NavList:

   

Reply

The phenomenon of horizontal refraction was well known in the era of
geodetic triangulation. The C&GS manual of first order triangulation
(1926) says, "Under ordinary conditions the air strata are of greater
density near the ground and lie roughly parallel to it. Over a sloping
terrain these strata of different densities are not horizontal, and a
ray of light passing through them will be bent horizontally as well as
vertically... The force and direction of the wind are also determining
factors, for with a strong wind the differences in the temperatures and
densities of adjacent air strata are less marked."

"It must not be thought that errors caused by horizontal refraction are
always of small magnitude. On first-order triangulation, where the
probable error of a direction may average about one-half second, cases
are not infrequent where horizontal refraction has caused an error of
from 3 to 6 seconds."

The manual notes that horizontal refraction also occurs on flat land, as
when a line of sight passes over a corner of a plowed field surrounded
by woods. The heated air rising from the plowed land forms a vertical prism.

Similar effects occur in urban surveying. The 1909 report on the
triangulation of greater New York says,

"Many large chimneys were in line with some of the stations and smoke
and hot gases arising from them rendered the observations very difficult
and in some cases impossible. By taking advantage of a favorable wind
and watching for a chance some observations were obtained on these
obstructed lines, but a few of them were lines bent from horizontal
refraction and were rejected."

"On many of the schoolhouses which were occupied in cold weather, some
one line passing over a chimney of the house in which a fire was burning
was impossible to observe. In some cases the fire was banked, at our
request, on Saturdays when there was no school, and the lines observed
at that time."

https://babel.hathitrust.org/cgi/pt?id=nyp.33433066326012;view=1up;seq=7

One day I spent a couple hours at the National Geodetic Survey site
looking up the old triangulation points named in the 1909 report. The
great majority are gone, but a very few survive and have been visited
recently. In at least one case, modern photos were included in the
datasheet.

   

Reply