Quote:
Originally Posted by chiaroscuro
Its incredible to think that daytime astronomy was used by surveyors before GPS. I suppose they would have needed some training in astronomy. Did they use their theodolite to look at the stars?
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Yes while it is possible to use conventional theodolites, and there were (are) astronomical theodolites with specific adaptations.
If nothing else was available as a reference to determine azimuth, surveyors sometimes used the sun by fitting a solar filter on the front of the telescope and using an almanac to determine its azimuth by date and time of day, though you also need an accurate clock for this, and corrections must be made for atmospheric refraction. Several measurements are usual too, both as a check and to reduce the probable error in the overall measurement compared to a single measurement,
Typical uses:
- determining time,
- determining the clock correction (it was usual to measure the error between actual time vs what a clock showed, and to measure the drift rate over several days, weeks, or even years);
- determination of latitude,
- determination of longitude,
- simultaneous longitude and latitude determination using two stars;
- determination of azimuth (to some distant reference object);
- various corrections to be applied for refraction, non-spherical earth, etc...
As regards accuracy... back in the 1970's-90's for those who participated in an experiment to measure the solar diameter accurately using Bailey's Beads at a total solar eclipse, USNO requested you put a peg in the ground under your telescope and take a photo of it (so it can be found again). If your data was good USNO surveyed your location to measure it's absolute position using an astronomical theodolite to one foot, better than GPS. Being USNO no doubt they had the best gear in the business, though I don't know what that was.