1. A method for passively determining altitude, comprising: receiving, at an image processing device, a first image from an imaging device, the imaging device having a known instantaneous field of view (IFOV); receiving, at the image processing device, a second image from the imaging device; receiving a groundspeed indication from a groundspeed system that is configured to determine a groundspeed (GS) of an aircraft; determining a Δt as a difference between in time between the first image and the second image; selecting at least one common pixel of the first image and the second image; determining a relative change in position of the at least one common pixel between the first image and the second image to determine an image distance (ID); and calculating altitude above ground level (AGL) above a portion of a surface depicted by the at least one common pixel using the following relationship: AGL=[(GS*Δt)/(IFOV*ID)]*cos (θ), wherein θ is a Nadir angle.