# How does TAP™ calculate and use the earth curvature correction?

TAP™ software allows elevation values to be corrected for earth curvature, including atmospheric diffraction effects.

The formula used for elevation correction is:

`Ecorr = E + (78.5/k) x 10-3 x D x (P-D)`
• Ecorr is the corrected elevation in meters.
• E is the actual elevation in meters.
• k is the curvature correction factor to compensate for atmospheric diffraction, typically 4/3 for most radio propagation applications (to ignore atmospheric diffraction, use a value of 1.0).
• D is the horizontal distance along the path from the primary site to the point being corrected (in kilometers).
• P is the distance from the site to the end of the path (in kilometers).

The equation can also be written using British units as:

`Ecorr = E + (D x {P-D}) / (1.5 x k)`
• Ecorr is the corrected elevation in feet.
• E is the actual elevation in feet.
• k is the curvature correction factor to compensate for atmospheric diffraction, typically 4/3 for most radio propagation applications (to ignore atmospheric diffraction, use a value of 1.0).
• D is the horizontal distance along the path from the primary site to the point being corrected (in miles).
• P is the distance from the site to the end of the path (in miles).

Since the term {P-D} is the distance from the point being corrected to the far end of the path, the equation can also be written in terms of the distances D1 and D2:

`Ecorr = E + (D1 x D2) / (1.5 x k)`
• Ecorr is the corrected elevation in feet.
• E is the actual elevation in feet.
• k is the curvature correction factor to compensate for atmospheric diffraction, typically 4/3 for most radio propagation applications (to ignore atmospheric diffraction, use a value of 1.0).
• D1 is the horizontal distance along the path from the primary site to the point being corrected (in miles).
• D2 is the horizontal distance along the path from the point being corrected to the end of the path (in miles).