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.

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Subject Tags: azimuth, observation, solar, surveying

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Are you familiar with a  document titled " Astronomical Observation Handbook" by Charles Ghiliani? It describes the techniques a surveyor needs to know for determining azimuths, north, etc using astronomic observations. If you send me your email address I can send you the pdf. It's only about 17 pages long.

Please send me the PDF . Much obliged Arthur Will

Here's the document.

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Here's the document.

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Hi Carlos, i saw your mail regarding the document, and it is an important helpful document. I kindly ask if it is possible to email the document to me as well. my email _ [email protected]


Thank you

Here are the two relevant documents.

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Here are the two documents discussed.

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Hi Carlos, maybe that would be helpful document. Please send at [email protected] Thank you bro.

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.

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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. 


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



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