Why Traversing is Still Essential in Construction.
For any construction project to be completed successfully it needs to have good dimensional control. We need to have a dimensional control survey. Dimensional control will enable us to build independent structures that can then be tied together with another structure. Think of building two bridge abutments separately and then linking them with a steel deck. Think of building two roads miles apart and then having to link them together. Anything that we build independently that then needs connecting will need to be accurately controlled dimensionally. Therefore, it imperative that the dimensional control points that we are using are as accurate as possible.
How do we achieve dimensional control?
Dimensional control on sites is now in the form of Temporary Bench Marks (TBM) on site. With the advent of GPS Surveying Equipment and the ability to get co-ordinates from national systems instantly, the old traditional Bench Marks are no longer maintained, that is certainly the case in the UK. Every construction site should have a network of control stations suitable positioned throughout the site. These control stations should be either ground markers, PK Nails or Hilti Nails all securely located in ground that is not susceptible to movement. This primary control would usually be established prior to any work commencing on site, with the co-ordinates of the control stations given. From these primary control points, the TBM’s, we can place all the permanent construction works in their proper locations. For this reason we need to have confidence in the site control we are going to be using.
How can we check the site control stations? Carrying out a dimensional control survey.
There are a number of ways that we can check the site control. The method we decide to use to check the site control will depend on the nature of the works that will ultimately be carried out. Most site construction control points are these days checked with GPS. This may well be done using network Rover GPS surveying equipment. This method is ok for sites without structures. But it is not the most accurate way for checking control stations. There can be some inaccuracies using network Rover GPS systems that are not readily identifiable. You can find out more by reading the real-world accuracy of network GPS systems.
But if we have a structure like a bridge spanning a River, a tighter set of control points would be required. For this we will need to do a traverse of all the control points. The traverse is the dimensional control survey and is the preferred method for checking and providing dimensional control points for construction sites.
What is a Traverse?
A traverse is a chain of measured lines which must start and finish on points of known co-ordinates, but which may follow any route in between. The included angles between the lines are also measured and at least the starting line needs to be of a known whole circle bearing.
What are the three main traverse systems that we can use?
The three main systems of traversing that we can use are:
1.) The open traverse.
2.) The closed traverse.
3.) The braced Traverse.
Of these three, the closed traverse is the most accurate. For the closed traverse, what we are trying to do is go around in a complete circle and end up back at starting point with the same co-ordinates. The closed traverse enables the surveyor or engineer to calculate the accuracy (or the error) of the whole traverse.
We can of course brace the closed traverse if possible. Also, bracing a traverse can help identify any errors that might be present in the traverse.
The Braced Traverse is our next preferred type of traverse. This is potentially less accurate than the closed traverse. But the bracing does enable some error checking of the traverse procedure.
The open traverse is the least accurate of the traverses that we would want to do. There is no way of knowing if the angles and distances we have taken are accurate. The only option here would be to do the traverse at least twice and on different days.
What Co-ordinate system should you use for a traverse?
In Great Britain, the Ordnance Survey has provided a network of permanent co-ordinated points. The easiest way to obtain these national grid co-ordinates is by using a GPS Network Rover. But if the construction site is small often getting national grid co-ordinates is not necessary. In these cases, and for small surveys a local grid only approximately orientated to north is all that is required. The traverse must start and finish on the same point in a loop. The point is given assumed co-ordinates (usually E:1000.000 N:2000.000 L:100.000) and the bearing of one line is estimated.
Guidelines for carrying out a traverse for a dimensional control survey.
Following these guidelines will enable you to get the best accuracy for your traversing work, thus ensuring a good quality dimensional control survey. When either checking existing dimensional control points or setting out new dimensional control points your work needs to be accurate. Careful planning and work at this stage will save a lot of stress at a later date.
1.) The first sight should be as long as possible. This is your back-sight. It may also be referred to as the base-line for the traverse.
2.) Lines of sight should be clear and unobstructed between control points. There should be nothing that might cause distortion to the angle or distance measurement. Edges of buildings, vehicles or sources of heat.
3.) All points ideally would be around the outside of the construction site or survey area. If possible, all dimensional control points would be visible to each other. This however is rarely the case.
4.) The more detailed the structure or building, the more dimensional control points will be needed.
To obtain the best accuracy, dimensional control survey traverses need to follow direct routes with long distances between stations, whilst detail traverses must run near to the detail to be located and necessarily involve shorter distances. Overall accuracy can be destroyed by the inclusion of short traverse lines. This is because centering the total station and prism stations becomes very critical as angular error is carried forward in the bearing. Short traverse legs would ideally be captured with an intermediate sighting or bracing to another control point.
Adjacent traverse stations must be inter-visible but the exact positions selected depend on whether the traverse is being established for control or detailing purposes.
What accuracy is acceptable for a Traverse?
The acceptable error for a traverse depends on whether it is a primary traverse or a secondary traverse. A primary traverse would be one that we are setting up before any construction has started on site. The accuracy for primary traverses should be at least 1 in 60,000.
A secondary traverse would be one that uses the dimensional control stations from the primary traverse to add additional control stations to the site. Secondary traverses would generally be used for areas that are not easily seen from the primary control stations. An acceptable accuracy for a secondary traverse would be at least 1 in 20,000. This check on accuracy is only possible with closed traverses.
How to do a dimensional control survey traverse.
This is just a brief overview of how to do a dimensional control survey traverse. This is what you would want to do if you are checking dimensional control.
With the total station setup on the first point, sight the prism station setup on the back-sight station and record the angle, height of the instrument and the height of the prism. Sight the prism station on the fore-sight station and record the horizontal and vertical angle, the slope distance and the height of the total station and the prism station. Repeat the above using the opposite face of the total station. Move the total station to the next station and repeat the process above.
How to calculate the dimensional control survey traverse.
From these bearings and distances, we can calculate the co-ordinates of the next dimensional control point. To do this we need to use trigonometry. I am not going to go through that process here. What you need to do is start from the your first setup point and use the bearings and distances to calculate the co-ordinates of the next point in the traverse. Keep doing this until you get back to your starting point. Here you should find the last co-ordinates calculated should match the first set of co-ordinates. Any difference in these co-ordinates is the closure error for this traverse. From here we can start to identify any potential errors we may have.
One check that can be done is calculate the internal angles. The internal angles of your traverse should add up to (n-2)*180 where n is the number of angles in the traverse. Any discrepancy in this should be shared with all the angles throughout the traverse.
What is the easiest way to calculate a closed survey traverse?
The easiest way to do the above calculation is through specialist survey software or through an excel spreadsheet. One of the best excel spreadsheets for calculating the closed survey traverse is the one found on the Lichfield Survey Supplies website. The spreadsheet will calculate all of your intermediate traverse points and at the end will give you a closure error. All you need to do is record your observations. If your closure error is not acceptable you can alter the angles in the spreadsheet and see what affect it has on the results. A handy way to find potential errors.
The survey traverse calculation spreadsheet is available from their website. Follow this link to their survey traverse calculation spreadsheet.
This is the description from their website.
The survey traverse programme spreadsheet has been specifically written to check the accuracy of a Survey Traverse. The traverse is one of the first steps to accurate surveying and setting out, as it is important to have a good, tight control network.
Using the survey traverse programme spreadsheet all you need do is enter your readings between the Control Stations around your site and let the spreadsheet work out the rest.
Just record the following items carefully.
- Instrument Height at the control station.
- Horizontal Angle and the target height of the backsight.
- Target Height, Horizontal Angle, Vertical Angle and slope distance to the foresight.
- Move the Total Station to the next control station.
- Repeat the above steps.
Once you have added the raw data into the first sheet you can then look at the results.
If the results were not as expected you can review the data via the computation sheet included. This may highlight any errors.
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