The Columbian. [volume] (Bloomsburg, Pa.) 1866-1910, 1890-11-07
The u. .. eeoioeica, survey.
ToPoeRAPHIolL SURVEY OF THE COAL
FIELDS.

FIKLDs.
The corps Of U. &. engineers now stop
ping at the Exc angc Hotel are engaged
In making a typographical survey Of
Bloomsbury and the surrounding country.
This is one of many parties scattered
through the coal fields from Harrisburg to
IIoncsdale. This work was begun some
years ago by the State geological survey
under Prof Leslie Of Lehigh, but after the
parties had been in the field three years it
was found to be SO expensive thai the leg
isature refused to appropriate any more
money for the purpose and the notes and
data collected by the parties were turued
over to the U. s. engineers with the request
that the U. = continue the work. Mr..
Robt. d. Cummin Is In charge, and he says
they expect to complete the field work this
season, and the maps will be drawn, en-
graved and ready for publication in about
one year. These maps will necessarily be
made on a small scale as the area surveyed
will cover over 1000 square miles; but they
will include detail map of all cities and
towns, all public roads, railroads, canals
and all public works. The topography, or
relief, will be expressed In contours of 90
feet interval The coloring of the sheets
will, be made consistent with nature as far
as possible, the culture in black, the topo.
graphy (contoArs) in brown, timber lands in
green, and the drainage and water surface
in blue.
Few people outside Of those directly in
terested in topography realize the amount
of labor and time required in the construc-
ton of such maps. The first step is to Sc
lect a level piece of ground and measure a
base line of at least 8 miles in length.
These measurements are made with the
greatest possible care, compensated stee |
rods are used and corrections made for
every change in temperature. Sometimes
it takes q whole party year to measure
one of these lines. After the base is meas
ured large signals are erected at the extre-
mites, and the latitude and longitude of the
points are then determined by astronomi.
cal observations. Then these points are
re-occupicd with theodolite aud the pri-
mary triangulation begins. Large triang-
les with the sides ranging from one to
twenty-five miles are thrown over the coun.
try until sufficient points are determined
to start the plaue-tables. The area to be
surveyed is then divided into atlas sheets
of 10 square (Lat. and Long.) and poly.
comic projection (allowing for the curva-
ture of the earth's surface)iare constructed
for each of these sheets and the primary
points are plotted upon them and the plane
table can then substitute the theodolite. This
is called the graphic method. With the plane
table great many points are determined a
shorter range until complete network Of
| triangles is thrown over the sheet, and al
each located point vertical angles are TAX
en- to determine the elevations anG thest
different systems of angles are connected
with line of levels run from the mean lie
vel of the sea at Perth Amboy. When thii
| is done it is called the control, or skeleton
map and the detail work begins. Filling
in the detail is largely a matter of common
surveying. One small triangle is taken uI
at a time, the roads are traversed ant
many prominent points are cut in, from
these traverse lines and the topography l
! sketched in between the located points
! Vertical angles for elevations are carrie'
along the traverse lines and the intermedi
ate points in the sketch are determined b
aneroid barometers and in this way tn
small triangle is worked up and then tn
next one taken up in the same way an
so on until the atlas sheet is finished
These maps are expensive, but they ar
also very useful. For instance, farmt
having such a map can tell at glanC
whether water can be taken froi
certain stream or pond TO his hous
or over his farm for irrigation pu
poses without spending cent for survey
ing. He can tell the latitude and long
tude of his house, its elevation above II
sea or of any point on his place. Eng
neers can often save the cost of pre'im
nary surveys for railroads and canals. Sue
map will show by the contours the be
, route to be taken and construction can bC
, gin at once. Biologists cannot make
satisfactory study without such maps, ar
they are very useful for military purpose
and in many other ways. Four of the el
gineers are now stopping at the Exchanl
, hotel: Robt. d Cummin, VIM. II. Atki
son, j. j. Mason and r. F. Mcmillan, q
, of Washington.