64 HYDROGRAPHICAL SURVF.YING [chap. n. 



and the soundings taken when this has been done. The 

 topography, or detail of the land, can be done at any 

 time. 



6th. Each piece of work is inked by the assistant on his 

 board, with all detail, and when complete, is carefully traced 

 on the above tracing of the " points." All bits are thus 

 collected together, and the total is retransferred to the plotting 

 sheet by means of transfer paper, and inked in as the finished 

 chart. 



These details must not be taken as unalterable. Some 

 prefer plotting everything on to the same original sheet, and 

 when a surveyor is by himself, or with one assistant, he would 

 probably do this, but the method described is calculated for a 

 number of assistants, and has been found to work well. 



It is not absolutely necessary to get a base before starting 

 a plan. Circumstances may make it imperative to wait a day 

 or more for this, and in the meantime, a distance between two 

 stations, to be finally measured, can be assumed and plotted, 

 and the whole system of triangulation built upon this. But 

 it must be remembered that no heights can be measured by 

 means of angles, until a scale is obtained. 



If an extended survey, and plenty of hands, some Avill 

 carry on the triangulation and marking in advance, while 

 others are putting in the detail, and sounding the part already 

 marked. 



The deeper soundings will be taken from the ship, to a 

 sufficient distance or depth off shore. 

 When to j^ ^^y[]\ depend upon circumstances when the astronomical 



obtain the . 



Astrono- observations for latitude and longitude are taken. If only 

 PosiT ^^ isolated plan is being done, the observations to fix some 

 definite point on it can be taken at any time. 



When an extended survey is in progress, that has been com- 

 menced on a measured base, they can also be taken when con- 

 venient. In this case the final scale of the chart will mainly 

 depend upon the observations taken at either extremity of the 

 chart, and they must consequently be done very carefully. 

 Circumstances of weather, time of year, etc., will therefore 

 influence the choice of the best time for these. 



Sometimes an extended survey will be originally plotted 

 on a base obtained by the astronomical positions, and in this 



