How do trig points actually work?

The way these were used was the same in every country. In America they’re called Trigonometrical stations, in the UK Trig Points, Australians calls them Trig Stations and in France they’re called geodesic terminals. .
This video will explain how the theory behind how they were used to create extremely accurate maps.
NOTE - at 15.05 there is a graphics typo. I dropped the results on to the wrong sides of the triangle when I did this in graphic in Photoshop. Obviously the longer side (on the left) should been 13 and the shorter side (on the right) shout have been 11, ooops 🙂, sorry

[@neilfoster814]

Fun fact. below each Trig pillar is a chamber, this chamber contains a piece of rebar or scaffold pole driven deep into the ground and the coordinates and height to datum noted. If the pillar was ever destroyed, or it moved, it could be replaced to the same accuracy as the original one. My late father was a surveyor for the Ordnance Survey for nearly 40 years. He retired just before GPS became the tool of the trade for the OS. I myself worked as a surveyor for an Internal Drainage Board for 15 years surveying ditches, rivers and the coastline.

[@ballagh]

Interesting to know. We’ve got a trig point on some land and I’ve often wondered about accuracy as it’s been hit by various tractors and towed implements over the years. I don’t think it’s moved much but it’s definitely been rotated by wheels rubbing against it.

[@Fatsimbacat]

This was made for radio

[@mikemiller209]

Are those scaffold poles magnetic ?

[@neilfoster814]

@@mikemiller209 no, they aren't magnetic

[@rogerwhittle2078]

My absolutely favourite story about the Ordnance Survey and how accurate the people who hauled that much concrete and brass to build a Trig Point were, all revolve around Major Martin Hotine. Hotine was directed to survey the entire country and ensure the grid system as it existed, was as accurate as possible. As the map man says, they started with the Liddington Baseline (or datum.) The country had been triangulated sometime in the 1790's using a short baseline on Hounslow Heath and later on Salisbury Plain, but Hotine's retriangulation used an 11 km baseline near Liddington in Wiltshire. Two trig points marked the ends of the datum and a third provided 'triangulation'. By using lamps and theodolites, the measurements were taken at night, as being more accurate. Most of the country was triangulated by the late 1930's, but war interrupted VERIFYING the survey. It wasn't until about 1962 that the checking got to the terminal datum near Inverness. What they found and calculated was astonishing. After all those tons of concrete, steel chains and dark nights with lamps and theodolites, they found the error was......this much. 17" or 42cm. In thousands of miles of calculated and measured distances, the error was less than an arms length. This meant Ordnance Survey maps are among the finest IN THE WORLD and every time I see a Trig Point - no longer used because we have satellites, GPS and laser beams - I think of Major, later Brigadier Martin Hotine and his precise mappers and thank them again and again. Don't forget him or them.

[@Megahegs]

Great information, but theres a common misconception that trigs are not still used with GNSS equipment. I use them weekly in Australia for this purpose.

[@welshpete12]

Very interesting , thank you for the information

[@monroe3004]

When I joined the OS in 1965 as a surveyor I was amazed that they were already working in metric.

[@Blagger3000]

The Corps of Royal Engineers (Survey) were amongst our earliest surveyors.

[@Ensign_Cthulhu]

How did he measure his baseline accurately, given that the ground between might have had obstacles? Or did he pick it for that purpose?

[@pnwxplored]

In the late 80s I GPS surveyed many of the New Zealand trig stations from helicopter for NZ GOVT. Been in mapping ever since and the basics of trig has never let me down. The traditional trig calcs were very good ! Very little error was identified with modern satellites use. I spent often half a day at waiting until 1-2am to get southern hemisphere says to lock in back then! Spent a night often on top of a mountain waiting for the helo to get me! Couple of times I had to walk out for poor weather. Best job I ever had.

[@klaasvanmanen8214]

So far, I've only watched the intro, and I must say: job very well done! Introduce the subject, tell what others say about it and then raise the question that so few tried to answer. Now I can no longer leave this video: I have to watch it! And b.t.w.: you have a terrific voice for telling stories.

[@TradieTrev]

I love how you've presented maths in a real world way. That's how you get the kids to learn!

[@ScouserBerny]

Of all the 300+ trig pillars I have visited in the UK, today I learned how they really work. Thank you for this. Totally enlightening,

[@tomvalentine4928]

Same here. Surprised to learn it's so relatively recent. so I assume the 1037 date quoted was part of the Retriangulation.

[@dickJohnsonpeter]

I've come across trig points in the US just hiking but they were always a metal circle like a medallion with geological info cemented into a round piece of cement on the ground. My theiry is they would probably just place a ruler on top of that since they are flush with the ground.

[@simongee8928]

I watched a documentary about the planning of the trig points in the 30s. All the concrete and other construction materials were taken to most of the sites by hand due to the remoteness of many of them. And all the calculations were done three times, each time starting from scratch, to ensure accuracy. No calculators then; slide rules and log tables - ! 😊

[@alexkalish8288]

I was looking all day for a trig point on the Ile d'Cavana in Brasil years ago. It was described as a square foot of concrete with a brass plate embedded in the top.This pre-GPS, we were going to be using transit satellites to resurvey the island.We located the exact spot but no marker. I gave up after 10 hours searching called for a chopper and went to get a beer at the local restaurant/shack at the landing field.After about 20 minutes speculating what happen to the marker we saw the trig marker in a rather smaller piece of concrete holding the front door open. Cheers , good video-

[@tfatcher]

As a 78yr old oldtimer, I enjoy dusting off some of my old math skills. This was fun and your explanation of using trigonometry to calculate distinces between two points was exceptionally well done. Thanks for the memory!

[@glenn7245]

Brilliant. Never knew the details of the ' flush bracket ' or what it was called for that matter. Your style of delivery is excellent. Keep up your excellent work.

[@crashnburnband]

If only my teachers in school explained this stuff as well as you did. Practical application is the key if you can see a purpose for something it becomes interesting and easier to learn.

[@philpaton3812]

Absolutely! In school I thought "when will I ever use this crap", but now I am interested in maps and navigation!

[@robt2151]

@@philpaton3812 we did learn this at school and actually used it on field work, but that was back in the 60s. Later, when I needed a job, I was able to become a surveyor setting out roads and buildings; it all started from those practical lessons.

[@fredtaylor9792]

Yes! It's easier to understand how to drive a car if you're told what the controls actually do, rather than being given a set of directions to move them in a specified way to get to point A.

[@rudolphguarnacci197]

Those that can, do. Those that can't, teach.

[@christopherschenk9108]

I came across this video by chance. What a gem! Charming and delivered by such an enthusiast. Whoever thought that algebra could be so entertaining! Thank you very much

[@carlthor91]

In Canada, we refer to them as Geodetic Stations. The instruments used before GPS, where WILD T-3. A friend originally from Nepal, was a instrument tech. One time he went to visit family, as soon as the government found out his trade, he was hired to calibrate all the T-3's in the country, he got a lot of trips back home paid for. Also, the little bit of trig-work, that I took part in, was a series of 20 observations, from each point, mining work, not Geodetic work. Noways it's all GPS, with Lidar, for mapping. Best wishes from the far North.

[@jefftheaussie2225]

The first trig site in Australia was done in May 1828 by Thomas Mitchell. It is on Mt Jellore in the Southern Highlands of NSW. He used a team of convicts to clear the trees off the top of it so it could be seen. That was the start of mapping here. Although they are not used anymore it is always fun to find one and means often you are in a quite remote place. I always think of the surveyors who found and built these too. Ours are often a short pipe mast with two steel discs about one metre in diameter at right angles to each other painted alternately black and white so a black and white circle would be seen. Australia can be both very rugged and extremely flat, it can be a very long way between them on flat country, trying to find a slight rise to put one on must have been a job in itself.

[@cabletie69]

Yes,apart from Tassie.

[@daztastic147]

No idea how or why this video popped up on my feed but it was extremely interesting and I enjoyed watching it. Very engaging the way it was produced👍

[@level1804]

FINALLY! After being an ‘A’ grade student in mathematics and arithmetic, someone shows me a practical use for trigonometry! I knew there was one, but no teacher ever explained the uses. Probably because they didn’t know themselves.

[@MrRedeyedJedi]

It's also used in phase diagrams/angles in physics

[@Anon54387]

Math teachers often don't know the practical uses, but they know the math so thoroughly that sometimes engineers, scientists, biologists do go to a mathematician for a specific answer on how to approach something. The joke about mathematicians is that they spend all day developing a problem in general to finally get around to the specific case people are working on, but in the process they do learn it thoroughly. People doing cancer and vaccine research sometimes talk to topologists, for instance.

[@jefftheaussie2225]

I thought the same thing when a builder showed me how to square up a building using Pythagoras. Sin,cos and tan used to haunt me at school too because no teacher I knew could show me a practical use for trig.

[@johnmckay1423]

I use it in DIY construction regularly. Although I often cheat and go back to the ancient Egyptian method of multiples of the 3,4,5 triangle which always gives a precise right angle. Saves needing a calculator and you look less of a geek!

[@srl6018]

I find triangulation pillars fascinating (yes I am a geek). And after watching this today I know a hell of a lot more about them than I did yesterday. Thanks.👍

[@arcadia1081]

I once had to match up GPS derived data to survey derived maps. The detail I had to get into was unbelievable, from what are known as Molodensky 3 parameter or Bursa Wolfe 7 parameter shifts. I was a humble GIS man using map reprojection systems and man was it a challenge!

[@dukesublime4178]

How times change. I’m 47 and was absolutely hanging on to every word you said lol. Brilliant video sir. Many thanks

[@GaryLaaks1]

Learnt something new today. 55 years old. Always wondered how it worked and then forget to Google when you get home etc. Tks mate.

[@vinnalls375]

The flush bracket demo is very good. Worth watching.

[@stupatrick]

Amazing. Like so many people, I've ended up navigating to trig points for years without knowing what their purpose was (other than having something to do with trigonometry). Now I know 😁 Thanks!

[@ephraimgadsbyjr.9441]

Thanks for the very interesting video. Here in Ingham County, Michigan, USA there is a park dedicated to a surveyor error that made the map of the county not be a perfect rectangle.

[@Anon54387]

Fun fact, no one knows exactly where the state line between Oregon and California are to this day due to some SNAFU from the 19th century (I forget the details off the top of my head) so the sign saying welcome to Oregon when heading north is about 100 yards from the sign saying welcome to California when heading south.

[@tararuaman]

Thank you for explaining 'trig points.' Here in NZ our trigs are built in the shape of a pyramid, which consists of a steel frame with wide boards fixed to it, they stand approximately four to five metres high. The top metre is painted in black and white squares. in the middle of the trig, on the ground, is a concrete pad and in the centre of that is a steel pin. If you Google NZ trigs you will see photos of them, but all serving the same purpose as the UK ones. Enjoyed the video. ATB Cheers from the mountains of NZ ☺☺

[@CoatsandGaiters]

What a fabulous video and brilliantly presented. Just shows how important the teacher is and it's not about buying the best tech to teach with. Hands on learning at it's best.

[@EMo5ive]

“Is knowing all of this going to make you a better navigator…no it’s not. I just find it interesting!” We are cut from the same cloth, my friend! :) I’m probably young enough to be your kid, but I’m guessing we’d make good friends. I went to college for GIS but never finished. Maps have always fascinated me, even as a kid. Thanks for the video. Very well done. Glad I found your channel!

[@jamescampbell7780]

Essential viewing! I’ve used a theodolite for drainage gradient once only and being an amateur, I found that the new drain actually works!

[@alexday5892]

This is brilliant ! I love your description and presentation. Making something curious feel quite understandable. I have had to navigate in white out circumstances with snow and fog obscuring all forms of references. Walking from a trig point allows you to know exactly where you. Superb video

[@MichaelOnines]

At 13:20 you made an error in calculating the unknown side lengths and got them swapped around. The T2-TN length is 13.258m, and the T1-TN length is 11.15m. You always want to use the angle opposite the side you are trying to calculate in your equation. You used an adjacent angle.

[@TheMapReadingCompany]

Oh. Sorry about that. Trying to do maths on a board in a tree is always slightly difficult. The basic idea is correct though - I've just watched it again and you're correct I got the "T's" mixed. Well spotted

[@MichaelOnines]

@@TheMapReadingCompany No worries. That example tool you built for the flush mount monument was really neat.

[@bimocular4312]

I hunted for somoene else who noticed!!

[@markbooth]

I had to come hunting through the comments to check this too. Great tutorial even with the minor maths mix up.

[@davidwilkinson1480]

Please correct me if I am wrong but isn't the length of T1 to TN = 11.15 and the length of T2 to TN 13.25?? The ratio of Sin 68.5 to 11.15 is not equal to the ratio of Sin 51.5 to 13.25 as pictured in 13:05 of the video.

[@terrypold]

Yeah, wasn't the shorter side longer than the longer side?

[@terrypold]

@@beeble2003 you know, I did not read the video description. Thanks for pointing it out.

[@davidwilkinson1480]

Thank you all. I really enjoyed the video and I learned a great deal. I spent the next two days researching numerous trigonometric laws and reviewed many other land navigation videos.

[@Baycliff]

Thank you. KZbin has some interesting suggestions sometimes and this was one I just had to watch from beginning to end. Thank you presenter and camera person(s) etc. What a teacher. So clear.

[@chrisbryden8102]

Thank you very much for putting together such a detailed explanation! It’s absolutely mind blowing how accurate they were when you would think even the slightest error would compound.

[@johnnytoronto1066]

I worked as a surveyor's assistant as a kid here in Toronto. We mostly measured urban projects for streets, bridges and tunnels. Our triangulation points were usually found in the concrete sidewalks (Pavements in the U,K,), and sometimes like your flush brackets on the sides of buildings. We called them all benchmarks. Our tools were the transit (your theodolite) and the level, a telescope with crosshairs and a levelling bubble. We relied heavily on tripods and plumb bobs. Horizontal measurements were made with the chain, which in my day had evolved down to a 100 foot steel tape marked in tenths of feet. The rod which accompanied the level for taking vertical measurements was similarly marked in tenths. We made extensive use of 2"x2" pointed wooden pegs about 14" long, which we drove into the ground with a sledge hammer. We would top the pegs with a small, dimple-headed nail to mark the precise point. That nail was generally the target for a later positioning of the transit. Where we had to drive one of these wooden pegs through a concrete sidewalk, we employed our bullprick, a two-foot long solid steel cylinder, pointed at one end and at the other, battered and bruised frommany stubborn battles with the sledge. All this was about 1965, the pre-digital age, but I imagine the terminology in Canada hasn't changed much since then. Thanks for the video and the great memories of one of the best times in my life!

[@HweolRidda]

FYI "Transit" is short version of "transit theodolite". I think the designation means that it has the ability for the telescope go to vertical and then flip over ("Transit") down the other side.

[@mikelong9638]

@@HweolRidda I believe you are correct. It is a transit because the telescope can be inverted.

[@pwblackmore]

An informative video presented in a way most people could relate to. What I found amazing is the accuracy of the OS team... just recently I came across the National Library of Scotland with its collection of maps dating back to the late 1800s, and the comparison with satellite imagery is spot on... originally, of course, all done from ground level. Thankyou, Presenter.

[@daklakdigital3691]

FASCINATING. What gives me vicarious satisfaction is notwithstanding the wonders of GPS, dotted all over the world are ground reference points used to correct GPS errors. I know of 3 in VietNam, where l live. Russia sought permission to install similar points in North America but we're denied.

[@bobwallace5257]

During my time on Canadian submarines we used theodolites to calibrate the periscopes and the torpedo tubes and all other machinery requiring precise placement so that they were all aligned exactly with each other. Much like the concrete pillars in the video we had datum plates from the time the ships/submarines were built.

[@nicholaskeep1605]

Brilliant!! That took me back to school in the mid-70s with my elementary surveying and the late 70s to the 90s navigating and hydrographic surveying... I'm now retiring to a small village in Dartmoor, so will keep a sharp lookout for the trig points...

[@user-pd5ot4zd4b]

Really cool viceo. I love stumbling on benchmarks. Id' argue that knowing all this would make one a better navigator! By increasing ones depth, intuition and practical knowledge of the art.

[@KarlWitsman]

Very interesting! In the US, we have geodetic survey markers and through time some have been lost or they were on a bridge that has been replaced. I used to go around, while geocaching, and try to find them and report those that still existed to a website. It was educational and fun. I need to look back at some of the photos I took of them to see if the elevation was on them too.

[@Anon54387]

One of those is set in concrete near a restored train station near to where I grew up in California. My brother, until very recently, worked as a surveyor and one of the first things they do is look for this at a property corner, they have a metal detector for this purpose as they very often wind up covered by dirt over the years.

[@LarsDahlin]

I did some civil engineering in Sweden for a decade in the 90's. Lovely reminder of the craft. Except you seem to have less mosquitos and bugs flying around at the moment. All the best!

[@jime8532]

nice demo, thanks. But if I may, the use of the sin law step -you had the wrong sides with each angle. IE the longer side is opposite the larger angle. Besides that, I learned a lot. thanks.

[@victorherrera5506]

I’m impressed. I always was curious how the surveyors would communicate with one another when one guy is looking through the eyepiece and the other is holding the elevation stick. Aerial photos show straight lines and traces of vantage points. The accuracy is amazing and I think the subject matter should’ve been shared long ago.

[@zabaleta66]

A video like this is exactly what the internet should be all about! Well done 👍

[@attadudepc]

What an extraordinary lesson, excellent in presentation. You, sir, are an amazing teacher and I'm in awe of the manner in which you've incorporated so much information

[@ourwetdogs5232]

I read ‘The Great Arc’ by John Keay some years ago and thoroughly enjoyed it and would highly recommend to anyone who has watched your vid and similarly enjoyed. Great subject and interestingly covered by you - thank you.

[@timothykeech7394]

There's an error there somewhere. Given the angles you have measured, the sloping side to the right should be shorter than that on the left. You should be calculating using opposite angles. The sides you have calculated should exchange positions. Apart from the mathematical error, this video is fascinating for its information and explanation and very well presented. I studied this stuff nearly 60 years ago when the job was very much hands on with microwave measurement in its infancy then, so it is all familiar and nostalgic.

[@Darisiabgal7573]

The formula which should have been written is Sin A/a = Sin B/b = Sin C/c We can rewrite this for the Application T1°/lenT2T3 = T2°/lenT1T3 = T3°/lenT2T1 Just as a side note here. If we were actually doing this on a large triangle we need to kniw that as the len of any side increases this measurement would not reflect statute distances because of the curvature of the earth surface. In that situation we need to recalculate given the cord and converting the cord to an arc (And the ancient babylonians would be smiling at you). T3° = 60° T2° = 51.5° T1° = 68.5° lenT1T2= 12.34 m The equation we select first is T1/lenT2T3 = T3/lenT1T2 we can then insert the givens and solve Sin 60°/12.34 m = Sin 68.5°/ lenT1T2 lenT1T2 * Sin 60°/ 12.34 m = Sin 68.5° lenT1T2 = 12.34m * Sin 68.5°/Sin 60° lenT1T2 = 12.34m * 1.074 lenT1T2 = 13.2575 m And so we can see he assigned the length he calculated to the wrong side. Good thing im not a cartographer or I might have got it wrong too.😎

[@timothykeech7394]

At a glance it didn't even look right. I'm sure if I'd done that I would have realised immediately just from the angle sizes - well, actually, I did. I am aware of adjustment for curvature but his distances were very short so not an issue worth raising for the purpose of the video.@@Darisiabgal7573

[@bluestormcloud791]

@@Darisiabgal7573 The three angles of a triangle on a flat plane add up to 180 degrees. on a sphere they will a up to slightly more than 180 degrees. Earth curves about 8 inches in the first mile.

[@madeljacky]

The first video I have seen that explains the trig towers and how they work, I often wondered what the flush plate was for but it was even better to see a practical example how the attachment worked, thank you sir.

[@malcolmstead272]

I did a presentation on this exact subject to my Explorer Scouts, this was to give them a better understanding of their maps; excellent video.

[@rogerspencer2292]

A couple of points of clarification if I may, both of which indicate the accuracy to which the Ordnance Survey works. 1. The small support ‘stick’, upon which the spirit level was placed, must be removed. The point upon which the bottom of the staff is placed should be at exactly the same level as the top of the arrow, otherwise an error would manifest itself. Old brackets were shaped (concave) so that a temporary reference point on the bracket stood away from the face of the trig point; so the support for the spirit level, which bridged between this and the top of the arrow, was necessary - the support pieced had to be removed before the level reading was taken. 2. The reference ‘arrow’ is only for establishing levels (the Z component) - neither the flush plate nor the staff is used in the measurement of angles. The top of the trig point/pillar has embedded metal grooves, set at 120° about the precise reference point (in X & Y) of the trig point. These grooves enable the siting of theodolites and/or their ‘targets’ with easily repeatable precision - repeatability over time and by different surveyors. Theodolites (and targets) sit on the top of the pillar, without the need for the usual tripod used by surveyors and engineers when measuring for, or setting out new, infrastructure. The central point on the top of the trig point is it’s reference point in X & Y. NB - I’ve learnt that links are not permitted in comments - my previous, now deleted, comment had links to images of brackets and the tops of trig points. These can be found by searching for ‘os flush brackets’ and ‘os trig point plates respectively.

[@loskop100]

I recall working as a chainman in the late 60's in New Zealand and the end of the wire was connected to a tension spring so as to be able to calculate the sag in the wire but and also the temperature recorded to allow for the expansion of the wire. Cutting bush and scrub so that the chain wasn't touching anything was also the job of the chainman but I was a lot fitter back then. Thanks for the video, it bought back many memories of a time that I enjoyed very much.

[@williamhall8683]

I was a few years behind you in NZ, working for the MOW conducting deformation surveys on the Waikato dams .. `Maraetai, etc Fortunately, the EDMs had just arrived, so triangulation became trilateration. Still, we had to clear the bush!.. Happy memories …….. I gave up survey when GPS took over; survey then became boring

[@grahamlait1969]

There is a lovely story (which we must hope is true) that when the OS were doing their original mapping of the UK in the Highlands of Scotland, they asked the locals what the name of various hills and mountains were. As in many parts of the Highlands the locals spoke only Gaelic and the surveyors spoke none, the surveyors would note the answers phonetically, in standard English. So far, so good. When the new super duper OS maps were about to be published, a Highlander read the proofs and realised that a lot of the Scottish mountains had very strange names, because if pronounced as written, in Gaelic, they were apparently called things like 'Who's asking?' and 'What business is that of yours?' or 'Why do you care?' and 'Just stop bothering the sheep'. Of course, the necessary corrections were made, but It may be that some Scottish hills on the OS maps still have such names and Hamish in Auchtermuchty is still having a bit of a laugh to himself.

[@bobwallace5257]

Absolutely love this video. I try to learn something new everyday and you are very entertaining.

[@LoftechUK]

This is brilliant and have joined the club. Happy to see more when posted.

[@Dudleymiddleton]

There is one on the A1101 near Nordelph( Cambs/Norfolk) and another at Mepal( on the way to Ely from March) Old Airfield, and at Beeston regis cliffs near Sheringham, Norfolk. Fascinating stuff! :)

[@1a1u0g9t4s2u]

Some of my memory cells are telling me I watched a TV show (yes before the internet) explaining how a team surveyors used this principle to calculate the height of Mt Everest. That team was very accurate but I don't remember to what that was. Perhaps one of your viewers knows this. Very interesting post, Thanks for sharing.

[@garymucher4082]

Thanks for the tutorial about how to figure things out like this. I knew it was going to be pretty precise mathematics to get the answers for these measurements and elevations. I like it too... Thumbs Up!

[@bowlingbuddie]

Why this should appear on my youtube recommendations, i've no idea but it was so informative and easy to follow and understand, just wish you were my teacher when I was at school (50+ years ago) studying Chart work & Navigation. Thanks.

[@oscarrox]

Great explanation of something I've walked past countless times. Your videos are very interesting and educational, thank you.

[@TheMapReadingCompany]

I'm glad you like them!

[@xxxthestuffxxx]

I've often wondered about they worked. Fascinating video, thank you.

[@wjf0ne]

As a child at school my maths teacher droned on putting us all to sleep with no connection with his droning to the real world. Imagine if he had put it into context of how it was used, heck we might have been interested and I may have had an understanding of maths and gone on to do great things rather than becoming the factory fodder we were destined to become.

[@Anon54387]

There is such a thing as personal responsibility. Just because your math teacher doesn't do say why or how it is used doesn't excuse you. Ask him, and if he can't or won't answer go look it up online. My parents taught me that I'm the one responsible for myself, not my friends, not my teachers, not even my parents when it comes to it. Apparently you missed that lesson somehow. If you rely on others you will always be disappointed. Teachers are sometimes, perhaps even often, boring but it is still in your interest to learn it despite that.

[@lyfandeth]

In the US, the survey markers are usually a bronze circle, about the size of your pslm, set into rock, or concrete flush with the ground. The problem is that every time a sidewalk is replaced, or a road widened, or some other routine work done, the old markers are either buried or removed, with no attempt to put in replacements or surrogates. So especially in urban areas, finding marks and confirming whether they still exist can be quite challenging.

[@TheMapReadingCompany]

We (in the UK) would call them a Bench Mark, not a trig point

[@akulkis]

​@@TheMapReadingCompany Same in the US. On US Geological Survey maps, they are literally marked with × symbols.

[@bareenlars]

Such a lovely video! Love how you explained it so well! These things is so strangely fascinating :D

[@TheMapReadingCompany]

Thank you so much!

[@buxvan]

I'm not a Hiker, Explorer, Fell Runner or Russian Spy, But I really enjoyed your Trig point explanation. Brilliant & well done. Takes me back to school & the horrible book of cosines, tangents & angles.

[@kimamey]

It's not something I need to know, but having seen a lot of them this was very ingesting. You did get one thing wrong when you said, "As you'll remember from school." All I remember from school, over 50 years ago, was that I didn't understand it, and that hasn't changed. Maybe I should try again. Thanks for a facinating video.

[@JM-jd7yp]

Absolutely fascinating. I enjoyed every minute of this video and I didn't like maths at school. Thank you and I wish you well.

[@jg2072]

This is very interesting. I did a presentation about the Public Land Survey System here in the USA. You do a much better job that I did. The other challenge to explain is how chaining can be done over elevation. As a Forester we used chaining when walking to determine locations. My chain is 12.5 paces to 66 feet. My last half pace was a little short.

[@MrBasildondave]

Very interesting, thanks. I figured how the position of TN was found myself, but I always wondered how to get the height. You cleared that up for me 😊. Great video

[@KidCurry666]

Love maps but never knew the full story of trig points, very enlightening, thanks for posting 👍👍

[@henrytomlinson3634]

Thank you for taking the time and effort to explain all this to all of us. All the best from Henry in Central UK

[@TheMapReadingCompany]

Central UK - are you sure ? kzbin.info/www/bejne/gZiqgKGnhcZnm5I

[@paulf2529]

Brilliant, by farr the best explanation ive read and one i could follow even with my school maths knowledge that i havent used for years!

[@mjf1036]

This is so cool to know. Thanks for the refresher course in algebra and geometry 😊

[@davidcousins3508]

An interesting presentation..I knew the basic theory behind trig points but this was packed with detailed information 👍

[@graememartin196]

One of the most interesting explaining videos on KZbin. Very interesting

[@Patriot-bn9om]

In 8th grade Trigonometry class we all probably thought it was useless math. I wish the teacher knew enough to tell us how it was used to map a country hundreds of years ago. We just needed an accurate measurement between just two trigs points, and a theodolite to accurately measure angles between points, then just simple math to calculate the distance between all points and the elevation of each.

[@kurtnowak8895]

This was so interesting, I watched right to the end. Even though you filmed it on the windiest day of the year. 😉 thank you.

[@evelearmont8545]

Thank you for your video, I have often wondered how/why trig points work as mapping. I am rubbish at maths but even I could understand your explanation. Brilliant !

[@123xqp]

Very interesting. I've seen the plates in the side of trig points many times but I never knew their purpose. I always assumed that the theodolite sat in the brass disc on top of the pillar and the plate on the side was for identification plus some random goverment marks. One small correction, which doesn't affect your video at all - you've swapped the calculated side lengths between Tn and the other two points. The longest side is opposite the largest angle. In practice this would have been noticed when the adjacent pillars were surveyed. Thanks for the video. I learned something new and that makes my day.

[@abstractapproach634]

Good catch

[@djs5074]

Brilliant, brilliant, brilliant. Thank you. My girlfriend is now going to get a full history and workings of trig points on our next walk, I'm sure she'll be thrilled 🙈

[@Jo-NZ522]

I have always known they were something to do with measuring land but didn’t know how. Now I do thank you very interesting.

[@compostjohn]

My friend Charles ('veggiebagger' to some) is a collector of trig points, and photographs them, sometimes repaints them. He will love this vid, and I'm sure his friends will find it interesting to find out what they're for and how they were used.

[@Bokooda]

Fascinating. I'm 56 and I 'sort' of knew what they were for and why they were there (ordnance = military) but not how they were created and not why they aren't all the same. It would be interesting to learn why those 2 particular reference points were chosen as the 'standard' for distance and average sea level... Cheers! 🙏🏽

[@SvenRayson]

Great video and fascinating. Keep up the good work and amazing info

[@dannycbe949]

Brilliant. 63 year old civil engineer here.... If onky our professors had explained this as good...

[@wilkoone9155]

In the early 1800s a mile long bi-metal strip was placed near the village of Bellerena at the mouth of the river Roe in Northern Ireland. This was the baseline from which the whole island of Ireland became the first country in the World to be surveyed to 6" to the mile in wonderful coloured maps, which were finished in 1846.

[@SuperNova-py1ec]

Fascinating thanks. I came across one of these whilst holidaying in Yorkshire.

[@Olan...]

Great info you are providing here, well done lad. Is the plate on the trig point put on the side that it is on for any specific reason ? I was in the military for 15 years and we would make/build a model of the terrain that we was going to cover out of sand so that we could discus things like potential ambush points and cover anti ambush drills or select observation point positions before we deployed so we had a good idea that we could see a target area from where we would be located. Sometimes the vegetation or tree line would hamper this but it was a good way of becoming familiar before we deployed and very often you wouldn't need to get a map out en route. It's something i still practice today and i am always selecting features to handrail while i am walking to a certain feature . I have tried GPS but always go back to a map because it gives me a better overall picture of the landscape rather than trying to look at a small screen that kind of blisters up when it's raining. Its like having a birds eye view in my opinion and i just prefer maps plus they dont cost an arm and a leg.

[@wisteela]

Very informative and interesting. I love that you made your own attachment. Subscribed.

[@geoffmatthews2353]

Just for your own (useless) information, the arrow is a “broad arrow” from the predecessor of the Ministry of Defence- the War Office. When Ordnance Survey was formed, maps were secret, your Ordnance Survey maps used to have GSGS - Geographic Section (of the) General Staff - printed on two corners of every map. I’m not sure when that stopped, it was certainly printed on every map I used in the Army as a Map Reading Instructor in the 80s and 90s. Keep up the good work, I really enjoy the channel!

[@TheMapReadingCompany]

A broad arrow. Thanks for that. I didn't know that. We learn something new every day 😉

[@sputumtube]

Came across this channel by chance. Very interesting - subscribed. :)

[@Takn835]

You are a wealth of good knowledge. Very insightful 👍

[@aigarslacis4433]

I see a mistake - the sides have been "mismatched", essentially - the largest angle has the longest opposing tringle side !

[@GemstoneActual]

You're a good teacher, dude. Good job. Thanks.

[@bengesell]

I find it interesting too. When we loose electrical grids and our phones become paper weights, we'll all want to learn this.

[@martintierney7187]

That was actually pretty informative, well done and a great explanation. Thank you for your time in doing this video. Not that it made my memories of the Army’s land nav course’s and practical field maneuvering any better 😢 but it make me think there were other n Knuckle heads out their before me laying it down. 😂😂😳😳 Thanks again and we’ll done.

[@Megahegs]

The 'ruler' is called a staff in Australia. Staves for plural. The arrow is a traditional broadarrow, signifying ownership of the Crown. Also most commonly seen on convict uniforms.

[@lambtonrob]

I’ve seen hole & broad arrows used as RMs instead of the usual drill hole & wing in a suburb where I’ve worked. A rare example of a surveyor with too much time on their hands.

[@Megahegs]

@@lambtonrob really old suburb?

[@alexandermenzies9954]

No use in South Australia and Northern Territory then! (No convicts allowed to settle in those colonies. :-)

[@matthewsheeran]

Strictly speaking the compass bearing is a magnetic asimuth and actual theodolite bearing i.e. angle measurement wirh respect to the baseline would be more accurate because of magnetic variation from place to place but the priciple is the same. I am not a surveyor but I am GIS guy who has been a surveyors' assistant. Great stuff. I once did a GPS assignment atop of a Trig Station so that I could be sure of the actual position although we ignored the plate tectonics way back then too, hehe!

[@jockcherokee5180]

Great video. Explanation made it simple. The use of sine rule was something i knew nothing about. Thankyou.

[@TheMapReadingCompany]

I'm glad it was helpful!

[@alan2804]

Very interesting and informative, ive often wondered how these things were used and what the height should be measured from. I never knew about that flush bracket and had forgotten about the line in the harbour innCornwall. The baseline was a new one on me too, i think years ago there was a docudrama on this surveying forcOS? However id forgotten most of what it taught!

[@frederickwoof5785]

We did this in our last year of geography and mapped the course of a local river. Great revision

[@saudistevo3002]

Thank you so much. Very interesting. I wasn't the brightest spark at school, however your explanation was brilliant.