Errors in Surveying: How to Identify and Calculate for the CA Surveying ExamAug 10, 2020
The data mapped in surveying can be used for many purposes, from defining boundaries in legal documents to determining the best route to run utilities or construct a road. With all of these implications, accuracy in surveying is so important. However, given the varying terrains and conditions for it, it's no surprise that accuracy can be hard to ensure in the field.
The two main styles of surveying covered on this exam are stationing with tapes and using a total station to create a traverse. In this blog post, we'll cover errors for both methods.
1. Stationing Errors
When stationing with a tape, each tape will be standardized to a certain temperature, tension, and length. You'll learn the correction formulas in the course, but here are some rules of thumb to remember:
For temperature, if the temperature is higher than the standard, the measured distance will be shorter than the actual distance. If not otherwise noted, the standard temperature is usually 68 degrees Fahrenheit.
For tension, if the tension is higher than the standard tension, the measured distance will be shorter than the actual distance.
For length, if the tape is found to be longer than the nominal length, the measured distance will be shorter than the actual distance. If not otherwise noted, the standard length is 100 feet.
There are two other corrections that you should also be aware of, and those are for sag correction (also related to tension) and elevation correction if the measurements are being taken at a high altitude.
2. Closed Traverse Errors
When conducting a survey using a total station, the path that the surveyor travels is called a traverse. A closed traverse is when the surveyor starts and ends in the same place. If the surveyor intends to do this, but the start and end points are slightly different, you have an error that must be accommodated for. This is called a misclosure error.
To adjust for this, consider the X and Y components of the misclosure error, and then apply those proportionally to each leg. So, if one leg is 50% of the total traverse, you would apply half the error in X and half the error in Y to the end point of that leg. Note that X and Y are handled separately for this.
In addition to linear misclosure, angular misclosure must be considered as well. The sum of interior angles of a closed traverse should be 180 degrees * (n-2), where n equals the number of nodes or sides. The sum of the exterior angle should be 180 degrees * (n+2). If the sum of the measured angles (interior or exterior) does not match the result of these equations, the angles will need to be adjusted. One thing to note is that all angles are considered equally, so the misclosure adjustment is divided evenly to each angle, and not applied proportionally like the length adjustments are.
School of PE offers comprehensive exam review courses for the CA Seismic and CA Surveying exams. Visit our website to register for a prep class that best fits your schedule.