Note how barometric pressure and MAP values compare to Vacuum readings. Absolute pressure vs vacuum gauge pressures are seldom explained. Absolute pressure is referenced to zero pressure, as in outer space. Vacuum gauge pressure is the pressure relative to ambient or atmospheric (baro) pressure. So vacuum gauge pressure is usually given as Pabs-Patm omitting negatives (I really don't know what this means in physics lingo). What I do understand is: the MAP scan tool value is the pressure difference between the engine's intake manifold (vacuum) pressure and atmospheric (baro) ambient pressure. When the engine is revved, MAP goes to atmosphere (absolute) while the vacuum gauge goes to zero. At idle the vacuum gauge's reading plus the scan tool's MAP value gives adds up to ambient or barometric pressure. This of course is impacted, to some degree, by altitude. These readings are what we see at 4,900 ft. elevation.