Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 378

Magnetic deflections caused by iron pipes, rails or anomalies due to ore deposits will
sometimes cause compass readings to be unreliable. In such cases the data should be
corrected to true north before evaluation of the readings (ISRM 1978e). If the local
azimuthal deviation of the field lines of the earth's magnetic field from the meridian
direction, referred to as “declination”, is known, the correction may be undertaken by
rotating the compass circle with the declination adjuster.
Discontinuity orientations can also be measured using a smart phone by means of
an app. The smart phone is contacted to a discontinuity surface in any orientation.
After touching the record button, dip direction and dip angle, as well as location of the
discontinuity, will be saved. As an example, Fig. 13.22 shows the use of such an app,
referred to as GeoID (see also Section 11.3.2).
Figure 13.21 Measurement of dip angle and dip direction by means of the Clar compass (Wittke 1990)