Are the AGL numbers measured from the Maximum Elevation Figure listed on the sectional?

  • $\begingroup$ Front the ground, hence AGL $\endgroup$ – Steve Kuo Jan 10 '18 at 21:01

So where is it measured from

Well, quite literally the 700 foot and 1200 foot numbers are AGL as in the distance between your aircraft and the ground at any point. If there is a mountain in the transition area, the 700-foot AGL figure would have a mountain-shaped "bump" in it. It does not take the AGL figure from some highest figure in the area.

The number you refer to, the Maximum Elevation Figure, is calculated based on the highest obstacle in the quadrangle, see the FAA Website chart legend:

enter image description here

So as you can see, the blue number in the quadrangle does not show the MSL of the "sector" (quadrangle), but it is a simple calculation based on the height of the highest obstacle in the quadrangle + 100 feet and some rounding.

The FAA does a similar calculation for natural terrain features:

enter image description here

The big take-away here, in reference to your answer on another question, is that you cannot use this number to determine if you are within Class-G airspace since it takes an elevation figure from some obstacle (man-made or natural) that may not be anywhere near you. So flying at the MEF + 1200 feet does not mean you are in Class-G since, as in the second example, that mountain is a long way away from Ute Park or Red river (at Red River you would be more like 6200 feet AGL).

The AGL of the highest man-made obstacles are shown on the chart underneath the MSL elevation:

enter image description here

But it may be omitted in congested areas of the map and only shown as MSL.

  • $\begingroup$ Since elevations at all points are not listed on a chart how do you know where the class E begins? ATC can't expect you to be carrying a topo map $\endgroup$ – TomMcW Jan 10 '18 at 20:08
  • $\begingroup$ You know where it begins because the map tells you where (either a transition area or within the "fuzzy blue" borders. The hard part is judging your AGL (as you say). VFR flight does not require prior communications for flying in class-E, so you can theoretically bust in and bust out as much as you want without getting somebody mad (although I wouldn't try that in a transition area). $\endgroup$ – Ron Beyer Jan 10 '18 at 20:15
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    $\begingroup$ @TomMcW The VFR sectional shows elevation, it is a topo map $\endgroup$ – Steve Kuo Jan 10 '18 at 21:02
  • $\begingroup$ @SteveKuo It shows elevations of obstacles, but not the ground itself $\endgroup$ – TomMcW Jan 10 '18 at 21:05
  • $\begingroup$ @TomMcW look at the contour lines, e.g. skyvector.com/… $\endgroup$ – Steve Kuo Jan 10 '18 at 21:16

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