The effective centre of the reflector does not lie on the vertical line passing through the station point on which reflector is being held. This is due to the refractive index of the prism glass causing equivalent air distance through which light travels more than the actual distance traveled. With reference to Figure 10.4, the distance through which the light travels in the glass cube during retro reflection is a+b+c, which is equal to 2t. The equivalent air distance through the light travels is 1.57X 2t on account of the refractive index of the glass. The effective centre of the corner cube is at R and represents the end of the line. Thus, an error c_r , known as reflector offset, gets introduced and needs to be subtracted from the measured distance of the line. The reflector offset is effectively eliminated by advancing the electrical centre of the EDM by a corresponding amount during manufacture.

When slope distances are measured, light rays striking the reflector are not perpendicular to the front face of the reflector and thus altering the path of the rays within the reflector and changing the position of the effective centre. Given the angle of slope, corrections can be calculated to compensate for this error. Some reflectors are designed with an adjustment allowing the front face of the reflector placed perpendicular to the incoming rays of light compensating for the error instrumentally.