The Washington herald., 1915-08-28
Coast and Geodehc Survey Has
1900 Miles to Supervise
Its Work an Important Factor in the Development of
Nations Commensurate of Statistics
Have Been Compiled

|
less changes.
1-Hydrograpny. which embrace
delineation of the bottom of any bod.
of wafer. showing the depths of wa
| ter, the positions of channels. shoals
etc, or. in general, il is the topog
raphy of the submerged portion q
/ the earth's crust.
The Coast and Geodetic Survey i:
| obliged to perform all of the fore
going operations in order to secure
the necessary data FOR making a nau
| tical chart of any part of the ccas
of appreciable extent
DeIineate. the Difficulties.
To assist the reader in getting a
clearer view OF the character OF each
! of the aforementioned operations, 4
brief account of each in the order
named is given.
Triangulation. In any survey it is
necessary TO know the relative posi-
ton of some principal points of ref
erence upon which TO base the work
When the area is small the distances
may be best asCertained by direct
measurement. but when a large area
is involved or high precision required
the method of trianGulatiom is used.
This involves the direct measuremen Mt
of one of the sides of one of the tri-
angles and the angles of the scheme
of triangles. The side measurement
is known as base line measurement.
and is done with great care.
Three classes of triangulation are
recognized, primary, secondary. and
tertiary, according to their accuracy
ence to the sizes of the triangles in
volyed. but to the accuracy with
which the work is done. Primary
triangulation is used IN extending con-
troll over the country and on accu-
rate township and city surveys. By
means of primary triangulation the
distance between two points may be
determined with an error not in ex-
cess of one ne-hundredth-thousa
part of the distance, or probably less
IMAM IlNc ltSl OIl d
On the Atlantic coast a system of
primary triangulation begins at the
eastern boundary of Maine and
stretches TO the Gulf of Mexico. A sec
ond extensive system extends across
the United States along the thirty
ninth parallel Of latitude. connecting
the surveys of the two coasts and
furnishing many fixed points, in the
thirteen States through which it
passes. that can be utilized for the
control of precise local surveys such
as those of the City of New York,
of Cincinnati, and the township sur
'ey of Massachusetts,. Other systems
If triangulation have been extended
,long the ninety-eighth meridian and
along the Pacific Coast from Mexico
10 Canada; from the ninety eighth
meridian in Texas across New Mex
GO, Arizona. and California, connect
ng with the Pacific Coast triangula-
ion; from the thirty-ninth parallel in |
Colorado northward to the Canada |
inc; and from the Atlantic coast tri-
Lngulation to the Mississippi River at |
,Iemphis. Tenn.
Much Work Completed.
A tertiary triangulation for topog-
aphIc and photographic surveys has
'een completed along the entire At
antic and Gulf coasts and that of
'orto Rico, and practically the whole
f the Pacific Coast except Alaska
iuCh progress has been made in the
after Territory, and in the Philip
ines and Hawaii the main triangula-
ion is practically completed.
The total linear extent of the primary
-isngulation done by the Coast and Geo
etc Survey, and the Lake Survey in the
nited States is about 1000 miles, and
le exact positions are known for about
ZOO objects. marked stations, spires,
Ipolas. stacks. towers. beacons etc.
incidentally the triangulation and as @:
onomical observations connected with a:
have furnished valuable data for the :
termination Of the figure of the earth
nd the variation of densities in the {:
lrth's crust. {:
Precise Leyeling. {:
IN lIS general execution precise levelinc +:
similar TO other spirit levelinE. differ {:
g only III such modification of the in !:
rum"nts and methods as will insure re- ::
Hts of high precision, the allowable e. ::
n now not exceeding one milIimeter pc ::
lometer in running leveIing lines. !:
network of precise levels covering IN ::
general way. the whole United States. ::
I'd connecting the Atlantic Ocean, the ::
ilf OF Mexico. the Great Lakes, and ::
e Pacific Ocean has been run by the !:
'ast and Geodetic Survey. the army en- ::
neers, the United States Geological !:
lrvey. and other organizations This !:
twork traverses forty-one States, and !:
ere are ten connections with mean sea !:
vel. six on the Atlantic. two on the :.
llf, and two on the Pacific Coast In :.
is system Of levels 81, 900 miles of levels ::
We been run and l2,0O0 bench marks ::
wing standard elevations referred 10 ::
ean sea level have been established for ::
e in engineering and industrial pur- ::
its. Much more leveling needs to be ::
ne to meet even present demands. (:
Study Movement of Tides. :;
n connection with its important duty !;
the PREPARATION OF nautical charts, ::
ling directions and tide tables. the !:
asst and Geodetic Survey has. from its !:
ginning carried on the observation and ::
'cussion of tides and tidal currents. !:
Observations of the tides are usually $:
lde by direct reading of a graduated ::
If or by a self-recording or automatic $:
e gauge. The latter is employed espe- ::
lily where long series of obserya- =.
ms Is required for purposes Of study ::
d analysis for the so-called harmonic =:
lStantS which are used in connection =.