AGL Vs. MSL: What Are The Key Differences Between These?
AGL and MSL are the two terms used in flying airplanes. These are used to represent the types of altitudes in aviation. Determining how high a pilot is flying an airplane is not exactly possible. AGL is known as absolute altitude. AGL is an abbreviation of "above ground level." According to AGL, the height at which an airplane is flying is estimated with reference to ground level.
MSL is the abbreviation of "mean sea level." According to MSL, the altitude at which an airplane is flying is estimated compared to the sea level. When flying an airplane, the pilots are always concerned about the altitude at which they are flying. Whenever they refer to the altitude, it is in accordance with the sea level, i.e., MSL.
The pilot is taught about AGL and MSL when he is starting pre-flight planning. He is well aware of the fact that how much above the Earth's surface he is going to fly. There is an IFR chart that tell about altitude. In this chart, the AGL value is calculated from MSL. As MSL shows height from sea level, its AGL shows height from the bottom of the sea to the airplane.
The weather conditions are also closely linked with AGL. The closer your airplane is to the ground, the more extreme weather conditions you would be facing. AGL and MSL are both essential in depicting the weather conditions. AGL and MSL are essential for pilots to fly an airplane appropriately and for a safe landing.
The AGL measurement changes as the topography of the ground are changes when an airplane is flying directly above it. It means that if an airplane is flying 20,000 feet above the Earth's surface and a mountain of about 20,000 feet comes in its way, the altitude of the plane with reference to the ground would become zero, so the AGL would be zero.
Similarly, the measurement of MSL changes with the tidal level and seasonal changes of the water level of the sea. When AGL is used as a reference point for measuring altitude, it means that the altitude of an airplane is measured with direct reference to the ground, and the height measured in digits is accurate.
On the other hand, when MSL is used as a reference point, it means that the altitude of an airplane is measured, taking the sea level as a reference. Still, actually, the height measured in digits is not accurate according to the ground level. It is because the sea level is well above ground level.
The benefit of using ground or sea as a reference to measure the altitude of an airplane is that the pilot knows his position above the Earth's surface and the position of other aircraft, so there is less chance of collision and air accidents. The MSL value is assumed to be constant universally.
The sea levels differ somehow with every region, but the difference is so minute that there is a globally accepted standard of sea level used by all pilots. In comparison, the AGL value differs immensely with every region because there is a great difference in earth level in various world regions.
MSL is used as a reference by an airline pilot, whereas AGL is used as a reference by a drone pilot. Drone pilots do not use AGL as a reference because it provides it may give them a false sense of security, so they do not rely much on it. MSL reference is a more secure option for them. AGL and MSL are used in correlation to determine an average value of altitude. The altitude of tall objects is measured with reference to AGL and MSL.
AGL is more dependent on atmospheric and weather conditions, whereas MSL depends upon the air and tidal pressure only. AGL is not used as a reference by the drone pilot because while flying, the earth's surface changes very quickly as there are mountains, hills as well as trees. So, the drone pilot has to look for the obstacles himself, and he could collide with something, so he is dependent more on MSL for navigation and measurement of altitude.