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and longj represent the last and the present DRAI longitudes, re- 

 spectively. The correction is added to or subtracted from the original 

 DRAI longitude to give the corrected DR longitude. 



In the second method of correcting longitude, the Dead Reckoning 

 Tracer (DRT) is used. "^Mien a fix is obtained the DRAI is set as 

 before and the table of the DRT is marked: the fix is also placed 

 on the chart, ^^lien the ship's latitude and longitude are required, 

 a 0°-180° line is drawn through the fix on the table. Then a 90°-2T0° 

 line is drawn through the present position on the DRT. The distance 

 measured in miles from the fix to the 90°-270° line gives the difference 

 in latitude between the two positions. The intercept measured on the 

 90°-270° line between its intei-section with the O'^-ISO" line and the 

 DRT position is the difference in longitude measured in miles. With 

 the difference of longitude and difference of latitude it is then quite 

 simple to determine the DR position on the chart by consecutively 

 stepping off these vectors in the appropriate directions from the fix. 



It is difficult, when navigating through ice out of sight of land, to 

 establish the ship's DR position. The fundamental factors, speed 

 and course, change continually and do not lend themselves to accu- 

 rate calculation. Even if a gyrocompass and automatic pilot are on 

 board, the distance run must be known. No device has yet been 

 invented which can measure continuously the speed of a ship in 

 ice. While maneuvering in ice, where the course changes almost con- 

 tinually, the average course should be noted over a relatively short 

 period of time during which deviations from this mean course are 

 inconsiderable (2° to 3°) or of short duration, e. g., when passing 

 round small floes. The ship's position must be kept up to date 

 while navigating in ice, otherwise after spending some time in ice 

 she will be out of position on reaching the open sea. 



For this purpose a careful record should be made of all alterations 

 of course and the corresponding times at which they are made for 

 subsequent plotting on the chart. For the continual noting of fre- 

 quently changing courses and speed, it is recommended that a special 

 notebook be kept, compiled in the following manner: 



Time in ^-i r^ t^ Duration 



hours and Compass Compass True ^^^ g ^^^ Distance Notes 



iiuuiE, anu goui-gg error course „„„_«„ 



mmutes course 



During frequent alterations of course it is extremely difficult to plot 

 on the chart, especially if it is of small scale. It is therefore recom- 

 mended to plot the general course and the distance made good on the 

 chart once during every watch. To obtain these data it is necessary 

 to carry out subsidiary plotting, for which squared paper should be 



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