Method for determining the average atmospheric refraction coefficient required for point visibility using the spreadsheet linked above.

Denotations:
point A – the point from which the observation is made
point B – point on the line of observation potentially obscuring the view
point C – observed point
The following values are needed: the altitudes of points A, B and C as well as the distances AB and AC. The heights of points A and C can be read off a map (for Poland, the most accurate data is available on Geoportal), distances can be measured e.g. on a Google map or calculated from geographical coordinates using one of the online calculators. Determining point B is more complicated. To do this, a visibility analysis has to be created for point C (or A – it doesn’t matter), using the website heywhatsthat.com, and then the position and height of point B should be determined using this method (tutorial in Polish). Measure the distance between points A and B. Finally, input the necessary values into the spreadsheet and obtain the refraction coefficient at which, from point A, point C is visible exactly behind point B.If the highest point of the observed object is taken as point C, a threshold value of refraction is obtained, above which the object starts to be visible. The stronger the refraction, the more of the object can be seen. By measuring the height of the visible part of the object in the photograph, the refraction coefficient corresponding to the conditions at the time the photograph was taken can be calculated.