The hyperbolic paraboloid and the hyperboloid of one sheet are doubly ruled surfaces. A hyperboloid of one sheet is "projectively" equivalent to a hyperbolic paraboloid. The crown of the famous parabolic vault is a hyperboloid. A straight line in hyperbolic "n"-space is modeled by a geodesic on the hyperboloid. Lattice towers are often designed as either a space frame or a hyperboloid structure. One-sheeted hyperboloids are used in construction, with the structures called hyperboloid structures. The two-sheet hyperboloid has one positive eigenvalue and two negative eigenvalues. The one-sheet hyperboloid has two positive eigenvalues and one negative eigenvalue. It is an orthographic projection of the hyperboloid model onto the xy-plane. The hyperboloid is stable under , illustrating the isomorphism with . The first hyperboloid cooling towers were built in 1918 near Heerlen. This line intersects the hyperboloid H in a unique point. This page is a list of hyperboloid structures. Each TDOA corresponds to a hyperboloid of revolution (see Multilateration). For simplicity the plane sections of the "unit hyperboloid" with equation formula 12 are considered.