Height Measurement Pitfalls

Point-to-point surveying is the practice of locating markable stations so that when the inclination measurement is made the instrument height above station always matches the target height above station.  Usually both heights are near zero (i.e., not actually measured) and neither is recorded in fieldbooks. Many cave surveyors would agree that any deviation from this practice often causes more problems than it solves. For one thing, instrument/target (IT) height measurements introduce yet another likely source for hard-to-detect blunders, such as reversed signs.  (Some of my past surveying efforts are a good demonstration of this.)  Furthermore, data screening with Walls is a bit more complicated since the raw vectors shown in the Traverse page resemble less the original data.

Another problem that comes into play with IT heights is the inevitable taping method confusion.  For me, instrument-to-target is the obvious method to assume by default; for others, station-to-station (the default in SMAPS) is the reasonable assumption.  Getting it wrong will introduce a systematic error, yet I don't recall ever seeing a set of field notes where the taping method was clearly indicated.

Finally, when interpreting fieldbook data, questions about units can arise.  Are the tape and units for IT heights the same as for the distance measurement?  If so, should a correction be applied to the heights if the tape was assigned one?  SMAPS, for example, allows specification of a separate "auxiliary" tape, the units of which determine the units of both the above-station heights and the LRUD passage dimensions.  During processing, however, it appears that the auxiliary tape correction is applied to the LRUD data but not to the height data!  Not only can field notes be ambiguous, but because of all the possible variations in length measurement data, assumptions made by software can be unclear as well.

Personal note: Despite the pitfalls, IT height measurements have been used in most projects I've participated in. I think they're so useful in some caves that trying hard to avoid them makes even less sense.