The vertical refraction in the atmosphere cannot be ignored for long-range observations. The exact value is difficult to predict as it requires a large number of temperature, pressure and humidity measurements all along the path of observation. However, the value can be measured using reciprocal zenith angles and other methods. This has been done and quantified many times at different locations under varying conditions.
- Account of the Observations and Calculations, of the Principal Triangulation; and of the Figure, Dimensions and Mean Specific Gravity of the Earth as Derived Therefrom
- Alexander Ross Clarke, 1833
- Pages 542-550 as marked on the pages, or 571-579 of the PDF.
- “…the coefficient required is simply the ratio of the excess of the observed above the true zenith distance to the angle subtended by the two stations at the centre of the earth.”
- This means the observation appears above its true location.
- Coefficient of Refraction:
- 0.0809 for rays crossing the sea
- 0.0750 for rays not crossing the sea
- Note that over sea observations are raised more than not over the sea.
- https://books.google.com/books?id=DKELAQAAIAAJ
- Direct PDF Download