There is also warm=and=moist air refraction in addition to dry air refraction in many realistic surveying circumstances.
---- Original message ---- >Date: Sun, 30 Jan 2011 13:25:40 -0800 (PST) >From: "John M. Steele" <[email protected]> >Subject: [USMA:49698] Re: [SI] A mole hill into Himalaya >To: "U.S. Metric Association" <[email protected]> > > Yes, EDM. I was a little surprised that the > Australian spec for EDM equipment wasn't better, but > I copied it faithfully from an authentic looking pdf > of a manual, only a couple of years old. However, > Wikipedia claims typical EDM spec is plus or minus > 1.5 mm + 2 mm/km over baselines up to 1.5 km. That > is more in line with your number, but is still 4.5 > mm over 1.5 km, not even a mile. 2 ppm is about 3.2 > mm over a mile. > > Since an instrument should be around 10X better than > what you want to discern, you would need 0.2 ppm EDM > equipment to reliably distinguish the Survey mile > from the International mile. > > I guess the obvious question is why ISN'T it 500 nm > or so? But apparently, it is significant that it is > not vacuum, the index of refraction varies as a > function of temperature, and the variation in light > path from refraction caused by the variation in > temperature slightly above the ground cause much > larger errors over real baselines. > > ------------------------------------------------ > > From: "[email protected]" <[email protected]> > To: U.S. Metric Association <[email protected]> > Sent: Sun, January 30, 2011 4:02:33 PM > Subject: [USMA:49696] Re: [SI] A mole hill into > Himalaya > John, > > Most high precision surveying is now done by *laser > theodolites* which are capable of measuring > distances within a few wavelengths of light, with > uncertainties of only plus or minus several > millimeters, well within 2 ppm in many applications. > > Gene. >...
