Crown pillar monitoring projects often involve a combination of Extensometers, Tiltmeters and geodetic measurements.
The length of Extensometers depending on the thickness of the remaining pillar and can be up to 25m
Horizontal Tilt Arrays, usually with 3-5m baselengths (please specify), can be used to monitor the settlement profile in addition to borehole extensometers. They can be integrated into the extensometer array to monitor solid-body translations which will not be monitored by an extensometer within the block. Horizontal Tiltmeter Arrays (HTAs) can eliminate the need for geodetic survey.
Monitoring is usually conducted from surface, most conveniently with wireless telemetry connection to a nearby surface location. 4G cellular uplink is also an option.