Since there is no absolute north in the Total Station, we measure relative angles. Angles can be measured more accurately than distances. This requires that you always measure the angle between two reflectors. If you measure a geometric shape then you have the ability to error-check your measurements.
It should be noted that there are many different methods for measuring angles accurately. Several methods are mentioned below. The first procedure listed is simple and straightforward to do with the SDR33 electronic notebook.
Note that the Sokkia Total Stations return angles in Degrees, Minutes, and Seconds. Many calculators have "time" functions for converting from hours, minutes and seconds to decimal hours. You can use these functions to convert angles to decimal degrees. Hewlett-Packard calculators will allow you to add and subtract hours, minutes and seconds, and give you the results in hours, minutes and seconds.
Microsoft Excel workbook showing correction of angles
All the methods listed below are designed to determine an angle as accurately as possible. All use double centering. This is the technique of making a "normal" or F1 reading, followed by a "reverse" or F2 reading, and combining them to give a single measurement. Double centering can eliminate 90% of instrument and operator errors. The difference in techniques is on the order that you make multiple F1 and F2 measurements, and the order in which you apply corrections. Pick the method that works best for you or the situation you are working with.
When using a Total Station for "plane-table" mapping it is not necessary to use the double centering technique. The Total Station in the F1 position will provide more than sufficient accuracy for this type of mapping.
Double centering should be used whenever you want to accurately determine angles and distances between stations. Such cases would be in surveying well locations, or strain markers on a landslide. When surveying in well surveys you should make observations in all directions. This allows you to average out elevation differences.
With the instrument is set up on point 1, and reflectors are at stations 2 & 3, the
following procedure should be successfully completed three times. Move to the OPTIONS
menu, and set the Combine F1/F2 field to YES. For this procedure all points in your
geometric shape should have a unique point number.
A. With the instrument in Position 1 (Face 1, V1, or normal position)
B. With the instrument in Position 2 (Face 2, V2, or inverted position)
The SDR33 will now average your two readings and make sure that they are within tolerances, if not, you will be warned.
C. With the instrument in Position 1 (Face 1, V1, or normal position)
D. With the instrument in Position 2 (Face 2, V2, or inverted position)
You now want to go back to step A. When you take a reading, it is important to make sure that you enter the correct point number for the target location. This way the SDR33 will average your new reading with existing readings. When done, you will have a record of all of your angles and the averaged angles. You can then check the quality of your data using the geometric shape you surveyed (see next section).
The following procedure can be used to measure the angle between two lines. In this discussion it assumes that you are out to survey a quadrilateral. This is commonly used to obtain very accurate angles. The procedure is such that each angle is checked each time. There are methods for the sequence of measurements that are faster, but the whole sequence has to be repeated if a mistake is found. Using this method is a little more time consuming, but you have less work to do if you find a mistake. It is perhaps a good sequence to use for inexperienced users. Experienced uses may prefer to work out a faster sequence of making measurements.
With the instrument is set up on point A, and reflectors are at stations B & C, the
following procedure should be successfully completed three times.
A. With the instrument in Position 1 (Face 1, V1, or normal position)
B. With the instrument in Position 2 (Face 2, V2, or inverted position)
C. Check the angle
Take the difference between BCI and BCII. Is the difference less than your pre-determined tolerance? If difference is less than your tolerance, accept the reading and continue. It the difference is greater than you tolerance, reject the data and repeat the measurements.
For the SET 4BII you may want to use an angle tolerance of 0.004 degrees (15 seconds).
When you have completed three successful measurements, average the three measurements to obtain a single angle for BC.
I observed the following procedure while working with surveyors from the Engineering Bureau of the City of Los Angeles. This is a good method to use if you are not closing a geometric shape. However, you should always close a geometric shape (that is measure all angles and distances within a shape). We were re-surveying sub-surface benchmarks looking for surface deformation from the 1994 Northridge earthquake.
In this method we make a F1/F2 reading to each target, turning the small angle between the two targets. We then rotate the lower base about 120° and make F1/F2 reading on each target, this time turning the large angle between targets. The two angles are then compared and adjusted to 360° if the difference is within acceptable limits. The procedure is as follows.
With the instrument on point A, and Reflectors at points B & C, we first determine the small angle between B and C.
Average the F1 and F2 readings to each point, then find the difference in angles. This is the small angle between B & C.
Now loosen the LOWER clamp on the horizontal circle and turn the instrument through 120°
Now repeat the above procedure, but this time turning through the large angle.
The two angles you have obtained should sum to 360°. You can sum the angles, take the difference from 360°, and then adjust each angle by ½ this amount, to that they will sum to 360°