4832 HYDROGRAPHIC MANUAL PaGE 430 



Straightedges should be cleaned with soap and water. Abrasive materials should 

 never be used as they tend to scatch the surface and make it more susceptible to the 

 accumulation of dirt. 



When not in use a straightedge should be kept in a canvas case or other suitable 

 container and stored in a place where it will be protected from moisture. 



4832. Opisometer 



The opisometer is a simple instrument designed to measure by revolutions of a small 

 wheel continuous linear distances on a map. In hydrographic sm-veying it is used 

 principally to measure distances run by the sounding vessel. The main advantages of 

 the opisometer are that distances along curved or irregular lines can be easily measured, 

 and that the total of numerous short distances can be measured without measuring the 

 separate distances. 



Two types of opisometers are in use at the present time, both being adaptations of 

 the principle of the odometer, by which horizontal distances on the ground are measured 

 by the revolutions of a wheel equipped with a counter. 



The type of opisometer most generally used (G in fig. 91) was first made by the 

 Instrument Division of the Coast and Geodetic Survey prior to 1895. It is simply a 

 small wheel, with a knurled rim, that is threaded on a small rod with stops on each end. 

 The wheel is mounted in a fork and held in place by means of the threaded rod along 

 which it moves laterally between stops, when rolled over a surface. One side of the 

 opisometer has a pointer, which extends almost to the outer diameter of the wheel, by 

 which accurate settings are made. 



To measure miles of sounding line with the opisometer, the wheel is revolved until 

 it brings up against one of the stops. The pointer of the opisometer is placed over the 

 first position of the line to be measured and the wheel is placed in contact with the paper. 

 The wheel is run along the line, maintaining the same direction of rotation even when 

 turns occur in the line or when the total of a number of unconnected lines is measured. 

 After the total has been traced, the distance is found by running the wheel in reverse 

 along a graduated scale until it is at its original position in the fork ; or, on a hydrographic 

 sheet, along a meridian line counting the number of parallels crossed, which will give 

 the distance in nautical miles, the fractional remainder being measured by spacing 

 dividers. In the latter case the distance is reduced to statute miles by multiplying 

 by 1.15, the resulting data being entered in the Sounding Record. 



4833. Map Measure 



Another and more elaborate opisometer made commercially is known as a map 

 measure (see H in fig. 91). Its principle is the same, but it is calibrated and registers 

 distances by a pointer revolving arouiid a dial containing two concentric circular scales, 

 the inner graduated in centimeters to 99 centimeters, and the outer graduated in half- 

 inch intervals to 39 inches. The dial is about Iji inches in diameter and is similar in 

 appearance to a small watch. The instrument has a swivel handle. Below the dial 

 a housing contains the very small diameter smooth-rimmed measuring wheel. This 

 wheel is connected by gears to the pointer so that correct linear distances will be indi- 

 cated on the graduated scales when the wheel is revolved over a surface. The swivel 

 handle is attached at an angle of about 15° so that the wheel will always be revolved 

 in the same direction and cause the pointer to travel in a clockwise direction. 



