blow a backsight
To occupy a point, then enter the wrong point for the backsight, or forget to enter the backsight.
blow a backsight: record the wrong setup point by mistake
To blow a backsight is to occupy a survey station, take observations back to the previous point, and then record or enter the coordinates of the wrong point into your notes or data collector. The error happens after the measurement is already made, not during it. You stand on point B, sight back to point A, collect the angle and distance correctly, but then write down point C's location as your backsight, or fail to record any backsight at all. The field work is sound; the documentation fails.
This mistake usually surfaces when the surveyor or technician is distracted, working in poor light, or juggling multiple instruments and field notebooks. A common variant is forgetting to enter the backsight entirely, leaving a gap in the setup record. Another is transposing a digit in the point number: backsighting to 1042 instead of 1024. The error is not obvious in the field because you have already collected valid observations; it only becomes apparent when the data is processed or when a later setup tries to reference the missing or wrong backsight.
Why it matters and how to catch it
A blown backsight breaks the geometric chain that ties your traverse or radiation together. If point B's backsight is recorded as C instead of A, then point B's orientation is referenced to the wrong direction. All subsequent observations from point B will be rotated or skewed. The error propagates through the survey, distorting areas, bearings, and coordinates downstream. Checking closure, plotting intermediate results, and reviewing field notes against the data immediately after collection are the main defenses.
Modern data collectors with drop-down menus for point selection have reduced this error, but it still occurs when points are manually typed in or when the technician bypasses the interface. The term is part of surveyor slang because it captures a specific type of careless mistake, distinct from calculation errors or instrument misalignment. It is survivable if caught early, which is why many surveyors re-enter the station to verify the backsight before moving on.