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Geodetic Surveying Made Plain

May 22, 2001
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Jerry McGray

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This is the third in a series of columns about the practical aspects of geodetic surveying for the ordinary practitioner. It is time for my first report card, thanks to E-mail. The convenience of this communications medium has helped a few readers to share their responses to the two previous columns.

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.”

So while I haven’t earned any trips to McDonald’s, I’m not sentenced to “time out,” either. We’ll look at another report card in a few months.

To the correspondents who wrote, I thank you. In writing a new column one wonders if there is anybody reading out there. Responses are certainly welcome.

Sun Mischief

Our sun is in the middle of a period of high activity, characterized by immense, violent solar flares and massive electromagnetic radiation. Many GPS users are painfully aware of this fact, since this activity can raise havoc with the ionosphere as well as interfere with terrestrial radio signal transmission. Most conversation I have heard has been related to RTK use with static datalogging being less affected. One defense for GPS users is to raise the elevation mask in the receiver. A really problematic aspect of solar interference with GPS is that an observation will appear to have reached a proper fixed solution, but the resulting positions are erroneous.

But these GPS problems are only a fraction of other mischief these sun storms can cause. Normal radio traffic can experience interference, and even electrical power grids can be affected. And GPS users can be thankful that the GPS satellite constellation is so far above the earth’s atmosphere. For closer-orbiting satellites, like geosynchronous telecommunication vehicles, the danger can be dire. In the past, huge solar flares have somehow caused the atmosphere to swell or to “puff out.” This can cause enough additional drag on a satellite to interfere with its orbit, and there are pieces of satellites in the ocean that got there in just that fashion.

One of the more dramatic results of these solar flares is the northern lights being visible from much lower latitudes than usual. On the rpls.com bulletin board, some posts have reported seeing them from New Mexico!

The sun seems to undergo cycles of such activity at an average spacing of around 11 years. These periods of intense activity have been dubbed “Solar Max.” Wonder if there is a rock band by that name yet?

For more—make that a lot more—information on solar activity, try spaceweather.com or just search on “solar flares.” A usually reliable source of web-based GPS information is the Coast Guard at navcen.uscg.mil. That site was down as of this writing and has been since February. Meantime, we could probably all profit from being aware of this situation, and for RTK users to be especially careful to run plenty of redundancy checks.

Musings About GPS Satellites

Ever get curious about how far a GPS space vehicle travels? Or how fast they go? If you think about it, we know enough about the orbits in general to approximately calculate those. We can even do the calculations in miles. Take an earth radius of 4,000 miles and a GPS orbit height of 12,000 miles. (Bear with some approximations here, OK?) That’s a circle with a radius of 16,000 miles, or a diameter of 32,000 miles. The diameter of a circle is pi times diameter, right? That’s 100,500 and change. They make two orbits per day, which doubles to more than 200,000 miles per day traveled per satellite! Divide that by 24 hours, and we see that their speed must be in excess of 8,000 miles per hour. I guess a used-satellite salesman would be reluctant to reveal the mileage on an aging bird. Maybe he would just say, “This baby’s only got .4 billion miles on her.”

Quality Assurance from Michigan DOT

Leica Geosystems had an interesting exhibit at the recent ACSM Convention in Las Vegas. Leica’s GPS Reference Station consists of heavy duty, extremely stable components (antenna, mount, cabling, etc.) specially designed for CORS occupations. With its ControlStation software, an automated system is continuously operating to log GPS data and to communicate with other similarly equipped stations. The system’s mission is to ensure the validity of CORS data. When operating at full functionality, reference stations within the targeted network will essentially run continuous checks on the positioning data of each other. When a station’s data is determined to be questionable for whatever reason, the data is disabled. In effect, by the time the data reaches the user, it will have been pre-processed.

The Michigan DOT is one CORS sponsor that has made a commitment to use Leica’s system to guarantee that the CORS data it supplies is free of anomalies or other errors. Such a commitment is a significant contribution to the NGS effort to upgrade the accuracy of positioning based upon the National Spatial Reference System.

NGS Website

If you’ve read this far, you are definitely interested enough in the subject of geodetic surveying to visit the website of the National Geodetic Survey (NGS). Most of you no doubt already have, but just in case somebody hasn’t been there yet, please let me urge you to check it out. There’s such a wealth of information there that we could spend a couple of columns describing it. Interesting and very useful, it’s found at www.ngs.noaa.gov.

See you in a few issues. Keep those E-mails coming; I’m determined to earn that trip to McDonald’s!

To read Jerry’s past columns visit POB’s website at www.pobonline.com.

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