High DA Engine died on rollout otherwise awesome 617nm flight!

[MikeOH]

1 hour ago, midlifeflyer said:

Depends how old the T-Shirt is.  Really. The "official" published airport elevation now (Sectional and Chart Supp) is 9934.  But it used to be 9927. 

Hmm, that begs the question: Has there been some 'seismic' or other event responsible for a 7 foot shift, or was the previous number just incorrect?  I know that elevations change a bit over time, but 7 feet seems pretty extreme, BWTHDIK.

[Ryan ORL]

2 hours ago, MikeOH said:

Hmm, that begs the question: Has there been some 'seismic' or other event responsible for a 7 foot shift, or was the previous number just incorrect?  I know that elevations change a bit over time, but 7 feet seems pretty extreme, BWTHDIK.

It almost certainly isn't an *actual* shift in the elevation, but rather survey error and/or an artifact of how we describe elevations above sea level.

Re: 'how we describe elevations', long ago, there was much less standardization of so-called datums, and it's very likely that the prior number was using a different datum.  You can think of the datum as representing something like 'the hypothetical idealized shape of the earth', and as the real Earth is quite irregular, different locations are more accurately represented by different datums.  Measuring with a different datum not only could result in a slightly different lat/lon, but also a different elevation.

Nowadays, everyone (including your GPS, all your maps, etc.) pretty much uses WGS84 + EGM96/2008 Geoid correction now, which is pretty much a 'best fit' for everywhere on earth, and the EGM geoid models (especially 2008) are quite detailed now.  Also, we now have highly accurate satellite-measured elevation data for the whole planet.  So until they decide to revise the ellipsoid and/or geoid model again, these numbers are unlikely to change much.

edit: This is also evident in, for example, anyplace that puts monuments to denote lat/lon locations such as the equator and such.  These can and do move over time since we have gone through a few datums over the years.

[midlifeflyer]

23 hours ago, MikeOH said:

Hmm, that begs the question: Has there been some 'seismic' or other event responsible for a 7 foot shift, or was the previous number just incorrect?  I know that elevations change a bit over time, but 7 feet seems pretty extreme, BWTHDIK.

Beats heck out of me. Airport elevation is defined as the highest point on the usable runways.  Aside from changes in measurement metrics as @Ryan ORL suggests, off the top of my head I can envision runway resurfacing and extensions as well as actual shifts in the earth (it is the Rockies after all). The only thing I know is that it's not unusual.  I found a March 1968 Sectional on the Library of Congress site. Here, side by side, is Lincoln, NE with a 21 foot change in published elevation between then and now.

 

[Ryan ORL]

Yeah that large change is definitely datum related, especially with the old number being in the 1960s.

If anyone is interested in the geodesy nerd type trivia of how this all works...  

Think about how we measure something like "altitude above mean sea level".  You can't directly measure your height above it... it's not like we can drill down and find a magic plane in the earth that represents "where sea level would be" at any given point.  Also how do you even decide what it might be, accounting for tides, etc?

The answer is, we develop a mathematical model that approximately represents the shape of the earth, which is an oblate spheroid aka an 'ellipsoid'.  (Because you can generate the shape by revolving an ellipse around it's semi-minor axis)   Careful measurements of the earth have determined that the Earth is very approximately 6378137.0 meters wide at the equator and 6356752.314245 meters tall at the poles.  (Earth is slightly squished)  These measurements have changed several times over the years, which result in slightly differently shaped ellipsoids.

Then, because differences in local density (and therefore surface gravity) exist, the actual "mean sea level" does not rest evenly on the surface, and the deviations here are as much as +80/-110 meters or so from the otherwise idealized geometric shape described above (which, if you think about it, is a fairly large deviation!).  These deviations are best described by a grid with thousands of squares, with each square having an offset which represents the deviation from the idealized shape expressed as meters plus or minus.

So you take the perfect mathematical shape, and you overlay this lumpy grid onto it, and in theory, that now represents the theoretical surface at which the sea surface would exist if Earth was a featureless mass covered entirely in water.  Or a more accurate way to describe it is, that theoretical surface is an "equipotential surface", i.e. a surface where all points have the same gravitational potential energy.

Now, you want to know how far you are above that magic point, it all comes down to accurate measurement.  We somehow (via traditional surveying or these days satellite-derived methods) measure your absolute position in space, and we subtract away the theoretical height of sea level at that exact position on the Earth.

Since the theoretical height of the sea level keeps changing as we get better measurements of the Earth, this difference keeps changing, but it's all fairly standardized now and the actual changes are getting smaller and smaller.

edit: An interesting result of this stuff is that it is entirely possible for two points which are both "at sea level" on the equator to be physically as much as ~200 meters different in terms of their actual distance from the center of the Earth (Geoid min to max) and points "at sea level" on the equator are as much as 21 kilometers further from the center of the earth than at the poles.