No you do not have to cut a trace in APM2.6 to use an external compass. The APM2.6 comes without an internal compass so you have to use an external compass that attaches to the APM2.6 I2C bus. Here are the Wiki instructions for attaching the 3DR GPS/Compass module to the APM2.6: http://copter.ardupilot.com/wiki/common-installing-3dr-ublox-gps-co...
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@Martyn,
No you do not have to cut a trace in APM2.6 to use an external compass. The APM2.6 comes without an internal compass so you have to use an external compass that attaches to the APM2.6 I2C bus. Here are the Wiki instructions for attaching the 3DR GPS/Compass module to the APM2.6: http://copter.ardupilot.com/wiki/common-installing-3dr-ublox-gps-co...
Regards,
TCIII ArduRover2 Developer