NJDEP - NJGS - First Annual Report of the Geological Survey of the ...

" l97the greatest care and accuracy upon a plain as nearIy level_s can be found. This is called a bc_e lble_from each extremityof which, by means of a theodollte_ the angles are measurcd_which are included between several prominent surroundingobjects previously selected as trigonometrical points. _'romeach of those points the angles are measured between the linesjoining the points and the extremities of the base line. Thereis thus formed a series of triangles, from the known parts ofwllieh the unknown are determined by calculation, and wethus ascertain the length of the sides of the triangles'or thedistafices between the trigonometrical points, l.i'rom thesesides as bases othm" trlgonmnetrieal points are determined_ untilthe entire tr,'mt of country to be surveyed is covered over witha network of triangles, of as large a size as is consistent wltlathe nmans employed. This is called a primary triangulation,within which a series of secondary triangles is formed in thesame manner. The interior details between the secondarypoints are filled in by moans of the plane-table. These dctailsconsist of the contour lines of the elevations, or horizontalsections of the hills, taken at cqui-distant intervals---the directionand sinuosities of .the ivater courses ; the houses, roads_farm lines, etc. Obsei'vations for latitude and longitude axemade at several of the most conspicuous stations, from whichthe geographical position of all the points can be dctermined_and their places projected on the map with extreme accuracy.For the purpose of the Geodetic Survey, the coast of theUnited States has been divided into eleven sections, in eachof which a base line of from five to ten miles is measured.The co,'_st of New Jersey is included in the second of thesesections_ the base of which is on Fire Island. The primarytriangulation cxtends across the State from Amboy to 'I_rcnton_and dowu the Delaware River. The secondary triangulationfollows the same course, and also extends along the wholecoast.. About two thousand square miles of topo_aphy havebeen completed, based on this triangulation. Observations forlatitude and azimuth have been made at a number of stationsof the primary triangulations. The difference of latitude,7NEW JERSEY GEOLOGICAL SURVEY