Redundant Backsights

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Redundant backsights are a method of improving the accuracy of a survey and avoiding blunders. (They should not be confused with ordinary backsights or reversed shots which simply reverse the station order.) With redundant backsights, you take two compass and inclination measurements for each shot. One is taken in the forward direction and the other is taken in the backward direction. Since both measurements are taken along the same shot line, the back sight should exactly match the forward measurements. (See corrected backsights.) If the measurements don't closely match, there must be an error in the shot.


Resolving Backsights. There are two approaches to using redundant backsights. The first approach only uses backsights in the cave. Basically, you keep taking fore and backsights until they closely match. When the measurements match, you only write one of the measurements in the book. In the second approach, you take two sets of measurements and use both of them. When you process the data, the two numbers are averaged, which theoretically, gives a more accurate survey.


Backsight Options. Compass supports both approaches. You can configure any survey so that it uses redundant backsights or only accepts normal foresights. To turn on or off redundant backsights select "Options|Survey Settings" from the menu bar and click on the Backsight checkbox to enable or disable the option.


Entering Redundant Backsights. If you have been using the redundant backsight option, the edit grid will be slightly different. In the grid, the forward compass angle is labeled "Comp1" and back compass is labeled "Comp2". Likewise, the forward and back inclination is labeled "Inc1" and "Inc2". Also, the second measurement of the pair appears immediately after the first.


Missing Data. In some cases, you may be unable to take both a fore and backsight at a particular shot. This means that part of the data will be missing. Compass can still process the shot as long as there is at least one compass reading and one inclinometer reading. You enter a missing data item by typing "M" or "*" in the field. The program will flag an error if you try enter missing data for both the fore and backsight item.