Point-Plane Distance

Given a plane

(1)

and a point , the normal vector to the plane is given by

(2)

and a vector from the plane to the point is given by

(3)

Projecting onto gives the distance from the point to the plane as

			(4)
			(5)
			(6)
			(7)
			(8)
			(9)

Dropping the absolute value signs gives the signed distance,

(10)

which is positive if is on the same side of the plane as the normal vector and negative if it is on the opposite side.

This can be expressed particularly conveniently for a plane specified in Hessian normal form by the simple equation

(11)

where is the unit normal vector. Therefore, the distance of the plane from the origin is simply given by (Gellert et al. 1989, p. 541).

Given three points for , 2, 3, compute the unit normal

(12)

Then the (signed) distance from a point to the plane containing the three points is given by

(13)

where is any of the three points. Expanding out the coordinates shows that

(14)

as it must since all points are in the same plane, although this is far from obvious based on the above vector equation.

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