I did further checking, including looking at Leica specs. Those numbers like 1.5 mm + 2 mm/km are not worst case specs, they are standard deviations. Statistically, we know there is reasonable chance of individual measurements having errors out to 2 or 3 standard deviations, rarely even more.
I continue in my challenge to Pat that I defy him to actually survey the difference between International and Survey feet with available field equipment. The difference is within the error bounds of the equipment. Apparently it is not practical to distinguish a molehill in the Himalayas, to play on the thread title. ________________________________ From: "[email protected]" <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Sun, January 30, 2011 4:37:41 PM Subject: [USMA:49699] Re: [SI] A mole hill into Himalaya 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. >...
