Hi fellows,
We have some queries to answer some technical issues regarding solar observation to compute/obtain azimuth of an observed station and how convergence angle applied to the computation of grid azimuth.
1. What is the azimuth reference computed from a solar observation?
2. How to apply convergence angle when the observed stations is in the left or right in the central meridian and how it differ from each other?
3. What is the difference between true north, grid north and magnetic north?
4. What is the azimuth reference used in executing a traverse for a meridional block survey?( Meridional blocks are defined by each geographic coordinates and later converted to grid coordinate.)
Thank you fellows.
Replies
The Radio Time Signals had warning blips prior to every minute completion / start and also corrections to the time as blips after every minute + or - depending on the sound. The latter was known as a DUT1 correction, and as I understand it this was a correction for minor variances in the earth's rotation dynamics...I don't even know if these radio signals are still transmitted but I guess these are as even Mariners equipped with GPS, Loran C, etc..still have to be able to perform celestial navigation as a backup.
Yes Scott, counting the clicks was important & just before the full minute there wold be a pause and a slight change in tone to alert you to be ready. We would us the word "UP" to alert the I man to take his reading as "UP" is a seldomly used word. Fun times.
1. The azimuth reference computed from a solar observation is astronomic north.
We always used to scale the latitude and longitude from a topographic map. These days I would encourage the use of an inexpensive GNSS receiver that is likely to provide a more accurate set of geodetic coordinates than can be scaled from most maps. I would encourage a comparison of coordinates scaled from the map and coordinates computed by any available GNSS receiver.
2. I always draw a little graphic to illustrate the convergence angle. In the northern hemisphere as the position moves east of the central meridian grid north is to the right of geodetic north because of the convergence of the meridian. As the position moves west from the central meridian grid north is to the left of geodetic north. It has been a very long time since I worked in the southern hemisphere but I would turn my sketch over and relate what I was doing to geodetic south if working south of the equator. For those of you who cross the equator with your traverse lines I offer God's blessings on you and your family. The challenges you face will keep your mind active for a very long time.
3. True North? I try not to define it because I am not always sure what is meant by the term. But in general it is probably either astronomic north or geodetic north. The difference between the two is usually quite small and often buried in the noise of the measurements made to determine azimuth. There was a time when I knew a lot about this topic but have forgotten most of what I learned. The National Geodetic Survey has some excellent resources to help with computation of the difference between astronomic and geodetic north if this small difference seems important to anybody.
I noticed that Carlos offered some documents that may be very helpful.
3. Grid north? The lines that define north are parallel and typically referenced to geodetic north at the central meridian of the grid zone. In the case of a tangent plane survey grid north is typically referenced to the point where astronomic or geodetic north was determined.
3. Magnetic North? This is north as measured by a compass that has not been adjusted to orient the compass to some other azimuth base.
4. I will leave this to others. I learned how to do this many years ago and promptly decided there were better ways to earn a living as a surveyor. One of the unfortunate complications of this method is that all traverse lines that are not oriented north are arcs. With that I will let everybody who disagrees jump up and tell me how wrong I am. That is only one reason why I will leave the answer to others. I am too old to argue about it any more.
Phil Stevenson
Professional Dirt Surveyor (retired)
I remember counting the "ticks" on the radio atomic clock in order to make the observation at a certain time.
It's been 30+ years since I last did one, so I will defer to the instructions provided by others. I will however tell you that the results we got were very impressive.
We had a 25 mile Geodetic control Survey in Upstate NY in early 80's. One of our party members was from Ghiana, (British Scholl of Survey), so if I remember correctly they could not observe Polaris or Southern Cross as they were so close to the Equator.
In any case he had a program in his TI 59 that we used. We scaled a lat/long from a quad and used a Radio Shack time cube (remember them?) and used a T2 and Rolffs solar prism. We observed azimuths on 4-5 of our legs that were adjusted from the 25 mile Geodetic traverse.
The results on the Solar vs. conventional were all with less than 10 seconds of the adjusted conventional control.
Pretty neat stuff.
Let us know how you make out.
Hi,
In brief terms Astronomic azimuth is true north; your meridional circuit is grid north, and magnetic north is the local magnetic north at any point on the earth's surface (not very useful in surveying terms but alas it is used as a reference in Australia for their cadastral surveys>>baffling!!)
True North and Grid North are related by convergence..Be very careful applying the correction you will need to draw a diagram..
At the central meridian of your grid system (meridional circuit) by definition grid bearings = astro bearings; and as we move east or west of this CM the convergence increases + or - depending on whether we go west or east.
You are in the Northern Hemisphere I take it so the convention is:
West of CM: Grid > True
East of CM: Grid < True
Southern Hemisphere:
West of CM: Grid < True
East of CM: Grid > True
To see this you will need to draw a diagram with pole (North or South depending on hemisphere), and draw location of observer P and a direction to a point say 'A'. Grid North or South is a line parallel to CM line (a meridian by definition), and True North or South the direction to the pole for the observer.
Remember that depending on the map projection you are using a different calculation of convergence may be needed but for a TM the general approx formula is:
Conv = dLong * SIN (Obs Latitude)
A scaled position of observer's LAT / LONG will be necessary to get a good approximation of the convergence at any point.
Hope this helps a bit
Grant
Separately, I sent you the document I originally noted. I just located another paper in my files on Astronomic Geodesy, which I am attaching here, and maybe provides more help.
PLEASE SIR SEND IT IN PDF FORM
Here are the two documents discussed.
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