An apparatus for measuring the velocity of a moving object or a collection of moving objects moving in a flow along the flow's stream-lines in a predetermined direction and at a predetermined distance, the apparatus comprising: a generator for generating excitation signals, an emitting transducer for transforming said excitation signals into wave energy and for emitting said wave energy in a predetermined direction of propagation, a receiving transducer for receiving, from said moving object or objects, signals generated by interaction with said wave energy emitted from said emitting transducer, wherein said emitting transducer and said receiving transducer comprise an array transducer including a plurality of transducer elements, wherein said emitting transducer and said receiving transducer have respective sensitivities which in combination give a focused sensitivity focused in a plurality of focal points on a line in the predetermined direction at the predetermined distance, and wherein the apparatus is configured to: obtain a representation of the line by computing a value for each focal point on the line, where a value for a focal point is the sum of values obtained from the elements of the array transducer at respective times, perform multiple emissions to get a representation of the line at different times, and perform correlation of representations of the line at different times to provide a measurement of velocity magnitude characterized in that the plurality of focal points is arranged along the flow's stream-lines, wherein the predetermined direction is inclined from the direction orthogonal to the propagation direction. a generator for generating excitation signals, an emitting transducer for transforming said excitation signals into wave energy and for emitting said wave energy in a predetermined direction of propagation, a receiving transducer for receiving, from said moving object or objects, signals generated by interaction with said wave energy emitted from said emitting transducer, wherein said emitting transducer and said receiving transducer comprise an array transducer including a plurality of transducer elements, wherein said emitting transducer and said receiving transducer have respective sensitivities which in combination give a focused sensitivity focused in a plurality of focal points on a line in the predetermined direction at the predetermined distance, and wherein the apparatus is configured to: obtain a representation of the line by computing a value for each focal point on the line, where a value for a focal point is the sum of values obtained from the elements of the array transducer at respective times, perform multiple emissions to get a representation of the line at different times, and perform correlation of representations of the line at different times to provide a measurement of velocity magnitude characterized in that the plurality of focal points is arranged along the flow's stream-lines, wherein the predetermined direction is inclined from the direction orthogonal to the propagation direction. obtain a representation of the line by computing a value for each focal point on the line, where a value for a focal point is the sum of values obtained from the elements of the array transducer at respective times, perform multiple emissions to get a representation of the line at different times, and perform correlation of representations of the line at different times to provide a measurement of velocity magnitude characterized in that the plurality of focal points is arranged along the flow's stream-lines, wherein the predetermined direction is inclined from the direction orthogonal to the propagation direction.