Evening star. [volume] (Washington, D.C.) 1854-1972, 1929-01-20
ALTITUDE FIGURES
!. GET COMPARISON
Indicated Recording Barom-
| eter Method May Be
Inaccurate.

Indications that the recording baron.
eter method employed by the Federation
Aeronautique Internationale in deter
mining official worlds altitude records
may be inaccurate by as much as per
cent below the altitude actually attained
are found in a calculation by photo
graphic methods of the remarkable alti-
tude flight made october 10 by Capt
st. Clair STREET. and Capt. Albert w.
Stevens of the Army Air Corps.
One of the main purposes of the
Night, which missed a new world record
by a small margin, was to test photo
graphic equipment and to obtain a COM
parison of altitude figures as calibrated
from the recording barometers by the
Bureau of Standards and the interna-
tional federation methods and those
calculated automatically from the
photographs.
The method of determining altitude
by photography is simple in principle,
according to the Army officials in charge
IF the flight. If the distance between
ground points shown in the photograph
taken from the plane is known, then, by
measuring the distance of the image
separations on the negative and know
ing the distance from the negative to
the optical center of the lens, or the
FOCAL length, the problem becomes a
geometric one Of similar triangles and
can be figured to within one-tenth Of
one per cent mathematical accuracy.
Comparison of Results Made.
A comparison of results made through
camera calculations with those obtained
from the Paragraph formula has been
decidedly Illuminating, according to an
announcement just received from
Wright Field, Dayton, Ohio, where the
test was made. The highest altitude
reached by CaptS. STREET. and Stevens
according to the Federation Aeronau-
tique Internationale celebration, was
37,854 feet. According to the Bureau of
Standards method it was 39,606 feet
and, according to the Photographic
method, $4,000 feet.
In order to obtain accurate calcula-
tons from the Photographic method
the Engineer Corps at Wright Field sent
a detachment to make six closed
traverses aggregating 1873, 98792 feet in
length OF the area of ground photo
graphed by the highest shots of the
camera. Capt. Stevens describes this
work as follows:
These traverses were run with tram
sit and tape and had an average error
OF closure of one part in 10,700. Fol-
lowing this survey, the altitude was
calculated by four different methods,
as a result of which one photograph
shows an altitude of 1890 feet above
ground, or $500 feet above sea level.
and photograph made several minutes
later in flight shows an altitude of 3D.
O60 feet above the ground or $1,000
feet above sea level. The ground ele-
walton of o70 was determined from the
records of survey of the C. C. C. 81 st.
L. r. r., Which passes Rushville, Ind.
(the point of landing). 1t is believed
that these values for elevation, as de-
termined through weeks of careful cal
culations, are correct to within 40 feet
plus or minus.
Tables OF Per Cent.
''It will be observed that the figure
Of 39,050 feet Is much higher than the
figure of 1884 feet obtained from the
Federation Aeronautique Internationale
tables, for barometer pressure of 152
nm. of mercury. A conclusion may be
drawn that the Federation Aeronautique
1nternationale tables are of about 8
per cent and too low for this altitude.
It Is also of interest to note that the
Bureau of Standards has computed,
from the tables of the National Ad-
visory Committee for Aeronautics, and
using tables of temperatures obtained
on the flight, that the elevation by ba-
rometer was 39,606 feet Again as
Stunning that the camera method is the
most accurate, the N. A. C. A. formula
Is approximately 1l2 per cent on and
too high.
THE methods of calculation used by
the Corps of Engineers are four: The
purely mathematical method requires
IL hours computation for a single
photograph; the Bagley method re-
quires hours; the pyramid graphical
method requires hours, and a second
mathematical method requires hours.
The results by the four methods check
within so feet and average of the val-
ues was taken"