It might also be noted that, in actual practice, the direction 

 of level bottom is usually determined by mentally interpolating an 

 extra contour midway between the two contours being considered, and 

 taking the direction of level bottom as a line tangent to this contour 

 at its intersection with the orthogonal. It is thought that this 

 method gives a more accurate determination than that given in H. 0. 

 Publ. No. 605 and referred to. by Mr. Kaplan, and, in addition, has the 

 advantage of giving the same direction for level bottom regardless of 

 which direction the orthogonal is being drawn in. 



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