Geodetic Surveying Made Plain

The column about surface adjustment factors (The 900 Lb Gorilla, April 2001) got a few thank-you’s for raising a warning flag. One surveyor, charged with establishing a local coordinate system for a large municipal project, questioned some values he had been given by a GPS contractor. Sure enough, the “scale factors” that were listed beside each point, with no further explanation or metadata, were the grid scale factors alone and did not include the application of the elevation component. It made an important difference. That same column pointed out that the elevation part of a surface adjustment factor should use the height above ellipsoid rather than the orthometric height. In my area, the difference is around 85 feet. It doesn’t make a lot of difference, but I sure don’t want to start out with a 2 or 3 ppm deficit in my error budget, do you? All in all, the responses to the surface adjustment factor were positive.

But the first column (Now Everybody’s Doing It!, January 2001) where we got all poetic about the mystique of round-earth surveying contained a couple of erroneous examples. Sharp-eyed reader John Nolton caught my error when I stated that an 11.5 mile arc on the earth’s surface subtended a chord only 0.05' shorter. John wrote that it should be 0.02' shorter. This time I calculated the chord from scratch (derive curve data for an arc of 60,720 and radius of 20,906,000) and found it to be—yep, 0.02' shorter, which makes an even more dramatic example, of course. Maybe that will teach me to copy “facts” from books without thorough checking. So on that mistake, I got caught understating the example. John’s next red mark came with my talking about lugging a 40-pound T-3 up a mountain. He said the instrument really weighs 24.6 pounds. I don’t know if I got my T-3 stories confused with my T-4 stories or if the weight just grew over the years. But I’ve got to ask this: which sounds more impressive, “lugging a 40-pound ...” or “lugging a 24.6-pound ...”? I rest my case. No, seriously, I appreciate those errors being caught. At this point I should say, “I’ll try to be more careful. So … “I’ll try to be more careful.”