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I just want to clarify what I said.My intent was not to tell you to use just two points.
I said that two points can work as good as three,the degree of accuracy is what matters.
The degree of accuracy was in reference to redundancy,as how many points you use.
As next, But always check into other points to make sure your in reason of a closure.
As referring, to shoot three or more points. Then I said next.
No matter what the closure is on the resection, tie other controls or property points.
I think everyone misunderstood what I meant or it seems that way to me. maybe not.
Two good points offer better results than two good points and one or more bad ones.
However there is no redundancy using just two points. That said, a correct Triangle is correct.
You cannot improve it,also redundancy is a two way street. The more data implied , good or bad, sometimes redundancies backfire and produce less, not more reliability.
While it is possible to use only two known control points in a resection (free stationing), it is recommended to use three or more control points.There is no redundancy for orientation, using two points only. Using five or more points of the control network, there is only a slight improvement in the accuracy. This also depends on how accurate the points are and your work and equipment.
Angular resection and Triangulation: only bearings are measured to the known points.
Trilateration : only distances are measured to the known points.
Free Stationing and Triangulateration: both bearings and distances are measured to the known points.
In a resection (triangulation) measuring bearings only, there can be a problem with an infinite number of solutions called: " danger circle" or " inscribed angle theorem" . With the Total Station,bearings and distances are measured to at least three known points of a Control Network .This with a hand held computer, recorded data is related to local polar coordinates defined by the horizontal circle of the total station. By a geometric transformation, these polar coordinates are transformed to the coordinate system of the control network. Errors are distributed by lease squares adjustment. The position and orientation of the Total Station in relation to where the control network is established.
The back-sight points of the control network should cover and surround the stationing site. The position of the total station is not part of the area. This is the area where you want to measure with this station setup. Topographic points or stakeout points should not be measured outside this area. If measured outside this area, the errors in orientation will be extrapolated instead of being interpolated.
I hope I have clears the two point thing up and this helps you .
Resections work great - depending on the precision of the resection points, and the precision required for the project.
NEVER proceed w/o shooting that 3rd (or 4th or 5th) point for confirmation.
After all - ANY fool can get TWO points to line up!
In theory, the closer to 90 degrees, the better your result. The resection uses the angle and the distances measured in your set up to resolve the position of the instrument. However, due to the set up and observation errors, mathematically, the lines of sight cannot be infinitely thin which is where the error comes from. Imagine (or sketch) two thick lines crossing at 90 degrees to each other...... The area where they overlap will be a square - mathematically, the smallest possible area. Imagine two lines that cross each other at an angle of about 175 degrees. Notice how the area for the possible solution is much bigger now? You can think about that as the area of the error. The closer to 180 degrees or 0 degrees, the worse your result. If fact if you set up too close to 0 degrees or 180 degrees between your backsights, you will get a message that the point cannot be resolved. This is because two parallel lines will never meet!
The above explains why mathematically you only need 2 points for a distance resection to work. However, you should always build in redundancy and take more points. The more points you take, the more accurate your set up. If it appears that your error gets bigger when you take extra points, who is to say that your original two points were good quality and your subsequent ones were poorer quality?
As other people mentioned, once you have set up you should always check to other control points an as-built features before continuing.
Never use a resection when you are establishing other control points. Only use it for tertiary surveying or setting out i.e. measuring survey points or setting final markers. Avoid using it for work of high accuracy e.g. structural monitoring.
I hope this helps.
Thanks Grant, quite helpful.
reading through the replies most is said I work in a wide fields of Survey
and Engineering projects.
Lot of times when using Total Stations I choose resection all modern TS have
good programs to calculate your setup using least square method some have options like Helmet.
I never experienced much difference between them. Most imported is always what accuracy you
need to achieve position and elevation wise and with all setups never work outside your longest backside.
As mentioned 2 points work to calculate "a" position but bad practice min 3 in good arrangement not 3 points in one line 20 meters apart and with all Survey tasks "Check Check Check".
Thank you guys for your contributions. I have picked up some principles regarding when to use a resection and good practices regarding the spread of the resection points.
I am a cadastral surveyor and use resection in 99% of my work as the residuals are minimised to zero.
A others have said you cant get away from the basic good geometry of your point selection.
Set up in the 'middle' of the point spread and for accurate work a minimum of 3 points is required and always check other control points for a 'sanity' check.
Another point to be aware of is the "Danger" circle where all control points are equidistant from the setup. very remote chance I know but a point to be aware of when in the field.
Cheers, Phil (Australia)