2263 HYDROGRAPHIC MANUAL PaGE 80 



more than 100 meters from the station, so that lines from them to the station will intersect approxi- 

 mately at right angles. The stakes must be located where they will not be disturbed by the local 

 construction, and spaced apart from each other and the station so that the focus of the instrument 

 will not have to be changed during the measurements of the angles. This is important, for any change 

 in focus when pointing from one station to another will likelj' affect the accuracy of the observations. 

 Drive a projecting nail vertically in the top of each stake. With a theodolite centered over each nail 

 and the station in succession, measure all three angles of the triangle formed by the two stakes and the 

 station. Three positions with a 6}^-inch direction theodolite or one set of 3D and 3R with a 10-second 

 repeating theodolite closing the horizon, will be sufficient. Measure accurately the horizontal dis- 

 tance between all three points in the triangle. 



After the construction has been completed, the station is re-marked by setting up again at each 

 stake, preferably using two instruments simultaneously, sighting on the nail in the other stake and 

 turning off the angle previously determined. At the intersection of the two lines of sight, mark a 

 temporary point at any desired elevation. Check this location by measuring the angle at each stake 

 with the same accuracy as before. If they are in agreement with the angles measured before con- 

 struction, the station will be in its exact original geographic position when replaced. If one or more 

 of the angles do not agree move the temporary mark slightly and repeat the measurements until an 

 agreement is reached. Before measuring the angles, the horizontal distances previously measured 

 should be checked. If more precise measurements are desired and if azimuth control is available, 

 the geographic position of the replaced station can be computed and compared with the original 

 geographic position. 



(6) If the station has to be moved horizontally, establish a new mark or move the old one to a 

 new position. The new location shall be determined by azimuth and distance measurements from 

 the old station with at least third-order accuracy. The azimuth should be measured with three 

 positions with a 6}^-inch direction theodolite or one set of 6D and 6R with a 10-second repeating 

 theodolite, and the horizontal distance taped twice (forward and backward) , checking with an accuracy 

 of at least one part in 5,000. Before or after the taping, the tape shall be compared with one which 

 has been standardized. In determining the azimuth, another triangulation station should be used as 

 the initial. Should an azimuth mark be available, it can be used in lieu of another station. If 

 neither is available, use a natural or artificial object whose azimuth frem the original station has 

 been determined or can be computed. Should this be lacking use a reference mark. When moving a 

 station mark, it is essential to hold the original position by a bench or other means, until the azimuth 

 observations and distance measurements have been made to determine the position of the new station. 



After the station mark has been estabhshed in the new position, occupy it to 

 measure a direction to the azimuth mark, if there is one. Another triangulation station 

 should be used as an initial, or the old station may be used if it is not too near. 



2263. Rules and Examples 



All triangulation stations shall be marked or re-marked, and the disks stamped, in 

 accordance with the following rules: 



(a) Each newly established triangulation station shall be marked with a standard station-mark 

 disk which shall be stamped with the name of the station and the year of establishment. 



(6) Each reference-mark disk shall be stamped with the name of the station, the number of the 

 reference mark, and the year. f 



(c) Each recovered 'station which is re-marked shall be stamped with the original name of the 

 station, the original date of establishment, and the year in which it is re-marked, 



(d) Additional reference marks may be established when a station is recovered and reoccupied. 

 The name and date, or dates, on the reference-mark disks shall be the same as those on the station- 

 mark disk at the time the reference mark is established. 



(e) Do not renew an old reference mark. If it is in poor condition, either reinforce it or destroy 

 it and set a new reference mark which will be numbered with the next consecutive unused nvmiber, 

 regardless of the existence or absence of any of the reference marks established previously. 



(/) The abbreviation "Ecc." (for eccentric) should never be stamped on a disk. Its use in the 

 records should be solely to indicate that the observations made at that point must be reduced to the 

 center station to close the triangles before being used in the subsequent computations. 



{g) All stamping on disks for new station and reference marks shall be done with ^le-inch dies. 



