I have control established on site but I need to add additional control to an area that wasn’t in my level run. What’s the best way to set new control points
What equipment do you have and what accuracy of control are you trying to create - i.e. what is the purpose of the additional control?
Typically you have the following options
1) Call in a Surveyor to set the additional control you need - if you want the highest accuracy and you need it as long term control and you do not want Legal Liability for the control created then this would be a good option.
The options below will depend on what equipment you have available to you and what accuracy you are chasing for the control point. The following guidelines can be used
1) For the highest position accuracy use a Total Station and Traverse the point in.
2) For the second highest position accuracy use a Total Station and Resect your position
3) For second highest position accuracy, you can measure the new control point from 2 or more locations and take an average location
4) For highest Elevation Accuracy use a Level and run a level run through the point to determine its elevation - you need to do the run from a known height point to the new point(s) and either forward to a second known height point or loop back to the start point where you know the height. Then you can compute the level run and an adjusted elevation for the new control point(s)
5) For second highest elevation accuracy you can use a Total Station. Keep the distance measurements to less than 500' if possible - and typically you want to measure the angles and distances in both directions along the baseline from the Known Point to the new control point, and compute the average elevation for the point. Total Stations are affected by Refraction (vertical bending of the light), that causes Vertical angle measurement errors, that can only be determined and fixed when you measure the same line in both directions.Doing a Traverse between 2 known control points and your new Control Point(s) where all lines are measured forwards and Backwards is the best approach and then optionally level the points if you want the highest accuracy.
6) If you have GPS only and you are doing "Dirt Work and Aggregates" then you can do a long occupation (measure the same location for say 30 seconds or a couple of minutes to create a new control point - this will create a control point of GPS level accuracy only - so do not then try to use it for Paving, Concrete or Curb and Gutter work etc.
If you have a Total Station, then depending on the accuracy you require, if you can see control points from where you need to set up and work then you can use the Resection command on the data collector to measure from the setup location to 2 or more points (where you set up a prism on a Tripod or on a Pole with Bipod) and it will compute the position of the Total Station for you using the best fit position to the angles and distances measured. Make sure that the distances to all the control that you measure vary (you do not want to be at the center of a circle where all your distances are approximately equal, and ideally the points have a angle between them greater than 45 degrees, or that you have 3 target points spread around a circle to get the best results. Typically measure 3 pointings on Face Left and Face Right and measure the distance using the averaging mode for 10 seconds to get a good set of results. If you are working in a building environment, you can establish self adhesive targets to piers or columns or walls and then measure them in from a known locations to establish them as "2nd order control" that can then be seen from the locations where you need to set up and again use the resection method to compute the stations location. If you need to reuse the setup location over and over, you should establish a control point using either a stake and nail in the top of the stake, or by sinking a circular pipe in the ground and filling it with concrete and placing a nail in the top of it, to give you a sable repeatable setup location. You can use Resection to establish its position, or you could measure to it from several locations and take an average XYZ for it from all the measurements, or you can traverse through it.
Let me know what equipment you have available (GPS, Total Station and Controller Software +/- a Level) and what accuracy of control you are trying to establish and for what purpose and I can maybe provide more specific response.
If you could also give me some context like you are working on a Highways Project, A Bridge, A Building / Warehouse, A High Rise Building etc. that also gives me a good indication of the accuracy you need etc.
If any of the terms I use are not understood, please let me know and I will try to explain what I mean.
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