Instrument Corrections

INCD / INCA / INCAB /  INCV / INCVB = <tape/instrument correction>

INCH = <height adjustment>

The expression INCx=n, with n a decimal number and x one of D, A, AB, V, or VB, specifies a correction that the program will add to the corresponding measurement entry when it is processed. Those measurement entries are, respectively, Distance, Azimuth, Azimuth Backsight, Vertical angle, and Vertical angle Backsight.  These corrections will be applied only to compass and tape (CT) data lines.

Note that only the distance correction, INCD, will be applied to pure vertical shots, where the inclination is +90 or -90 degrees.  Also, corrections are not applied to zero heights or distances, both of which are allowed.  (Zero-length vectors can be defined.)  However, if applying a correction to a distance would cause a change of sign, an error message is displayed.

Like measurements on data lines, the correction units can be given explicitly with suffixes (e.g., IncA=1.35d).  Otherwise, the units are assumed to be the current default for the corresponding measurement.  In this case, if measurement units are later changed without a corresponding change in the correction, the correction keeps its original meaning.  For example, a six-inch tape correction will stay six inches, even after switching to a meter tape.  (This is done to be consistent --  we don't want some types of settings to be automatically zeroed when their units change.)

The height adjustment, INCH, is a special case in that it is not an instrument correction, but a vertical adjustment that will be applied to all non-vertical shots.  It is available to compensate for a possible systematic error in vertical target positioning -- something I've personally encountered in a few highly interconnected surveys where techniques and personnel remained constant. Its default units are the same as that of the distance measurement.  For example, "INCH=0.04m" would cause four centimeters to be added to the computed elevation of the TO station with respect to the FROM station.  The mainframe program, Ellipse, actually provided a "confidence interval" for this height error as a product of the adjustment.  Walls currently doesn't do this, but you can get the same result by trying out different INCH values while homing in to the smallest vertical UVE.

For an illustration of the effects of INCA and INCH corrections, see Tree Survey Example.