?
But how do you align and set a total station to True North?
In order of increasing precision:
1. Assume north (or some other azimuth) in some convenient direction, such as up a street.
2. Use a compass, such as the one on your phone, to determine a direction.
3. Find a couple monuments for which the bearing between them is known and use them as a basis of bearings. In my experience, this is the most common by far.
4. Observe the sun. With an ephemeris, your latitude and longitude scaled from a map, and the precise time of observation, the azimuth to the sun may be calculated. NOTE: DO NOT LOOK AT THE SUN THROUGH THE SCOPE. YOU WILL BE BLINDED!!!! Also, do not aim the total station at the sun without the proper objective filter, you may burn out certain components.
5.Observe Polaris, the north star. Again, you will need an ephemeris, your location, and precise timing because Polaris is not exactly true north. You have to do this in the dark on a clear night, but at least the risk of blinding yourself is less.
This all how it is done without using GNSS. Nowadays, in practice, I always do at least enough GNSS to establish a basis of bearings.
> But how do you align and set a total station (i.e. old Leica TCRA1103 Plus) to True North?
Assuming by 'True North' you mean Astronomic not Magnetic...
By direct stellar observation.
Which hemisphere are you in?
Yeah I would go with star shot or sun shot as well if by "true" north you mean astronomic. Depending on your latitude you might need to use a right-angle eyepiece for a star shot.
You could get geodetic north from GNSS and convert using DEFLEC18 to get the LaPlace correction at your location.
Some NGS monuments have azimuth marks, or are intervisible, and astronomic north can be computed from a setup/backsight between two of those.
In some areas there are mountain peaks and building/antenna spires that have NGS geodetic coordinates. In theory one could perform a resection to determine position and from there "true" north, as long as you have three points plus decent geometry. Long shot (literally) and not super accurate.
> But how do you align and set a total station (i.e. old Leica TCRA1103 Plus) to True North?
Assuming by 'True North' you mean Astronomic not Magnetic...
By direct stellar observation.
Which hemisphere are you in?
I guess I should have "known" to be a little more clearer... with which I have now "learned" from these wonderful answers!
Yes, I was looking for True North in terms of Geodetic North in relation to the ellipsoid. Northern hemisphere.
But obviously from the answers referring to the astromical observations method, great to understand this along with those other methods!
Thank you Norman and Rover for the concise help! ; )
In a place or two in the above discussion there was confusion between geodetic north (which I would equate true north to) and astro north.
They differ by the Laplace correction which was also mentioned. That is because of deflection of the vertical caused by variations in gravity around your location that tilt your apparent vertical.
I always used two methods, solar observation or occupying existing government control monuments.
Frankly for day to day "true north" solar observations the Laplace corrections were irrelevant.
I do know that using NAD27 monuments worked better since I've retraced my control networks with GPS.
Often I see less than 10 seconds between the two methods (NGS monuments and GPS), more like 1/2 minute for the solar observations.
Adjusting for Laplace correction of 3 seconds is swamped by errors in the solar observations.
Of course it depends on how much time you wish to invest in a solar, how many observations, how accurate you can get time (one second of time translates to about 15 seconds of arc,,,,I think that's right), how good your instrument is.
Today it's only GPS for me, I've done the other methods way more than most for real work and I have no interest in doing it again.
I only did star shots for curiosity, they aren't practical when you're working.
Amazing how frequently the phrase "True North" is used and yet it is almost a non existent concept.
I've done astronomical observations just for my own education. If you're just doing it out of curiosity, you'll want to keep the cost down. The procedure is spelled out in some elementary surveying books, and it covers perhaps 20 pages.
Since you're in the United States, you can skip most of the difficult calculations because the US Naval Observatory does all the difficult math for you. What you want is Topocentric Positions of Major Solar System Objects and Bright Stars
For Position Type, select "Topocentric Zenith Distance and Azimuth". For Celestial Object of Interest select Polaris, a planet, or a bright star. Polaris will probably give the best results because it moves slowly across the telescope field, but depending on where you are, it may be too high in the sky to see without an angle adapter on your eyepiece, which can be expensive. Also, it might be blocked by a tree or something. So pick some object other than the Sun that you can see at the time and place in question. A really bright star or planet is great because you can be pretty sure you've got the right one while looking through the telescope. The moon is super easy to see, but needs additional calculation to allow for the fact that you're observing the edge rather than the center.
Also, depending on your instrument, it may or may not be great at letting you see the graticle and the star at the same time. If your instrument is marginal at this, a really bright star or planet will help.
The next thing you need is a real stopwatch, or a helper. Observe an accurate time source, such as your cell phone while it's outside and receiving GPS, or WWV. Start the stopwatch on some easy-to-work-with time, such as the top of the hour or a 5 minute mark. Try not to let too much time pass between when you start the stopwatch and when you make the observations, because some digital stopwatches reduce the resolution of the display when the duration gets larger. For example, mine displays to the hundredth of a second up to 30 minutes, and then reduces the precision to the nearest second. For best results, you want to be correct to a few tenths of a second.
The reason you need a real stopwatch rather than a cell phone stopwatch is you can't control the cell phone while you're looking through the telescope, but with the real stopwatch you can press the right button without looking at the stopwatch. If you have a helper, the helper could control the cell phone but there's a lag between when you say "mark" and the helper stops the stopwatch.
After you've done all the steps associated with a traditional astronomical star observation, you are left with astronomical north. You can convert this using values for local vertical to geodetic north, which is what you get from GNSS.
@mightymoe I decided to check how much the time matters by using the US Naval Observatory web page I mentioned in another post in this thread. The azimuth of a star changes fastest when it is near its zenith or nadir. For Vermont and Polaris this works out to about 0.3 arcsecond of azimuth per second of time. For a bright star near the equator, Betelgeuse, it works out to 10 arcsecond of azimuth per second of time.
For the sun it's basically 24 hours vs 360 degrees of arc. So divide 360/24=15, obviously that's a more or less calculation, depends on time of day, latitude of observation, ect. But if you've done this enough you know how fast it's moving across the sky, you've tracked it many, many times.
Amazing how frequently the phrase "True North" is used and yet it is almost a non existent concept.
Bingo.
I've laid out true north for more cell towers than I can remember and while a base and rover were always at my finger tips, they were not the correct tool to get the job done. The best, easiest and most accurate way to do this is through either solar or star shots.
I carried a laptop with me loaded with C & G software. C & G had some great routines to calculate the solar shot azimuths quickly and easily with the proper inputs. Aside from having a good solar filter for your gun lense and a precise timer, all that is needed is a little training if you have not done it before.
Once you have determined your initial azimuth and have done the observations from a hard point, set a second hard point to fix your azimuth on the ground and from there you can transfer it anywhere you want and be able to set a baseline on true north.
@chris-bouffard Really? Then please tell me what is True North? I don't know how to orient myself in regards to True North.
Amazing how frequently the phrase "True North" is used and yet it is almost a non existent concept.
There are many "True"s...what is YOUR Truth?
😉
When using GNSS equipment (base & rover), True North is obvious. But how do you align and set a total station (i.e. old Leica TCRA1103 Plus) to True North?
I can use a cell phone and "roughly" align the total station using that using either True North or Magnetic North, but what is the proper way to align the station exactly to True North best as possible please?
Thanks guys!
Total stations don't know north, they only know coordinates.
Or at least that's how I look at it.
But if you've done this enough you know how fast it's moving across the sky, you've tracked it many, many times.
Yeah, we did a solar observation once in school (still haven't had to do one in the working world) and I was surprised how fast the sun actually moves. In hindsight it made some sense though because it's what... about a thousand miles an hour at the equator.
I was surprised how fast the sun actually moves.
Hmmm? Does the sun move? That really complicates things! ; )