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On a sectional chart legend, the airport icon with a solid magenta or blue disk is labeled as "Hard surfaced runways 1500 ft to 8069 ft in length". This seems like an extraordinarily arbitrary number to use as the upper limit before a different symbol is used. When I convert this to meters, or yards, or furlongs, or any other common unit, it doesn't correspond to any even number so it doesn't seem like a conversion from another system of units. What is the reason for this unusual dimension for a runway length?

Sectional chart legend

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  • $\begingroup$ Interesting question........ $\endgroup$ – Carlo Felicione Dec 13 '16 at 20:54
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Apparently you aren't the only person who wondered this; the FAA actually has it in an FAQ:

What is the significance of a runway 8069 feet in length and why are two different aerodrome symbols used to depict hard surface runways on Sectional charts?

For purposes of airport depiction, specialists represent a runway between 7970 and 8069 feet in length as 8000 feet, which equates to a line 0.192 inches in length on the Sectional chart scale. In this case, a circular aerodrome symbol is used.

If a runway is between 8070 and 8169 feet in length, specialists round to 8100 feet, which equates to a line 0.1944 inches in length on the sectional chart scale. This line is too long to fit into the largest circular aerodrome symbol FAA has available. Therefore, specialists place a line-work around the runway pattern forming a polygon (enclosed shape) for anything over 8069 feet in length.

Specialists also place these polygons around the runway pattern of aerodromes with multiple runways that are less than 8069 feet, in cases where the multiple runway pattern does not fit into the largest, circular aerodrome symbol.

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    $\begingroup$ I think that's only a partial explanation. It gives the impression that it's simply something to do with how rounding is applied and the practicalities of drawing charts at a particular scale. And that rounding is biased so that rounding downwards is more likely than rounding upwards. But doesn't actually explain why the rounding works that way to begin with. I imagine that if they used a more traditional rounding approach (8050-8149 -> 8100) then 8049 would be the "magic" number. Why did they decide to round from 8069 instead? $\endgroup$ – aroth Dec 14 '16 at 0:32
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    $\begingroup$ Apparently, when performing some calculations for recommended runway length for a given configuration, "lengths of 30 feet and over are rounded to the next 100-foot interval."... and given the importance of not running out of runway, that does seem to make more sense than rounding +/-50. $\endgroup$ – Michael - sqlbot Dec 14 '16 at 1:58
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    $\begingroup$ Aeronautical Chart User's Guide: "Runway length is shown to the nearest 100’, using 70 as the rounding point; a runway 8070’ in length is charted as 81, while a runway 8069’ in length is charted as 80." $\endgroup$ – Ryan Mortensen Dec 14 '16 at 3:07
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    $\begingroup$ @RyanMortensen Is this different from just subtracting 20 feet before rounding to the nearest 100? $\endgroup$ – Random832 Dec 14 '16 at 4:31
  • $\begingroup$ @Random832 - Nope. $\endgroup$ – Wren Dec 14 '16 at 6:13

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