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raw_ultrasonic_measure measures the duration of an echo staying high. It is not related to the distance in any way, it is just a duration of trig signal.

You need a time elapsed from the beginning of a signal to reception of echo - that is, a duration of echo being low.

In a quick and dirty way,

      signal_start = micros();
      digitalWrite(Trig, HIGH);

      while (digitalRead(Echo) == 0) {
          ;
      }

      echo_received = micros();
      digital_write(Trig, LOW);

      return (echo_received - signal_start) / (2 * speed_of_sound);

In a real world application you would want to debounce echo, provide for a timeout, etc.

You may also want to account for the time taken by a first call to digitalWrite, but I doubt it will make any perceptible difference.

ultrasonic_measure is in fact a loop:

      double value = 0.0;
      for (int i = 0; i < 5; i++) {
          value += raw_ultrasonic_measure(Trig, Echo);
      }
      return value / 5;

One benefit of such approach is an ability to pass a number of samples as a parameter.