[TowerTalk] FAA & Private Airstrips

[Jim Lux]

On 12/30/11 9:45 AM, Al Kozakiewicz wrote:
> That's really interesting.
>
> Jim Lux already mentioned the pitfalls of GPS and barometric measurements of altitude.  The technology of ILS doesn't require a functioning altimeter to land and I don't believe IFR rules allow landing without some minimum visibility - enough to render a functioning altimeter "optional" on approach.  The point being that no one expects a barometric altimeter to be accurate enough to be the only reference for height above ground as you land.
>
> Many, if not most, USGS elevation benchmarks were set using a rod and level starting in the 19th century.  They are surprisingly accurate considering that those in the middle of the country are more than a thousand miles from the nearest ocean.
>

USGS references to NAVD29.  A note I have says:
"Regarding the actual leveling undertaken by USC&GS/NGS, the 1929 
adjustment (NGVD29) entailed about 75,000 km in the U.S. and about 
32,000 km in Canada. By contrast, the 1988 readjustment (NAVD88) 
involved re-running much of the first-order vertical control network -- 
about 81,500 km -- to First Order Class II specifications."

First Order survey is typically good to 1 ppm.. so, for 1000 miles, 
that's good to 5 feet.

Imagine doing this carrying all your gear on horse/mule, etc.  Those 
guys were tough.

http://www.ngs.noaa.gov/web/about_ngs/history/ is an interesting source.

(A great book: "The Great Arc" describes the Great Trigonometric Survey 
of India.. 5 foot diameter theodolites, precision chaining, crew eaten 
by tigers, etc.)


> As an aside, I thought I had read that Google Earth had a problem with elevations.  I don't remember the exact circumstances, but the context was some sort of modeling as GE assumes a spherical earth. Basically, GE underestimates altitudes and sea levels at low latitudes due to the "bulging" of the earth at the equator.  That sound familiar to anyone?