A graphical derivation of the perpendicular part of the ray Jacobian for planar rays is presented and found to match that derived via paraxial raytracing. The graphical method provides the reader with a clear picture of how the ray Jacobian can be factored (for planar rays), how the perpendicular part arises, and that it is easily computed if the ray paths are known. The Jacobian is incorporated into a transfer function that approximately converts 3D measured data into 2D equivalent data that is useful in 2D full‐wave inversions.

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