3. EQUIPMENT AND INSTRUMENTS 



45 



end of the field season, worn out, damaged, 

 and surplus instruments should be returned 

 to the Washington Office, or stored at a 

 District Office in accordance with Bureau 

 Regulations. 



Equipment installed where it is exposed to 

 the weather should be kept covered when 

 not in use. During the layup period, or if 

 the machines are used infrequently, they 

 should be examined periodically to see that 

 motors are operating properly and all mov- 

 ing parts move freely. 



3-6 Instrument requisitions. — On receipt 

 of instructions for a survey project, the in- 

 strument inventory should be examined to 

 determine what additional instruments, if 

 any, will be required to accomplish the work 

 (see 2-17). All items of equipment and in- 

 struments are obtained from the Washington 

 Office by requisition of the Chief of Party, 

 using Form 12, Property Requisition. The 

 requisition should be submitted sufficiently 

 in advance of needs to allow adequate time 

 for packing, boxing, and shipping. 



Immediately on receipt of a shipment of 

 instruments, the Chief of Party should have 

 the items checked with those shown on the 

 invoice. Each instrument should be closely 

 examined and tested to see that it is in good 

 working order. Any discrepancy, or any 

 damage to instruments, should be reported 

 to the Washington Office at once. 



3-7 Shipment of instruments. — Instru- 

 ments that are returned to the Washington 

 Office, transferred to another ship, or placed 

 in storage, shall be reported on Form 573, 

 Property Storage and Transfer Report. Each 

 instrument to be shipped must be firmly se- 

 cured in its box, but in such a manner that 

 delicate parts will not be injured because of 

 excessive pressure. Special precautions must 

 be used in preparing some instruments for 

 shipment, loose pieces should be securely 

 lashed, the glass face of a clock must be well- 

 padded with shredded paper, a compass 

 should be removed from its gimbals, and the 

 balance wheel of a chronometer must be 

 locked with small cork wedges. Instruments 

 generally are expensive precision mecha- 



ni.^.ns and are easily damaged in shipment 

 if not carefully packed. 



3-8 Position locating instruments. — 



There are two general methods of locating 

 a sounding vessel when engaged in hydrog- 

 raphy. Inshore surveys are usually con- 

 trolled by three-point fixes using hydro- 

 graphic sextants to measure two angles be- 

 tween three known points, one of which is 

 common to both angles. The position is 

 plotted by a three-arm protractor. Offshore 

 hydrography, and some inshore surveys, are 

 usually controlled by an electronic distance 

 measuring system and may be Shoran, Ray- 

 dist, or Electronic Position Indicator. Posi- 

 tions are plotted by an Odessey protractor. 

 The various instruments used to determine 

 and plot positions are described in the fol- 

 lowing sections. 



3-9 Sextants. — The sextant is a hand- 

 held instrument used for measuring an angle 

 between two points and has a maximum 

 range of about 140°. There are two general 

 classes of sextants: (a) the navigating sex- 

 tant which can be read to 10 seconds or 0.1 

 minute according to the design, and (b) the 

 hydrographic sextant which is usually de- 

 signed for measuring an angle to the nearest 

 minute. Sextants are also classified as ver- 

 nier or micrometer drum according to the 

 method used to obtain the final reading. See 

 Chapter 15, Bowditch (1958) for a com- 

 plete discussion of the various types of sex- 

 tants available. 



With the clamp screw vernier sextant, the 

 index arm is set at the approximate angle 

 and the arm firmly clamped to the frame. 

 The two objects are brought to coincidence 

 by use of a tangent screw of limited range. 

 Modern vernier sextants have a worm gear 

 attached to the arm which engage a match- 

 ing gear on the frame of the sextant and 

 permit unlimited movement of the arm with 

 the tangent screw. 



The micrometer drum sextant (Fig. 7) 

 also has an endless tangent screw but the 

 final reading is made on a graduated drum 

 rather than a vernier. The drum may be 

 graduated to 10 seconds, 0.1 minute or 1 min- 



