פילוס דיפרנציאלי

Consider point A, which is a benchmark with an elevation of 60 meters. 

To determine the difference in elevation of another point C located away from point A, differential leveling is performed.

First, set up a level at a convenient location near point A and take a reading on the level rod positioned at point A. 

This reading, recorded as the back-sight in level notes, is used to calculate the instrument's height.

Next, the rodman moves towards point C and places the level rod at point B. 

The surveyor takes a reading at point B, records it as the foresight, and determines point B's elevation. 

The surveyor then shifts the level to a new location closer to point C, takes a back-sight reading to point B, and uses it to calculate the new height of the instrument. 

Finally, the rodman places the level rod at point C to record the foresight, which is used to calculate point C's elevation. 

The difference in elevation between points C and A is then determined.