Using surfaceToTransducer to calculate water depth from surface not bottom of the keel.
(eg. Airmar DST810 provides surfaceToTransducer.)
Current DPT sentence uses transducerToKeel which has to be derived (in my case) and measures the depth at the bottom of the keel not the depth of the water.
https://gpsd.gitlab.io/gpsd/NMEA.html#_dpt_depth_of_water
Personal preference... for extra confusion DPT allows for either.
Using surfaceToTransducer to calculate water depth from surface not bottom of the keel. (eg. Airmar DST810 provides surfaceToTransducer.)
Current DPT sentence uses transducerToKeel which has to be derived (in my case) and measures the depth at the bottom of the keel not the depth of the water. https://gpsd.gitlab.io/gpsd/NMEA.html#_dpt_depth_of_water
Personal preference... for extra confusion DPT allows for either.