In this equation, the origin is the midpoint of the horizontal range of the projectile, and if the ground is flat, the parabolic arc is plotted in the range 0 ≤ ≤ ϕ ϕ ≤ ≤ π π . This expression can be obtained by transforming the Cartesian equation as stated above by y = r sin ⁡ ⁡ ϕ ϕ and x = r cos ⁡ ⁡ ϕ ϕ .