"For example, with Regional QNH set and the altimeter indicating 2000ft the aircraft should be at 2000ft amsl or slightly higher. No aerodrome QNH in that region will be lower than the value of the Regional QNH."An alttitude "above mean sea level" (AMSL) is true altitude. Notice it is NOT an absolute ("above ground level" (AGL)) altitude, as the actual sea level may be above or below mean sea level (tides etc.).Now, imagine that actual sea level is higher than mean (high tide, storm, etc.), and you can see that your AGL altitude (one example) would be lower (read less than) true altitude, and your indicated altitude. There are other cases as well, but I'm too busy to detail an example right now. "If, as in the example I gave, two aircraft use their closest reporting stations then could be a loss of separation when they meet half way between them if the stations report different QNH."You are absolutely correct, if two aircraft are at the borders of the "reporting stations" or "regions" and the regions have different QNH (in the US we call that altimeter setting) then the altitude separation will not be the difference between their indicated altitudes. This is one reason why the required minimum vertical separation is 1,000 feet (in the US at least). "Where are the regional boundaries in the US and how are they defined?"They are not clearly defined (with lines on a chart) as they are in the UK, VFR pilot's are just required to use the "nearest" reporting station and if memory serves me correctly within 100 nm of the aircraft. If no station is that close (you are flying over Area 51 and don't have the freq's of the military station that does not exist) you would use 29.92, if I recall correctly. (See the AIM on the FAA web site for details).So, they are an imaginary system. When IFR, the controller gives you the altimeter setting to use as you enter and fly through their airspace, and maintains your separation for you.

"An alttitude "above mean sea level" (AMSL) is true altitude. Notice it is NOT an absolute ("above ground level" (AGL)) altitude, as the actual sea level may be above or below mean sea level (tides etc.)."It is an absolute level because actual ground levels are referenced to mean sea level. My original post, that you said was wrong, referred to ground level. End of discussion.

>"An alttitude "above mean sea level" (AMSL) is true>altitude. Notice it is NOT an absolute ("above ground level">(AGL)) altitude, as the actual sea level may be above or below>mean sea level (tides etc.).">>It is an absolute level because actual ground levels are>referenced to mean sea level. My original post, that you said>was wrong, referred to ground level. End of discussion.I love how you do not quote the part of your original post, or the definitions to support your statements. Of course, you can not do so when the facts do not support what you are claiming. Again I refer you to the post of definitions above:TRUE ALTITUDE - The exact distance above mean sea level (MSL). ABSOLUTE ALTITUDE - The exact altitude of an aircraft above the surface over which it is flying.The altitude, as you have clearly also stated, is above mean sea level and hence true altitude, not the altitude above the surface (the water) and therefore not absolute altitude. In fact, the only time that true altitude would be the same as your absolute altitude over the water would be a brief period during which the actual sea level happened to match the mean sea level.So, again you are in error. But then these are confusing issues even to licensed pilots, and you probably have not trained and recieved your pilots license. Even if you had, don't feel bad, most pilots do not have this level of understanding of these issues, and fly along quite happily just following procedures.