Magnetic Anomalies
The SURVEY/MAP module can automatically adjust for magnetic anomalies if both fore and backshot azimuths are entered (and the Automatic Magnetic Anomaly Adjustment is set using the Advanced - Options button in the Traverse Entry Options dialog). In segments where the compass readings are suspect, the adjustment is implemented by using the difference between fore and backshots as a relative turning angle to calculate the azimuth from the previous absolute azimuth. At the end of such a segment the absolute azimuth is again defined and the difference between it and the calculated azimuth (the adjustment error) is distributed over the relative segment and reported to the user.
To initiate the magnetic anomaly adjustment you must mark the stations where the azimuth is NOT to be assumed absolute. This can be done automatically by setting a maximum Foreshot/Backshot Tolerance in the Advanced Mode Options dialog. All shots where the foreshots are different by more than the tolerance from the equivalent backshot will be calculated as relative turning angles.
Another way to mark stations for relative turning angle calculation is in the Foreshot/Backshot dialog. Selecting the check box forces relative turning angle calculation even if Absolute Azimuths is selected in the Advanced Mode Options dialog. If the station has already been marked because the foreshot/backshot difference is greater than the tolerance (when Automatic Magnetic Anomaly Adjustment is selected in the Advanced Mode Options dialog), then the Calculate Azimuth using Turning Angle and last Azimuth item will be checked and disabled. You can clear all stations marked as relative turning angles here by using the Traverse | Remove Relative TA menu item.
The Foreshot/Backshot dialog allows you to enter backshot data and choose shots that should be treated as relative turning angles regardless of settings in the Advanced Mode Options dialog.
Note that the foreshot and backshot azimuths in the Foreshot/Backshot dialog refer to the same traverse segment. Once the relative turning angle stations have been marked, the Map | Adjustment menu item will reduce the traverse. During this process you will be notified of the size of any azimuth error adjustments (when a calculated azimuth at the end of a relative segment is reconciled with an absolute azimuth).
Immediately after the adjust procedure the stations that were calculated as relative turning angles will be marked with a ~ in the station field (the ~ symbol is temporary and will be cleared on various operations including save). Use this information to determine whether the adjustment makes sense. If there are large calculated azimuth versus absolute azimuth adjustments (more than .5 degrees per station) then it is probable that the azimuths in the relative turning angle segment are actually deviated by the local attractor and should be considered relative. If the angular compass deviation of adjacent stations is large but equal then the fore and backshots will be the same and the Automatic Magnetic Anomaly Adjustment procedure will assume that the azimuth is absolute. To correct this error find the offending stations (usually between two relative segments) and use the Foreshot/Backshot dialog to set them as relative turning angles.