A control system in combination with a tool carried by a construction apparatus, said control system for establishing grade and elevation control for the tool and for maintaining the vertical position of the tool relative to a reference, the construction apparatus including at least one positioning device that responds to signals from said control system to adjust the position of the tool, said control system comprising: a sensor assembly being arranged to be mounted on the construction apparatus; a first ultrasonic transducer configured to be carried by said sensor assembly, said first transducer being arranged to transmit and receive acoustic waves; a second ultrasonic transducer configured to be carried by said sensor assembly and spaced a distance from said first ultrasonic transducer, said second transducer being arranged to receive acoustic waves; a first reflectors and a second reflector, said second reflector reflecting an acoustic wave from said first reflector to said second transducer; a controller arranged to drive said first transducer to produce an acoustic wave, to monitors the time reflections of the acoustic wave received by one or both of said first and second transducers, to determine the local speed of sound based on a first in time reflection from said reflectors (72,74) received by said second ultrasonic transducer acting as a receiver only, to determine the distance from the construction tool to a reference based on the local speed of sound and a second in time reflection received by both of said first and second ultrasonic transducers and to produce at least one electrical signal as a function of the distance of said ultrasonic transducers from the reference, characterised in that, said reflectors (72, 74) are placed on an emitting surface or on or near screens (52a, 54a) overlying the emitting surface of the respective first and second transducers (52, 54) so as not to protrude from the body of said sensor assembly and wherein at least one of said first and second reflectors is positioned in close proximity to one of said first and second transducers, and in that at least one of said first and second reflectors (72,74) comprises a cylindrical member (76,78). a sensor assembly being arranged to be mounted on the construction apparatus; a first ultrasonic transducer configured to be carried by said sensor assembly, said first transducer being arranged to transmit and receive acoustic waves; a second ultrasonic transducer configured to be carried by said sensor assembly and spaced a distance from said first ultrasonic transducer, said second transducer being arranged to receive acoustic waves; a first reflectors and a second reflector, said second reflector reflecting an acoustic wave from said first reflector to said second transducer; a controller arranged to drive said first transducer to produce an acoustic wave, to monitors the time reflections of the acoustic wave received by one or both of said first and second transducers, to determine the local speed of sound based on a first in time reflection from said reflectors (72,74) received by said second ultrasonic transducer acting as a receiver only, to determine the distance from the construction tool to a reference based on the local speed of sound and a second in time reflection received by both of said first and second ultrasonic transducers and to produce at least one electrical signal as a function of the distance of said ultrasonic transducers from the reference, characterised in that, said reflectors (72, 74) are placed on an emitting surface or on or near screens (52a, 54a) overlying the emitting surface of the respective first and second transducers (52, 54) so as not to protrude from the body of said sensor assembly and wherein at least one of said first and second reflectors is positioned in close proximity to one of said first and second transducers, and in that at least one of said first and second reflectors (72,74) comprises a cylindrical member (76,78).