map surveying question

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Ray(GB)
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map surveying question

Post by Ray(GB) » Thu Mar 09, 2006 1:06 pm

Hello All,
What trig formula is used to make maps?.
I know they measure a base line on the landscape,then sight from each end to a point to make a triangle,and they know the internal angles of the triangle, and the length of one side, but how do they calculate the length of the other 2 sides?.
I know the formula for right angled triangles, but not irregular ones,
Thanks,Ray.

skyfrog
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re: map surveying question

Post by skyfrog » Thu Mar 09, 2006 1:38 pm

Hi,

You can use this equation to calculate the length of any sides. Yes it holds for irregular triangles.

A/sina = B/sinb = C/sinc

where A, B, C = length of sides,
a,b,c = corresponding angle of side which length is A, B, C respectively
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larry cottrill
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re: map surveying question

Post by larry cottrill » Thu Mar 09, 2006 4:18 pm

Ray -

Note that a Registered Land Surveyor will NEVER use triangulation just to save steps IF the actual chain measurements can be made. It is used where it is impossible to chain, such as two of the points are on opposite sides of a lake or something. This is the case even though the chain has to be pulled tight, held level and corrected for temperature. Of course, the art of chaining is dying out anyway as distance measuring is now all done with laser devices (in any situation where line-of-sight is possible).

Once a survey has been run clear around a site (which may be many measurements of distance and angle), there will always be some (hopefully small) error of closure. The surveyor applies special formulae (using a computer program, nowadays) which distribute the net error uniformly among all points measured. Once this is done, ALL the distances and angles will be slightly different from the as-measured values, but the figure WILL close!

I know this stuff from working on a surveying crew one summer. We would do ANYTHING necessary to be able to stretch a chain, clearing brush with machetes and so on. The "chain" of course was really a VERY sturdy SS tape on a large real, with special tools to lock onto the ends to grip it for pulling it tight; also to hold a plumb line so you KNEW you were measuring right on the point. Most of that practice is probably dead art now, because of the modern laser-based instruments that have become affordable.

I stupidly dropped a beautiful Zeiss theodolite one time on the pavement. All survey work was stopped and the instrument had to be sent in to a Bureau of Standards "depot" for testing and possible recalibration.

L Cottrill

Bruno Ogorelec
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re: map surveying question

Post by Bruno Ogorelec » Thu Mar 09, 2006 8:27 pm

I live close to a construction trade school and see the students there doing the survey work, which is an important part of the curriculum, in the traditional manner. So, the students at least learn the art. Whether they use it outside school is another matter.

It's like navigation. Every self-respecting sailing school will teach you navigation by sextant -- despite the fact that no one actually navigates by sextant anymore. But, the art is important, as it gives you an insight into navigation that is otherwise simply not there.

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re: map surveying question

Post by Al Belli » Thu Mar 09, 2006 11:50 pm

Hi Larry,

I had a re-survey on my 25 acre parcel done last summer as part of a subdivision to sell a strip of about 8 acres to my neighbor.
About two hours with GPS, and laser shot lines , cost me $1,400 !!!!
They had exsisting pins to work off of, and it appeared that the process could have been performed by trained simians.

Al Belli

larry cottrill
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Re: re: map surveying question

Post by larry cottrill » Fri Mar 10, 2006 4:24 am

Al Belli wrote:Hi Larry,

I had a re-survey on my 25 acre parcel done last summer as part of a subdivision to sell a strip of about 8 acres to my neighbor.
About two hours with GPS, and laser shot lines , cost me $1,400 !!!!
They had exsisting pins to work off of, and it appeared that the process could have been performed by trained simians.
Al, you have to pay for that equipment somehow ;-)

Seriously, though, these kind of professional services always seem to be priced out of sight. One of the few jobs I did for money as an independent DP consultant was a "Consultant's Cost Accounting" system for a land surveyor / civil engineering company. This was decades ago, of course, but I remember the markup on time for the crew was considerable. A lot of it, Al, is mandatory regulatory stuff that you and I wouldn't usually even think of - continuing education, liability insurance, "malpractice" type insurance, equipment maintenance. Even the cost of just maintaining an office is staggering nowadays in most places. Every job has to pay some piece of all that, not to mention wages and benefits.

Of course, it seems like an almost trivial job with the kind of equipment they use today - but they're selling more than what you see at the time. At the end of it all, the head surveyor's signature goes on the drawing, and if it proves out wrong in a court action someday, his career is on the line. One thing I learned on that crew is that there's a lot more to it than tromping around with a stick and a "gun". After all, there's a reason the law doesn't just let you go out there with a steel tape and do it yourself, even though a lot of people would probably be able to do an adequate job of it, in very simple cases. I think when it's done the old way with a theodolite, level and chain, it just looks like you're getting more for your money - overall accuracy with today's technology is probably far better.

Remember, as they say, "What you're buying is peace of mind." ;-)

L Cottrill

Cita
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re: map surveying question

Post by Cita » Fri Mar 10, 2006 9:34 am

I used to work in a surveyor team back in the old days when you needed 3 man and a lot of equipement.
When the electronic theodolithe came on the scene a lot of things changed (one of them was that the team was reduced and that I was transferred to the design burea!!) and the time in the field was reduced dramatically!
This tends to think that it must be cheaper today for a complete survey!!

The equipement is rather expencive to start with,15,000 to 20,000$ for a good theodolithe is not unusual and a descent computer and software will set you back another 10,000$ In the old days it was common that a surveyor bought one good theodolithe and,apart from regular calibration,this theodolite was used throughout his whole carreer.Today this is not possible any more so a new advanced theodolithe must be purchased on a rather regular base in a carreer.
Use of satelite's is costly too and keeping up with the newest technique through courses is not cheap as well.
Insurence for liability has gone up skyhigh as it's not uncommon these days that a surveyor has to defend his findings/conclusions in court.

It is like Larry said,in the old days a team of 3 people crossed the field for a whole week,making seemingly very complicated measurements,and the next week a plan was delivered to a satisfied costumer.He payed a lot of money but it appeared that the surveyor team had worked hard.
Today the surveyor does the same job in one day and delivers the plan next day....for the same amount of money and all he did was walking around with a prisma and typing some figures on the keybord.

Ray(GB)
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re: map surveying question

Post by Ray(GB) » Fri Mar 10, 2006 3:14 pm

Thanks for the info., and anecdotes,everyone.
Ray.

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