ENG 97-258

TABLE NO. 4.-Triangulation of tertiary stations and lighthouses.

TABLE NO. 4.- Triangulation of tertiary stations and lighthouses—Continued.

D.-REPORT OF MR. H. VON SCHON, ASSISTANT ENGINEER.

UNITED STATES ENGINEER OFFICE,
Sault Ste. Marie, Mich., June 30, 1897.

SIR: I have the honor to herewith respectfully submit my report for the fiscal year 1897, relating to those operations in connection with the survey of the Northern and Northwestern Lakes-resurvey of the St. Marys River-of which I have had charge. Synopsis. The approved season's programme consists of the location of all "topography and hydrography" required for the completion of the resurvey of St. Marys River and of the extending of a line of "precise levels" to the shore of Lake Huron. This programme was fully carried out."

The scene of the field operations was all that territory which is embraced in the projected chart No. 1 of the St. Marys River, which extends from the latitude of Old Fort St. Joe south to the mouth of the St. Marys River at Detour Light, a distance of about 74 miles, and from the shore of the Upper Peninsula of Michigan easterly over Potagannissing Bay to Sulphur Island, off the north entrance to False Detour Passage, a distance of about 19 miles.

The character of the field operations was:

First, extending the established secondary triangulation system to Sulphur Island Light, being the most western point in these waters which has been geodetically determined during the last admiralty survey executed by the Dominion of Canada; Second, extending the established tertiary triangulation system over the islands of Potagannissing Bay, the south shore of St. Joseph Island, the north shore of Drummond Island, and the Detour Passage;

Third, locating all shore topography within the territory described;

Fourth, sounding all the waters within this territory sufficiently to develop fathom submarine contours; and

Fifth, extending precise levels from a point on the Charlotte River near the Hay Lake shore to Detour Light on Lake Huron shore.

Narrative. The field organization under my charge consisted of a party of sixteen members, which was housed and subsisted on the U. S. quarterboat Hay Lake with the U. S. steam tug Myra attached. This party left Sault Ste. Marie, Mich., on July 1, 1896, reaching the first camp at Harbor Island the same day. On July 16, 1896, camp was moved to Burnt Island; on August 20 back to Harbor Island, and on September 3 to Detour, where the party remained until October 6, when it returned to Sault Ste. Marie, Mich., and was disbanded on October 7. On September 23 a party of five members was detached to continue the work of carrying the precise levels over territory too distant from the camp at Detour. The level party remained in the field until October 30, when it also returned to Sault Ste. Marie, Mich., and was disbanded October 31, 1896.

FIELD OPERATIONS IN DETAIL.

Secondary triangulation.-The reconnaissance and planning for this work was executed during the preceding season. The approved system consisted of three quadrilaterals departing from the established secondary line Harbour-Bacon as follows: Harbour-Bacon-Burnt Point-Burnt Island, Harbour-Burnt Island-KocruishChippewa, Chippewa-Kocruish-Serpent-Sulphur, the latter being the objective point.

Two additional quadrilaterals were planned from this system, one northerly from line Kocruish-Serpent to Grande Point and A 252 on the south shore of St. Joseph Island, and one southerly from line Serpent-Sulphur to A 343 and ▲ 345 on the north shore of Drummond Island, in order to locate the terminal points of shore topogra phy at the eastern limit of the surveyed area by secondary determination, as well as checking the tertiary extensions in this direction.

Of the observing stations required in this secondary system, those at Harbour, Burnt Point, and Chippewa had been erected by a construction party during the season of 1894, the first two being described in the Annual Report for 1895, Appendix EE E, page 4177, in the report of Assistant Engineer Glen E. Balch; station Chippewa is located on Drummond Island, on the eastern end of the point of the same name, being about the northernmost point of the island; the top of the tripod is about 12 feet above the surface.

The observing stations at Kocruish and Serpent were constructed during the season of 1895, and are described in my report on page 4058, Appendix E E E of the Annual Report of 1896.

The other stations of the secondary system, namely, Burnt Island, Sulphur Island, A 252, Grande Point, A 343, and ▲ 345 were established and constructed during the season covering this report.

Station Burnt Island is located on the island of the same name, being in American waters, in section 21, range 5 east, township 43 north, on the highest point of land in a small clearing on the land of Mr. Spencer Hill. The geodetic point is a 3-inch hole drilled in the top of a cut stone 6 by 6 by 24 inches between the letters U. S. The surface mark is a nail in center of 10-inch wooden post. The top of the observing tripod is about 35 feet above the surface. Station Sulphur is on Sulphur Island, being in Canadian waters, about 3 miles northerly from the northeast end of Drummond Island and off the north entrance to False Detour Passage. The geodetic point is the center of the light tower located on the south end of the island.

Station Grande Point is on St. Joseph Island on the point of land of the same name, which is the easternmost point of the island. The geodetic point is a hole drilled in the top of a large granite bowlder inside of a square. The top of the tripod is about 12 feet above the surface.

Station 252 is located on Caradoe Point, on the south shore of St. Joseph Island, the geodetic point and the tripod being the same as described for station Grandé Point.

Stations 343 and 345 are located on the north shore of Drummond Island, the geodetic points being holes drilled in large bowlders and the observing tripods being 5 feet high.

The methods followed in the secondary triangulation work were those usual with direction instruments, the angles being measured with the Pistor & Martins transit No. 218. The system contained 20 separate triangles, involving the measuring of 60 different angles. The longest linein the system is Kocruish-Sulphur, being 13,679.62 meters.

TERTIARY TRIANGULATION.

The tertiary triangulation covering the territory was planned as a continuous system of quadrilaterals, departing from and closing on the established secondary line Harbour-Bacon. This system embraced 51 stations which were occupied by

surface tripods firmly anchored, the station point being marked on a stone below surface with wooden post above. Each tripod had its corresponding station number cut on the face. Thirty-seven of these stations are located on as many different islands, 4 on St. Joseph Island, 8 on Drummond Island, and 2 on the upper peninsular shore. The system contained 98 triangles, requiring the measuring of 285 separate angles. In addition to these, 45 angles were measured for the determination of 15 lights and buoys. The instrument employed in this work is the Buff & Berger transit No. 245, and the methods followed those usual with repeating instruments. All pointings were made to targets fixed in top of station tripods.

TOPOGRAPHY.

The topographical field operations consisted in the location of all the shore lines, of details important to navigation, of elevations for development of 20-foot contours, of roads, houses, timber limits, and all other topographic inshore features within the territory of the programme, resulting in the complete survey of all the islands and shore lines within the area of projected chart No. 1, St. Marys River, as far as the mouth of the St. Marys River at Detour Light and extending along the north shore of Lake Huron from Detour Light westerly to Carlton Bay, and easterly to and including Whitney Bay on the south shore of Drummond Island. The instruments employed in this work were the transit, plane table, and Wye level, and the general methods followed those of the stadia survey referred to and controlled from triangulation points.

In the course of this work the following points were instrumentally occupied: Eleven secondaries, 55 tertiaries, 387 transit, and 329 plane-table stations.

The distance of developed shore line thus located was about 121.5 miles. The highest elevation found was near Detour, about 724 feet above mean tide, New York. All topography located was plotted on 29 plane-table sheets of the approved form on a scale of 1:10000.

HYDROGRAPHY.

The hydrographic field operations consisted in sounding over all the waters between the islands in the territory covered, and of the detail surveys of all shoals near navigable water. The method employed in shallow waters-less than 3 fathoms-was to sound with poles from small boats being rowed along lines between fixed shore ranges, and in deep waters by sounding with lead lines from the steam tug running at the speed of 3 miles per hour and being pointed to by 2 transits from triangulation stations simultaneously, at intervals of five minutes or less. Sufficient sounding lines were planned to thoroughly cover the territory, the individual soundings being taken at 50-foot intervals or less. Only suitable calm days were devoted to this work.

The total distance of developed sounding lines was about 99 miles in shallow waters and about 178 miles in deep waters, making in all about 30,000 individual soundings. Nine shoals were surveyed by control from nearby triangulation stations and fixed buoys.

The deepest soundings were found to be about 150 feet in the Detour Passage. All hydrographic field work was reduced and platted on the before-mentioned planetable sheets.

PRECISE-LEVEL WORK.

The extension of precise levels to the shore of Lake Huron was entered upon during the season of 1895 and reported on by me as on pages 4058, 4059, and 4060 of Appendix E E E of the Annual Report for 1896. The work was completed during the season covered now.

A special report treating of this work and containing description of lines run, of permanent bench marks established, of methods employed, and of the results secured is hereto attached.

The office operations which were carried on under my charge during the period reported on have consisted of the reduction and adjustment of the secondary and tertiary triangulation, the computation of geographical coordinates for secondary points, and of rectangular coordinates for tertiary points, and lights and buoys, and of the tabulating of the resulting data in tables 1, 2, 3, and 4 hereto appended; of platting all topography located and of all soundings made; of reducing, adjusting, and tabulating the data resulting from the precise-level work, and of continuing the compilations on the approved form of record sheets No. 29, Milford Haven; No. 30, Serpent; No. 31, Sulphur Island, which leaves record sheets No. 32, Chippewa; No. 33, Potagannissing; No. 34, Maud Bay; No. 35, Detour; No. 36, Detour Passage, yet to be compiled in order to fully complete the uniform office records of the entire topographic and hydrographic surveys executed during the seasons of 1893, 1894, 1895, and 1896 on the project of the resurvey of the St. Marys River.