Evening star. [volume] (Washington, D.C.) 1854-1972, 1930-07-13
ALTITUDE GAUG|NG
! DEVICE PERFECTED
Standard of Accuracy Is Bet
ter Than g. MiIlionthS
of an Inch.
BY THOMAS r. HENRY.

standard of accuracy better than 60
Irillionths of an inch, thus permitting
the finest measurements ever attempted
In field work, has been designed and
constructed for the United States Geo.
logical Survey by Capt. r. L. Atkinson,
chief OF the Division Of Field Equip
fnent.
The instrument is a precise level for
Use in determining attitudes above sea
level, and its use already has reduced
the official altitude of Mount Whitney,
California, the highest peak in the
continental United States, Az feet. The
extra altitude was due to an accumu-
lated error impossible to avoid with the
old instruments. Changes of like mag
nitude are expected for other noun
tains.
Precise measurements of the attitudes
IN the Rocky Mountain area are essen
tial for the study Of earthquakes. Loss
within the next few years Of few
inches, which could not be detected by
the old instruments, would indicate
that Mount Whitney Is slipping"
Errors Tend to Balance.
The altitude of any spot in the United
States is calculated from leveling OBJ
servations taken at intervals OF from
8 to 800 feet from a base station at
"sea level with due allowance for
such factors as the curvature of the
earth. The error Of any single observa-
ton is very slight, and such instru-
mental inaccuracies tend to balance
each other. but in any long line of
observations the accumulated error may
be considerable. The method of MAX
ing the observation is simply the sight
ing of the top and bottom as a scaled
rod through telespoce with cross-hairs,
thus obtaining the elements of a tri-
angle which can be calculated.
But for this purpose the observing
instrument itself must be absolutely
level, and this hitherto has been accom-
plished by q level attached to the side
of the telescope, consisting Of bubble
in an alcohol-ether mixture. This was
o somewhat clumsy device, because the
surveyor was obliged to set his inStru-
ment independently of taking his sights.
Now by a mirror arrangement both OPP
erationS are practically simultaneous
without the Observers changing his posi-
ton. At the very second the sight is
taken the observer knows that the in
strument is level.
Another element Of error came from
The use Of wooden surveying rods, which
changed in length slightly through alb
sorbing and giving off moisture. The
difference In such a rod at sea level
Of Mount Whitney might even be few
thousandths of an inch, thus throwing
out badly long series of observations
But most metals were even worse than
wood, because of their tendency to ex-
pand with heat. Capt. Atkinson devised
an indar rod-a nickel-steel alloy with
o coef1cient of expansion of less than
one two-millionth of an inch per de-
gree centigrade-which could be scaled
in hundred thousandths of an inch by
use of dividing machine.
Instrument Perfected Further.
The work. which Capt Atkinson only
can do, is carried out in a closed room
kept at a temperature constant to an
almost unbelievable precision. Another
person entering the room will affect
the temperature and thus the length
OF the metal enough to affect the &C
curacy of the machine: Even change
In the emotions of the operator, ac-
cording to Capt. Atkinson, might cause
q change. He must be careful not to
lose his temper. He also must make
allowance, almost instructive, for the
variations in the length Of the several
kinds of metal at the observed tempera-
ture.
With these possibilities of error elimi-
mated, the instrument is still further
stainless steel, whose expansion with
heat has been determined to be almost
precisely that of glass, so that there Is
no warping due to the pull of the diff
ferent elements. Then the cross hairs
are of platinum wire drawn to a fine
ness of one ten-thousandth of an inch
diameter. Hitherto the material gen.
orally used has been a strand from a
cocoon, but this has tendency to sag
slightly in hot, moist climates, so that
an element of error Is introduced.
BY an ingenious mounting of the lens
Capt. Atkinson has been able to make
such good use of light that the work.
ing day of field surveyors, which hither-
to has been at an end practically with
the first signs of dusk, can be extended
by approximately one hour. This Is of
especial importance because Of the
shortness of the season during which
parties can remain in the field.
Capt. Atkinson long has been the
mechanical genius of the Geological
Survey, and his inventions have result
ed in greatly increasing the accuracy
Of the surveyors tool chest. His work,
especially with the precise level, 'has
been almost revolutionary and has re-
suited in setting up new standard for
surveying accuracy" according to a re-
cent statement Of the Geological Survey.
$50 Parties Now in Field.
There are now approximately $50
parties in the field. all equipped with
instruments perfectly adjusted, many of
them Of such delicate mechanism that
q 17-jewel watch is like a crude me-
chanical top compared with them. But
despite the delicacy, they must be
adapted to rough field conditions. All
the accuracy of the sheltered laboratory
must be obtained in the open with all
sorts of weather conditions and all sorts
Of natural obstacles to observation.
Until the use of stainless steel in
these instruments was introduced by
the research laboratory under his direc-
ton, Capt. Atkinson explained, it was
customary for surveying parties to have
umbrellas handy on cloudy days to
avoid the errors that would result from
the sun coming from behind q cloud
and falling on the metal parts. Now
the expansion with heat is so insignifi-
cant and so perfectly adjusted to the
expansion Of the glass that this is not
necessary.
More than 81 tons of instruments
have been sent to the field this year.