What altitude is maintained after a Portland Low departure at AGL?

The correct altitude maintained after a Portland Low departure is 500 feet MSL (Mean Sea Level). This altitude is specifically designated for the initial stage of the departure procedure to ensure safe vertical separation from obstacles and other traffic in the area. After reaching this altitude, departures will usually transition to a higher altitude or continue with the specified flight plan, which ensures that they remain clear of any potential hazards within the airspace.

Typically, initial altitude restrictions like this are part of published Standard Instrument Departures (SIDs) that enhance safety and efficiency for both pilots and air traffic control. This specific altitude allows aircraft to gain altitude safely and avoid conflicts as they depart from busy airports like Portland. The other options may represent altitudes used in different contexts or other procedures but do not apply as the initial altitude following a Portland Low departure.