Partial excerpt from survey notes / 3 wire leveling, benchmark circuit corrections and survey planning

 EDM - Electronic distance meter built in Total Station - uses infrared light (Emitter) to Prism (reflector) and it comes back to receiver.

Station Backsight HeightofInstrument Foresight Elevation

for every reading whether its a backside or foresight, were actually taking 3 readings. for instance

the BS on the first BM 9904 the 3 readings are 2.90 2.29 1.68, those are on the upper stadia hair , center horizontal crosshair, lower stadia hair. 

we use all 3 readings and we will average 2.290. also we must compare interval, between upper hair and center to distance between lower hair and center. should be equal. in this case both are 0.61

2.90-0.61 = 2.29 = 1.68+0.61 . თუ არ დაემთხვა მაშინ სამივე თავიდან უნდა შემოწმდეს და ციფრები ჩასწორდეს.

DIST(distace can be computed simply subtract lower hair reading from upper hair reading (1.22) and multiply that difference by 100 so = 122)

notice that since we are using averages computed to 3 places, we're going to carry our elevations and HIs to 3 places (tp1 tp2 tp3) 

as u can see  HI (tp1 row) we have expressed it to 3 places and BS that was used to compute that HI the average of 3.127 is defined more finely than the smallest increment (ნამატი) on the rod when we average these 3 readings (4.04, 3.13, 2.21) , then the result of 3.127 will be the reading that we use to compute HI. likewise for FSs and use that as the basis for our calculation