Triangulation, and other forgotten skills

Can't say I've ever done triangulation, though I have certainly studied it. A number of years ago we came across some GLO notes where they stated that a pair of flags were set across the water and they observed them, similar to a subtense bar scenario- that was interesting for sure.

I remember triangulating across a wide stream at my first surveying job when still in high school in 1968. That might be the only time triangulation was part of a traverse.
I still do it every other year or so when my old TCR305 doesn't have the reflectorless range to an inaccessible point. I sight the target, turn on the reflectorless and hold my hand in front of the scope to store a point a foot away. Then I can intersect lines with cad to avoid straining my memory with trig.

I have done triangulation on hydrographic surveys in early 1980's. Out of town for 3-4 weeks,working everyday for 10-12 hours per day,and hopping from island to island on a Boston Whaler,Tofino,British Columbia.Beautiful scenery.Miss those days.

Locating building corners of a difficult neighbor's house. If the ridge is difficult for the reflectorless laser, we will run a B/B intersection for a Building Height Cert.
I tried to setup the high school to get a class to the beach to locate the point of a rapidly eroding barrier using B/B. I did not want to look in a book, so I sat down for an evening and ran through the COGO by hand (excel counts as by hand, right?). With CAD, it is wicked easy to run the geometry, so should the methodology should never be forgotten.

Started my survey career triangulating flags on sheet glaciers from nunataks. We hit them once at the beginning of the season and again at the end looking for movement.

Last time was both vertical and horizontal about 15 years ago to determine if a power transmission tower about a half a mile away penetrated the glide slope of a General Aviation Airport. It was close enough to require traversing to the pole. The results were very close.

Before I had reflectorless I used it occasionally to locate remote building corners and the top of transmission towers. The danger of reflectorless is you aren't getting what you think you are getting so in some situations it's wise to get two shots from two control points but only if they are 80d spikes with a dimple 😉

I'll repost the extreme example of triangulation I gave about a week ago. There's a tip here in case you ever need to work with a too-short baseline.
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Some years ago my brother was speculating on what an object (A) on his horizon might be. It could only be seen from his yard between points (B) and (C) a distance of 26 feet, before disappearing behind trees. Trying to measure ABC and BCA was useless because the short baseline made the tolerance on the angles and centering prohibitive.

Instead I picked an insulator on a power pole a mile away (D), measured ABD, set point (C) as near as I could to the line BD, and measured ACD. Then the centering error on my setup points (B) and (C) was manageable and I got a solution of 11.8 miles. We found it on Google Earth at a distance of 13.2 miles. So 8.5% is quite a bit of error, but still pretty good given a 26 ft baseline and an angle difference of 80 seconds.

Try this in your least squares program to get a feel for how much the long BD improved it.

Did triangulation, traverse, and measured base-lines in Japan, Okinawa (now part of Japan ), Philippines and in the USA. Starting in 1962.

JOHN NOLTON

Back in 2007, I was teaching math to high school and community college students and teaching myself what I could about surveying and geodesy. Communication towers were a convenient means for verifying some of the principles that I thought I learned. The FCC maintains location data on all registered towers and they are easy to find on county GIS systems, so very good location data is easy to come by.

My tools were a consumer-grade gps and a bearing compass. I recorded three azimuths by sighting the target tower with the bearing compass from three well-spaced locations and recorded the gps coordinates of each location. Choosing the observation locations in pairs, I used NGS Inverse to calculate the ellipsoidal distance between them. After adjusting the azimuths for magnetic declination, I calculated the angles from each observation point to the target tower and used the Law of Sines to calculate the distance from each observation point to the target tower. I used NGS Forward to compute the position of the target tower from each observation pair and averaged the results for a final position. The picture is from the Guilford County, NC, GIS in 2007. The blue dots are my observation points and the red star is my computed position for the communications tower.

I had no clue as to what ellipsoidal distance is nor how using it in the place of a ground distance affected anything. However, when I checked computed coordinates with GIS or Google Earth locations, the computed ones typically fell within the fence around a tower. The computations and source data for the one pictured are long gone, but I think that its computed location fell within its fence.

I used variations of stuff like this to teach Pre-calculus students about real-world applications of trigonometry. In Geometry, I used GIS pictures of the trailer (double-wide, thank you very much) I taught in to teach about scale. Students would measure the distance between two objects both on the ground and on the photo and then determine photo's scale. Then they would measure the sides of our trailer on the photo and determine its ground dimensions by applying their scale factor.

Personally, I think that it's very important to maintain some basic skills regardless of technology. The key is to choose the right ones to maintain and everyone has his own opinion as to which ones they are.

Other than for some exercises in my classes in college, I've only used triangulation to get remote heights of inaccessible things (tops of towers, usually). Of course, most of the stuff for which I need to measure remote height is not a critical measurement (nearest foot) so reflectorless often works just fine for that.

I still know how to work it all out by hand, though. Of course, I'm kind of a math nerd.

I used a bastardized form of triangulation to locate a wild bee hive for a bee keeper. Usually you just sight a bee flying to its nest, take the bearing, then start walking (hopefully at the same time as someone else who has begun from a different starting point). Private property prevented this so I calibrated my compass to Google Earth using features observable in the field and Google Earth. We set bee traps and dusted the backs of the bees with black soot. Once released, the bees bee-lined it to their nest and with some trial and error mostly relating to visibility (it must be overcast to bee-line) I took four bearings and drew the lines on Google Earth using the measure tool and observing the "heading". Where the lines roughly converged I obtained a UTM coordinate entered it into a recreation grade GPS and we found the nest without any difficulty. Many states still have some rather specific laws regarding access to lands for purposes of reclaiming absconded hives or even for locating wild hives as I did.

It doesn't come up very often but we've done it when it represented a major time saver.

Nate The Surveyor, post: 362182, member: 291 wrote: How many of you have triangulated, to obtain a distance?
It's an old skill, that basicly involves b/b int, to derive a long distance measurement, without the use of an edm.
I've done a number of them. I've not done one since buying an edm.
And, I don't pull out the total station very often either, with the advent of rtk GPS.
It's becoming a forgotten skill, only found in old field books, and text books.
Our world is changing fast. I still know how to hang a plumb bob, under a transit... Did it for years.

Nate
Ps, please don't abuse the moderators. They are giving us a forum. Be thankful.

The control network I'm currently developing is nearly completely triangulated...it's a great check on the distance measurements, and provides a ton of additional observations to put into LSA. Hard to imagine what I'd do if I ever did jump into learning GPS (rtk or otherwise). Didn't know it was a forgotten skill. The best example I've seen was that survey of the Himalayans someone linked to here. Can't find it at the moment, but it was pretty awesome.

Locating remote points via triangulation occupied much of my early career. My favorite project was locating a remote navigation aid from a baseline atop mountains in the vicinity of Aspen. Beautiful scenery, challenging observing conditions and fun getting to the sites. This was a summertime project.

Years later, in the early days of GPS, I made observations at a number of mountaintop sites. It was not the same. As GPS enabled the shift from interesting vistas to along roadways, field work lost much of its charm.

For those interested in how the USC&GS computed and adjusted triangulation work in the 1930's see: http://docs.lib.noaa.gov/rescue/cgs_specpubs/QB275U35no1381934repr1955.pdf

I cannot find the more recent "Manual of Geodetic Triangulation" on line.

I hope those practicing this technique keep in mind the fundamentals like base expansion ratios and strength of figure. Those determining height differences should certainly correct for curvature and refraction effects.

Well, rfc above, I certainly applaud your efforts. What kind of distances are you talking about?
I've got 3 locus L1 units gathering dust here. With a download ir wire. And some software.
You'd have to have an older 32 bit xp or Windows 3x machine to process it.
I tried to sell them on ebay, but tiger whatever, (inst dealer in Florida) tried to play hardball, to buy them. I have them still. I think one has battery corrosion issues. But the other 2 are good.

Nate The Surveyor, post: 362363, member: 291 wrote: I tried to sell them on ebay, but tiger whatever, (inst dealer in Florida) tried to play hardball, to buy them.

What does that mean? Was there only one interested bidder?

Several intrested bidders. But the guy in Florida was high bidder, and second bidder. Multiple ebay names.
If I put them back on, I'll cancel any of his bids. He goes back a ways. It's not the first time he's tried stuff.... It wastes your time and effort.

Nate The Surveyor, post: 362182, member: 291 wrote: How many of you have triangulated, to obtain a distance?
It's an old skill, that basicly involves b/b int, to derive a long distance measurement, without the use of an edm.
I've done a number of them. I've not done one since buying an edm.
And, I don't pull out the total station very often either, with the advent of rtk GPS.
It's becoming a forgotten skill, only found in old field books, and text books.
Our world is changing fast. I still know how to hang a plumb bob, under a transit... Did it for years.

Nate
Ps, please don't abuse the moderators. They are giving us a forum. Be thankful.

Triangulated last week to bridge structures...got all the equipment to do it reflectorless, but triangulation was more effective in this case.