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Let's say a plane flies 10m above some trees, that themselves are 10m above the underlying ground. Would that be 10m or 20m AGL?
Or, in more abstract terms: What exactly is the reference point for AGL? The actual ground that you would stand on as a pedestrian? The point where you would first impact something (even if it's just a very lightweight sprig)? something else?
How does it work with objects like high bridges, that you could fly below but could still impact on a higher altitude?
AGL is "Above Ground Level". Ground is the reference. Trees, buildings, towers, and such are obstacles. Pilots need to be aware that obstacles exist. Most of the smaller ones can be addressed by just flying at a safe altitude. This is covered (in the US) by the following Part 91 rule:
§91.119 Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
(d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—
(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA; and
(2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph (c) of this section.
Following (a) and (b) is the safest course. Para (c) provides a great deal of latitude to fly low but has significant risks. Agricultural pilots fly very low all the time. But ask any one of them and they'll be glad to discuss all the stuff they've hit.
These rules are primarily directed at VFR flight. Under IFR, ATC clearances are designed to ensure operating at safe altitudes, clear of both terrain and obstacles.
For larger obstacles such as towers and bridges, they are charted. As you can see on the clip below:
There are a number of towers in a relatively small area. In this case there is the underscored BITHLO TOWERS label to bring attention to them. Adjacent to the tower icons are two numbers. The number not in parentheses is the height in MSL. In parentheses is the height AGL.
The AGL is mostly taking reference to the Radio Altimeter. The RA is measuring the distance between the ground and the aircraft through waves sent to the ground and receive the rebound of it. The time between the ping and the pong gives you the distance between the aircraft and the ground (like a sonar).
See: https://en.m.wikipedia.org/wiki/Radar_altimeter
That being said,the reference of AGL is random. Because the RA should catch the top of the tree and if you were flying at a constant ASL with trees or hills below you, you will see your RA change all along your leg.
