When the target centering feature of my servo-driven total station is switched on and I do a visual check through the telescope, I often find that the telescope cross hairs are not on the target. There seems to be a constant offset above and to the left of the target. My instrument dealer has told me not to worry about this. He says that the instrument has been calibrated and that a software correction is applied. Is this true? Should I be concerned?
The answer to your question depends on the manufacturer and model of instrument you are using. You should vigorously pursue a comprehensive answer that is detailed enough so that you can check it yourself. You could run one or two simple tests. The easiest is to sight on a tripod-mounted target about 100 m away in F1 and F2 modes. First, sight with the automatic target centering feature. Then sight conventionally with your eyes, cross hairs, and instrument locks and target screws. But don't sight on the center of the prism; use the triangular shapes or other ways of centering on the target accurately. In both cases, take several readings in each position of the telescope and average them. Assuming you have not shifted the instrument, the average of all the F1 and F2 readings with automatic target centering, and the average of all the F1 and F2 readings using your eyes and hands should be the same. Be sure you record each circle reading by hand; also record horizontal and vertical circle readings. You may have to check your instrument manual or ask your manufacturer to find out whether the angle values recorded in memory are different from the ones shown on the display.
A bit more complex procedure for checking your target centering feature is to set up a second target and repeat the same process that you performed with the first target. The averages in circle readings with each method (automatic and manual) to each target, when subtracted from each other, should give you identical angle values within the expected uncertainty of the instrument you are using. When doing this analysis, consider how your instrument's target centering feature is designed and the applications for which it is designed. Some work well in topographic mapping conditions but should not be used for accurate measurement of the angles between traverse legs, or triangulation or resection (free stationing).
On a long leveling loop, a passerby drove his car over our level rod, which was lying on the ground. The rod (a wooden one) snapped around the 0.9 ft mark. My rodman told me that he had seen something similar to this happen when working for another employer, and that his party chief had neatly sawed off the rod at a full foot mark and simply subtracted the number at the "new" bottom of the rod from every reading to enable work to continue with only a small interruption. Is this a valid thing to do?
An undamaged rod is always best to use for leveling, especially if the route is long and the accuracy of the results is critical. However, your rodman is correct in his procedure except that it is unnecessary to subtract the value at the "new" bottom of the rod from every reading. As long as the "new" rod is used for the backsight and foresight(s) at the setup where the accident occurred, you should be able to appreciate that a non-zero value at the bottom of the rod does not affect the difference in elevation determination between the backsight point and the foresight point(s). In other words, if the backsight and foresight would have been 7.00 and 6.00 with the unbroken rod, and it was cut off at the 1.00 point, those readings with the same instrument setup would be 8.00 and 7.00. Both give the same difference in elevation. If you are doing this, make sure that the cut is square, i.e. perpendicular to the long axis of the rod.
Also, when a rod undergoes trauma such as this, it is critical to check all the components of the rod to make sure that nothing else has been damaged. With extending rods, it is particularly important to extend them fully and measure the distance between each pair of foot (or meter) marks to ensure that the stops and locks on the rod haven't moved or been damaged.