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. ---- Original message ---- >Date: Sun, 30 Jan 2011 12:14:10 -0800 (PST) >From: "John M. Steele" <[email protected]> >Subject: [USMA:49694] Re: [SI] A mole hill into Himalaya >To: "U.S. Metric Association" <[email protected]> > > For the US, it is up to each state. Many still use > the Survey foot, some have gone to the International > foot since the release of new geodetic data to the > NAD83 datum. > > Anyway, I defy you to survey the difference which is > 2 ppm. Long steel tapes, when corrected for > temperature and at proper tension are certified to > 0.02% (200 ppm). A little Googling turned up an > Australian standard for EDM equipment, plus/minus 5 > mm + 30 mm/km over length of baseline, so 35 ppm > over a kilometer. Surveying simply doesn't have the > precision to resolve the difference. >..
