This option allows you to control horizontal angle entry and how elevations are calculated. It is accessible from the Traverse Entry Options dialog (found in the Edit Menu) by pressing the Options button.
Horizontal Angle
Angles can be either Absolute Azimuths or Turning Angles or a combination of the two (Automatic Magnetic Anomaly Adjustment). Usually the survey instrument (compass or turning instrument) define which mode is appropriate. However compass surveys are vulnerable to magnetic anomalies and it is possible to use a compass as a turning instrument if you record both foreshot and backshot.
Absolute Azimuths (the default) can be vulnerable to magnetic anomalies but have the advantage that azimuths are independent - error can't accumulate. Conversely, the Turning Angles mode corrects the influence of magnetic anomalies but is vulnerable to accumulating error. You can take advantage of the strengths of both of the above modes by applying them to sections of the traverse where they are best suited. You can do this either globally with the Automatic Magnetic Anomaly Adjustment mode or manually, station by station, by enabling Calculate Azimuth using turning angle and last azimuth in the Foreshot/Backshot dialog.
Absolute Azimuths
With Absolute Azimuths is selected, the values you enter into the Fore Azimuth (Fore Bearing, Fore Grad) column of the Survey window will be interpreted as compass bearings. You can work in magnetic or true bearings (see Magnetic Deviation from North below) and you can select the appropriate angle format using Edit - Traverse Entry Options. If you enter both foreshot (to next station) and backshot (from next station), the values will be averaged.
A double-click on the foreshot field will display the Foreshot/Backshot dialog which allows you to change to a turning angle shot by checking the Calculate azimuth using turning angle and last azimuth item.
Turning Angles
If you choose the Turning Angles option, the foreshot column heading will read Fore Right and the angle will be the turning angle measured to the right from the back sight to the fore sight. If both backshot and foreshot values are entered, then the turning angle is the difference between the previous backshot and the next foreshot.
Automatic Magnetic Anomaly Adjustment
Use this option to automatically correct traverses affected by magnetic anomalies; you will need to have backshots as well as foreshots recorded. You can enter angles in the foreshot and backshot columns or in the Foreshot/Backshot dialog.
The backshot from the next station and foreshot to the next station are automatically compared. If the difference exceeds the Foreshot/Backshot Tolerance (see below), a turning angle will be calculated and it will replace the averaged absolute azimuth. The anomaly affects both fore and backshots equally at a given station and thus the turning angle is accurate.
After a series of turning angles, the final bearing of the turning angle segment will be compared to the first absolute azimuth (first foreshot of the next segment of traverse having absolute azimuths). If these two bearings differ, the last bearing of the turning angle segment is forced to equal the first bearing of the absolute azimuth segment and the angular difference is allocated evenly backwards throughout the turning angle segment. Note that you can identify which segments are turning angle segments because the station column will have a "~" symbol shown between stations. In addition, a double-click on the foreshot field will display the Foreshot/Backshot dialog which allows you to see whether the Calculate azimuth using turning angle and last azimuth item is checked.
Foreshot/Backshot Tolerance
This value is only used when Automatic Magnetic Anomaly Adjustment is selected. For a given shot, if the foreshot and adjusted backshot (180 degrees added) differ by more than this amount, then the azimuth is calculated from the previous azimuth by adding the turning angle.
Magnetic Deviation from North
This angle is added to the recorded compass bearings.
Elevation Calculation
Use previous TP elevation (standard)
Elevation at a point is calculated by adding the elevation difference defined by the previous shot to the elevation at the previous turning point (TP).
Use Setup Elevation (level re-survey)
This mode is used when a separate level traverse is performed to define elevations. Elevation at a point is calculated by adding the elevation difference defined by the current shot to the Setup Elevation (defined at the current point). In this mode the side shots are interpreted as being Height of Target (HT) values.
Note that side shots should not be entered until the centerline elevation is defined correctly; the side shot HT values that you enter are reduced to elevation differences from the centerline before they are stored in the database.
