Distance Between Two
    Points
 Pacing
 Chaining
 Temperature
 Standardisation
 Catenary
 Slope
 Tension
 Tachaeometry
 Stadia System
 Measurement of
    Tacheometric Constants
 Refraction and Curvature
 Theodolite
    Tachaeometry
 Field Tachaeometry
 Electronic Tachaeometry

Measurements of Tacheometric Constants

The values are usually given by the makers but this is not always the case. It is sometimes necessary to measure them in an old or unfamiliar instrument. The simplest way, both for external and internal focussing instruments, is to regrad the basic formula as being a linear one of the form:

D = Cs + k

1. On a fairly level site chain out a line 100 to 120m long, setting pegs at 25 to 30 metre intervals.

2. Set at up at one end and determine two distances using tacheometer or theodolite, one short and one long. hence C and K may be determined.

   i.e.	D1 (known) = Cs1 (known) + k
   	D2 (known) = Cs2 (known) + k

Distance	Readings			Intervals
	Upper Stadia	Centre	Lower Stadia	Upper	Lower	Total
30.000	1.433	1.283	1.133	0.150	0.150	0.300
55.000	1.710	1.435	1.160	0.275	0.275	0.220
90.000	2.352	1.902	1.452	0.450	0.450	0.900

D =Cs + k

30.00 = 0.300 * C + k

90.00 = 0.900 * C + k

therefore C = 100 & K = 0

Any combination of equations gives the same result, showing that the telescope is anallatic over this range, to all intents and purposes.