You only need elevation if you don't know the local air pressure, just the sea level pressure. In other words, you either need sea level pressure and elevation, or local pressure at the location in question. The latter is more accurate since the former relies on an approximate model of the atmosphere - in reality pressure decreases with height according to the complete temperature profile of the intervening air mass, something you probably don't know.
Helicopter flight manuals normally use graphs that allow entry of pressure and temperature. Normally dew point (humidity) isn't taken account of because it's affect is small compared to other errors / approximations.