It is not; there are more precise measurements.
It depends on the size and mass of the rock. For a "normal" rock (as opposed to merely a stone) I suggest a crane with some means for measuring the rock's mass. A number of pulleys, each with a weighing machine should do the trick. Alternatively, you could use a weighbridge, if there is one in the vicinity. To obtain its volume, you probably need a 3-d laser-profiling device. That should give the most precise measurement of its volume.
foot is the most precise way to measure any unit
2.5cm is the most precise measurement of the two measurements.
98.6 degrees is most precise.
98.6 degrees is most precise.