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I am about to start my PPL soon (still flying in FSX) and I do not understand one thing. As per FAR 91.159 if my altitude is above 3000' AGL I must maintain certain altitude depending on my course, BUT what would be my reference point to know that I am above 3000' AGL? Departing airport? MEF from chart? or just look out my window? Thanks!

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  • $\begingroup$ Find the highest point near you on the chart (building or mountain), add 3000 ft to its elevation and stay above that. $\endgroup$ – Peter Kämpf Feb 8 '18 at 8:02
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    $\begingroup$ AGL means above the point you are flying over, hence this is an irregular surface parallel to the ground, measured using a barometer. But you don't know the local QNH in general, so you can use the regional QNH or the QNH of the highest point in the area. $\endgroup$ – mins Feb 8 '18 at 13:09
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    $\begingroup$ As far as I know AGL really means AGL and not as measured from the highest point in the grid. Unless you are flying in an area where ground height varies a few thousand feet in a few miles, you will be fine with the altitude of the departing airport or the contour colors as depicted in the sectional. $\endgroup$ – Prashant Saraswat Feb 8 '18 at 21:48
  • $\begingroup$ I use ForeFlight to let me know my AGL altitude. You can display your current AGL and, with the profile view, the AGL on the route. Before I started using ForeFlight, if I was not sure of the height of the terrain I was going to fly over, I’d use my Garmin GPS to toggle over the map. And before that, just guess based on the chart, nearby reference points, and the contours. $\endgroup$ – JScarry Feb 11 '18 at 18:30
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FAR 91.159 speaks for itself. The reference point is directly below your current location.

There are other regulations that use "AGL" (sometimes stated as "Above the surface") as a reference (see FAR 91.155 or 91.119).

As a side note, FAR 91.159 is used to mitigate the risk of midair collisions but not at the expense of prohibiting necessary close to the surface operations (e.g., helicopter EMS, telephone line patrol, crop dusting).

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On bigger aircraft there's a radar altimeter that gives you precisely that (AGL). Typically the radar altimeter reading is only shown when below 2500ft or something similar. In little planes you don't have that information.
You estimate. Using a sectional chart the MEFs are one way to go, but we should think about what the AGL figure is meant for.
For instance MEFs will include buildings, towers, anything that raises obstructions to flight.
But that's not AGL. AGL would be above the ground.
For instance there are rules on maximum speed near airports up to a certain altitude. For that I would use the airport elevation, since its meant to produce a veil around the airport with a fixed height. AGL is used to make it clear that you are not looking for MSL altitude.
If its something to do with terrain clearance then MEF for VFR and MSA for IFR flight is clearly the best option.
And there's the rule about separation from ground objects. Those are to be estimated visually and given a wide berth !

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