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trmcconn

How significant is refraction?

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Light travels slower in denser air than in less dense air, an effect known as refraction. If a beam of light is directed parallel to the surface of the earth, the parts of the wavefront in the denser air closer to the surface of the earth will be slowed relative those further away, causing the beam to bend slightly in the direction of the curvature of the earth. One effect of this is that distant objects appear slightly higher in the sky than they really are. Faraway ships at sea, for example, can be spotted by observers when they are still below the horizon. Since the deflection is vertical, this effect should not be a problem for triangulation surveys. Indeed, there could be a slight benefit in that lights over station marks could be mounted a bit lower than if the survey were done on, say,  the Moon. On the other hand, observations made in the presence of horizontal pressure gradients should experience horizontal deflections. For example, if you're looking north with a high pressure system to your West, then light will tend to curve around the high, deflecting the apparent position of objects slightly to the East. I wonder if this effect is significant at the level of accuracy of our historical surveys, and whether the surveyors corrected for it? Could this be the reason relatively nearby azimuth marks were established, rather than relying on quite distant objects - church steeples and such?

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Azimuth marks were set for most triangulation stations because intersection stations (e.g. church spires, water tanks etc.) were in many cases not visible from the ground (VG) from the triangulation station, and in many, many cases there were no intersection stations anywhere near the triangulation station.  For a surveyor to efficiently use the geodetic network it was required to have position coordinates (latitude, longitude and/or State Plane Coordinates) and a corresponding source of azimuth of orientation.  The U.S. Coast & Geodetic Survey (USC&GS) started setting AZ Mks in the mid-1930s as increasing numbers of surveyors were being required to use them mostly for government programs such as road construction and local mapping. 

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I think triangulation measurements were mostly done on clear nights, sighting to lights on the other towers.  The atmosphere would be fairly uniform then.  You may be right that working when a front was moving in could introduce significant errors for that kind of work.

 

Locally, we're about 40 miles from any point that was used to constrain the the horizontal network adjustment to GPS data.  Using my antique professional GPS, I find most of a foot difference on local horizontal stations between NAD83(HARN) from the data sheet versus NAD83(2011) OPUS reports from my measurements.  That's an angular difference in the network of about one arc second.

 

Under stable conditions and ordinary temperatures, the effect of vertical refraction in feet is 0.003 (D/1000)^2 or in feet and miles, 0.09 D^2.  Earth's curvature is larger and is offset a bit by refraction, for a combined correction in feet of 0.021(D/1000)^2 or feet and miles 0.57 D^2.  This is large enough that leveling operations are usually limited to sights of 100 yards or meters and sight lengths are balanced for careful work.

Edited by Bill93
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