The stadia method determines the horizontal distance to points through the use of readings on the upper and lower $($stadia$)$ wires on the reticle. The method is based on the principle that in similar triangles, corresponding sides are

$.$target $1$ of $4$

Thus the equation for a distance on a horizontal stadia sight reduces to D$=$KI $().$target $2$ of $4$

In differential leveling, the actual sight distances to the rod

important. All one needs to balance is the rod intervals on the plus and minus sights between benchmarks to ensure that the sight distances are $.$