Which projection uses rays that are perpendicular to a plane tangent to the sphere?

• A. Mercator
• B. Gnomonic
• C. Stereographic
• D. Orthographic

Answer: (D)

Rays perpendicular to a plane tangent to the sphere describe an orthographic projection. In this type of projection, all projection lines are parallel and strike the projection plane at right angles. If that plane is tangent to the sphere, the lines are perpendicular to the tangent plane, which is exactly the setup described. This is different from other projections: gnomonic uses rays emanating from the sphere’s center to a tangent plane, so the lines aren’t parallel; stereographic projects from the opposite pole onto a plane, again not with perpendicular rays to the tangent plane; and Mercator maps onto a cylinder tangent to the sphere, with projection rules that involve the cylinder rather than a tangent plane with perpendicular rays.