These methods were joined by radar in World War II; while this could detect a shell in flight the gun that fired it could not usually be seen and the shell's elliptical trajectory made it impossible to extrapolate backwards with the technology of that time. However, mortar bombs have a parabolic trajectory (as do guns firing in 'high angle') defined by a simple mathematical equation with two points on the parabolic curve. It was therefore possible to deduce a mortar's position by tracking its bomb and recording two points on its trajectory. Another method that emerged was crater examination, this could reveal the azimuth back to the hostile gun or mortar and study of fragments could reveal its type. However, while it was a useful source of information it was not sufficiently accurate to give a location for the firer.