to mesure the earth resistance we need to find a specipic 
constant of the earth soul.
so by using a formula.

                       R=P(L/A) Ohm
where,
      p = Secific gravity constant
      L = lenth of Earth
      A = Area of Earth.

Dear Friend
the R = Pl/A is not used to measure earth resistance. is is 
used to measure conductor resistance.
to measure earth resistance. We need an DC power source, 
Ampere meter & volt meter & the formula R = V/I
The earth resistance testers are available in market, they 
are been used by three leads on connected to the test 
object & Current & Potentional probes are conneceted 20 & 
10 meters away from object under test.

using two methods:
1.fall of potential method
2.earth tester(device)

factors affecting earth resistance are
1. Shape & material of the earth electrode.
2. Depth of the electrode at which they r burried in soil.
3. Specific resistance of the soil surrounding the electrode

(Specific resistance of the soil varies depending on 
moisture content in the soil..... varies from 80*10^3ohm-m 
to 80*10^6ohm-m)

earth resitanse is also measured by meggar

Bu meggar