Geodetic Surveying Made Plain
I clawed my way out of a deep sleep, a sleep filled with dreams of renegade units of distance: meters, U.S. survey feet, international feet, varas, chains, smokes…they had all joined forces to attack me and my fragile forces for truth. The good thing about the dreams—if you could call it good—was that it was a relief to wake up. Yeah, sure...to wake up to a bleary-eyed hangover caused by way too many diet Dr. Peppers—and sure, I admit it—more than one Snickers bar. (Way more than one.) I hadn’t slipped all the way to the hard stuff: the KitKat Bar followed by the Reese’s Peanut Butter Cup, but temptation was never far away...in the file cabinet behind the jukebox, if you must know.
I blinked until the stenciled letters on the outside of my office door were readable: “Deifitsmy...” Wait a second, those letters are mixed up. What it really says is Discrepancies, Inc. Devilishly Difficult Distance Difference Dilemmas De-mystified. I tried to come up with an attention span and to think of a word to substitute for “Inc.” that started with “D” as I listened to the rain pounding on the composite asphalt shingle roof. I wondered if my detective agency was about to be washed away. Nobody told me being a private eye-man was so tough. Or that I would have to sleep in the office…but that seems to be the way all us private eyes do it. How else would you start your stories? But I digress...
Right about then was when she walked in the door. Her gaze could melt ice cream. Actually, I guess ice cream would pretty much melt anyway, sooner or later. But this lady had the look of a professional; in fact, she might as well have carried a sign saying, “I’m an engineer.” It was obvious that I was in the presence of not only an engineer but probably even a civil engineer! More importantly, I think this might even be…a client! I resolved to behave myself.
She broke the silence: “I have a problem.”
“Don’t we all,” I smirked. But when she rolled her eyes, I remembered my resolution and asked,” What seems to be the problem, Toots... I mean, Ms. Engineer?”
She sighed and plopped down into the only decent chair in the office, an Office Warehouse 31628 Deluxe Plastic w/Arms. I could see she was genuinely troubled.
“We got our surveyors to tie into a monument, and our coordinates are 2 1/2 feet different from the parish’s values.”
I thought, “Oh boy, this is gonna be too easy.” I asked, “Do you happen to know whether your surveyor was using NAD83(93) HARN Datum?”
“Oh, of course!” she answered. “Our surveyors use nothing but the latest and greatest.”
“And were your coordinate values slightly less than the parish published values?”
“Why... yes! How did you know?”
“To answer that will require a check made out to me for $28.50 per day expenses, plus my usual fee of $12,000.”
She almost knocked over the plastic bowl containing RoyBoy, my Siamese fighting fish, as she shoved the check my way. “Here, take it! Now what’s the answer?”
“You and your surveyor are in the two-bit, I mean two-foot world of HARN versus non-HARN. Probably because of timing, the parish positioned its monument based upon the NAD83 values established before the High Accuracy Reference Network (HARN) was created. In this geographical area, that difference results in around 2 1/2 feet, with the more recent (HARN) values being the smaller, State Plane Coordinate-wise.”
She almost smiled as she got up to leave. “Thanks, private eye-man,” she said. I feebly waved a goodbye as she walked out.
I flicked the last of the fish food into RoyBoy’s bowl while I made a mental note to get a fresh can next trip to Safeway or Walgreen’s, your—I mean my—neighborhood markets.
I had barely popped the top on a Pepsi One (by the way, if you’d like your product featured in our column…oh, never mind) when my Nokia digital phone started chirping. I answered, “Discrepancies, Inc. What’s your problem?” The frantic babbling on the other end of the line could only mean one thing: Surface Adjustment Factor Syndrome! (Believe me, I’ve heard enough bawling and whining about SAFS to recognize the symptoms right off.) When the caller finally got coherent enough to continue, I was able to figure out that this firm had done an engineering design project tied to a Huge State Transportation Entity project. Upon project completion, they shipped a copy of their drawing file to the City Prettymuch Geographic Information System. And, of course the coordinates of the HSTE project were a long way from fitting properly into the CPGIS (I don’t have to keep explaining these initials, do I?) They didn’t fit by a long shot, as in 1,300 feet! A classic case of Surface Adjustment Factor Syndrome (which I once heard an elderly surveyor refer to as the 900-pound gorilla of problems. Wonder whatever happened to that old codger?).
Long story short, all I had to do was explain to the afflicted patient that the application of a surface adjustment factor not only nudges the points apart a tiny bit as was intended, but also plops them unceremoniously several hundred feet from their rightful State Plane location. After helping him figure what the factor was, and advising him how to “fix” the problem in a simple spreadsheet, there was nothing left to do but collect my exorbitant fee—plus the $28.50 per day expenses, of course.
I was about to get my sea legs and begin my usual late-morning debate with myself: Wendy’s or Burger King? That’s when the computer beeped with an E-mail. Yep, another distance discrepancy. This engineer had received a mapping file and loaded it into his computer –a Dail PowerGrabber running MicroSolvitKwik v.83.4. But when he tried to overlay his alignment file onto the mapping, he was off by around 20 feet! I put in a phone call to him right away.
I tried to soothe his hysteria by repeating a surveying mantra... “turn right, read left, turn right, read left.” But he didn’t seem to get it—probably too young. So I went right to the nasty truth: Was there ever a metric component to this project? He calmed down, sniffled and replied, “Why, yes. As a matter of fact, our design was originally done in meters. We just converted it to feet the other day.”
“Umm, hmm. And do you happen to know whether you used U.S. survey feet or international feet?” I asked.
“All I know is, the difference in the two is negligible. Why do you ask?”
I said, “You’re right. The difference is negligible…
except when dealing with the massive values of State
You could have heard a pin drop. No wait, I think that’s Sprint with the pin-dropping thing, and we were on AT&T. Anyway, he was quiet. I went through the math with him...the U.S. survey foot is officially 39.37 divided by 12 feet per meter, while the international foot is 2.54 cm per inch. We proved that the difference in the two can make roughly 20 feet of difference in the coordinates by the time you multiply them by several million.
I pulled the shade down for my afternoon nap, feeling a twinge of guilt that these clients’ problems were so easily solved by simply looking at the magnitude of the discrepancies. I wondered if my ride down Easy Street would someday find me in a cul-de-sac if these folks ever figured it out and could do their own Distance Discrepancy Detecting (DDD).
Let’s see, in one short morning in private eye-land I had solved a discrepancy due to epoch differences between NAD83(86) and NAD83(9x), a discrepancy between grid coordinates and scaled “surface coordinates,” and the difference between the U.S. survey foot and the international foot.
I mentally checked off the other common discrepancies, and found only one left: NAD83 versus NAD27. Most folks can figure that one out pretty quick. To put it on the list of magnitude = identification: around 100 feet.
As I slipped back into sleep, my dreams began to turn to a whole nation of surveyors who had a solid handle on the numerical conversion problems that can creep into the surveyor’s work. A nation of surveyors who want to get it right, yet keep it plain. I have a sinking feeling that they are wising up. I began to envy RoyBoy in his isolated, watery world.
Maybe it’s time for that Reese’s/Reese’s cocktail after all. Maybe even make it a double.