This is probably the first time I??ve been asked to do this but someone wants true north laid out for a metrology tower. ?ÿI??ve got a Trimble R10 running Survey Pro. ?ÿI??m working in Illinois State plane zone west. ?ÿI??m not sure if the info is available in the data collector or not. ?ÿI thought there was a program for it on the NGS site but I didn??t see one. ?ÿAny help is appreciated. ?ÿ
That term is too ambiguous - it has a number of definitions. You need to find out what the REALLY need. It generally, but not always, refers to astronomic north.
I think I found a program called NCAT on the NGS website that gives the convergence angle.
GPS north, astronomic north or "true north" are so close I doubt the difference between the two would matter much for any practical purposes. What I've done in the past is to locate the base as close to the tower site as possible and use a local ground project, no standard projections, no state plane, no mapping angle, no nuthin. "No project/no datum'. Give the base a value of 5000,5000, whatever. The bearings will be determined from your 'here' position relative to GPS north. The difference of 10' one way or another with an autonomous position will make no distinguishable difference. If your forced to use state plane I suppose you could get an opus solution and determine from the report the mapping angle to use to correct from your grid bearings to "true north" for that specific location.?ÿ
Easy peasy.
Sin of lat times delta long for TM. Sin of lat times lat sub o for lambert.
The other option is work in some basic flavor geographic system.?ÿ
Butchered that lambert formula but i cant edit...
The first step is to find out how close it needs to be to "true north" ? (at what confidence level?)
If that was metrology, it might need to be tight.?ÿ If that was intended to say meteorology, probably much less so.
How will the line or end points be marked?
Then nail down which kind of north they want.?ÿ I'm betting they never heard of State Plane, and most likely want geodetic, but if they are sighting things from the tower it could be astronomical.
Make a plan to get better than the required tolerance.
GNSS will require a fairly long baseline to get decent azimuth accuracy.?ÿ If the GNSS points are within 2 cm of being on a true north line and you want 10 arc seconds, then you need over a quarter mile spacing (412 meters).
My method of choice would be to get a rough azimuth with GNSS and then do a Polaris sight for final marking.?ÿ The total station would let you set points much more accurately with a shorter baseline than GNSS. at 42 degree latitude you can probably do the sight without a right-angle eyepiece but you will be craning your neck at an odd angle.?ÿ Right now Polaris approaches east elongation in the evening (central US) so time isn't particularly critical, and the star azimuth will be somewhere around 0d 40' (must find its azimuth, not 90-declination).?ÿ That gets you astro north.?ÿ If they want geodetic north then use the Laplace correction of a few arc seconds as found by NGS tools.?ÿ?ÿ You should easily be within 10 arc seconds with this method.
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I might add that I just recently did a similar task for a local astronomy club who are putting in a newer telescope and needed to check the alignment of the telescope base to be sure its adjustment range would let them do the final positioning.?ÿ My "baseline" was limited to about 11 feet, so fortunately the mount's adjustment range was large.
It for the foundation of the tower. ?ÿI already know based on the past history of this project no one will be able to answer any of my questions. ?ÿI set a lath at the tower center and then a lath at a 50 foot offset. ?ÿI??m not too concerned about accuracy. ?ÿ
It has been awhile since I have used Survey Pro, but I believe when you calculate a point by bearing you can select what it referenced as North. When in the line by bearing menu, I believe that you can change grid north to true north.?ÿ?ÿ
Store a point at the tower, then key in a point by lat long with the same longitude and add 1 minute to the latitude. Stake a line from the tower to the new point. Problem solved. You could inverse the two points for a bearing and that would give you the theta angle if you are curious.
James
I used JaRo's method, which is of course brilliant in its simplicity.?ÿ I'm west of the principal meridian in Illinois and after I used this method an inverse gave a bearing of North 0 degrees, 20', 19" East.?ÿ?ÿ I looked up a NGS station that is a few miles away and it listed the convergence angle of?ÿ negative 0-23-33.?ÿ I'm pretty confident in the end it makes little difference.?ÿ Its a foundation that is about 20 feet square and the difference between grid north, magnetic north, and geodetic north would probably make a 0.10' or less on the location of the corners of the foundation.?ÿ This is for a met tower, as in recording the weather.?ÿ They may have me back to align the instruments on the tower but I doubt it.
- Many years ago I had a couple of radio masts to align to a high precision. They were out on some nearshore islands, beyond mud flats. I was picked up by a military landing craft from a nearby harbour and set off on what ought to have been a 10 minute journey. Landing craft had high sides so I couldn't see out. After about half an hour of cruising around I commented to the skipper that when we got there I would know where it was in the world to a few cm. "Oh, OK" he said, changed course and we were there in a couple of minutes!
- I never did find out what the masts were going to do, but the clue lay in the fact that all the clocks in the buildings on the island were several hours ahead of my watch.
- Didn't we have fun back in those far off days.
I set my system and datum as WGS84 and let my GPS do the rest as I?ÿam a flatlander as compared to most others here.
Then again, I rarely have to give a report that has to comply with state plane or any other projection.
Does the client actually know enough about all the different norths to know what they are asking for?
Also whats a metrology tower? Typo for meteorology? Just intrigued.
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No the client hasn't got a clue what they are asking for.?ÿ I just wanted to do it correct even though for a foundation nobody would be able to tell the difference.?ÿ It a tower for recording the weather.?ÿ Spelling and vocabulary aren't my strong points.
No the client hasn't got a clue what they are asking for...
Years ago (back in the chain & transit days) I was hired to lay out the construction of a large sundial at a local "institute of higher learning".?ÿ What was initially required was control for constructing the concrete foundation aligned with astronomical north.?ÿ ?ÿThe only glitch was there was a multi-story building north of the site and Polaris was obscured.?ÿ It didn't take too much "surveying" to give them what they wanted.?ÿ After they had the concrete poured we came back to "check" everything.?ÿ The concrete crew hadn't been real careful about the placement of the poured-in-place mounting bolts.?ÿ We took care of that.
After a few months they finally got the behemoth mounted with the help of a rubber tire picker.?ÿ It all looked good.?ÿ A few days later one of the supervising faculty staff (a Doctor in Philosophy) called and was concerned because the structure had been "checked" with several different compasses...and although each one varied slightly they all indicated that the sundial wasn't aligned with "north".?ÿ?ÿ
That was the day I realized "smart" people are harder to enlighten than "dumb" people.
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Things may not have changed as much as we might think. One thing on a met tower that needs to be aligned to true north is the anemometer. But this guidance from California in 2017 doesn't seem to be overly precise. See pages 10 - 12.
But if the foundation is right, then .....
because the structure had been "checked" with several different compasses...
I would think the proper check would be an accurate time telling device:
Walk out there at "High Noon"; if the shadow of the Gnomon, isn't hitting the XII on the Dial Plate on the Sundial; you probable should do a little adjusting.
Don't bother the surveyor; he has better things to worry about...?ÿ
