360 I. P. KOCH. 
Azimuths and “courses” are computed from the south, positive 
towards west, negative towards east. As the compass line between 
two points (the course) on the northern hemisphere is always more 
southerly than the great circle between the points, a small practical 
facilitation is effected, in that the angle indicating the compass line 
must always be numerically smaller than the one indicating the azimuth. 
The correction from azimuth to “course” is made by the formula 
d= = sin g'), where 4 is the correction sought, d the difference of 
longitude between the station and the point measured at, and ф is 
the latitude of the station. As in this case sing is very near 1, d 
may with sufficient accuracy be put at 12 а, taking dı from the 
working map by laying down at an estimate the point in the latter. 
The zenith point correction is + 0'.5. The values of tanv, which 
are taken from a four-placed table of the numerical values of the 
tangent function, are determined by о = 2- 90, where = equals the 
reading off on the vertical circle + 0.5. 
The column of distances in kilometres is, as far as the coast 
points are concerned, filled in according to a computation of the 
distance by means of an engineer’s scale in the manner indicated 
on pp. 304—307. As far as the other points are concerned, the 
distances are determined by intersection of bearings and taken from 
the map. 
After that everything is ready for the final laying down of the 
points in the working map, which is done with a good station pointer. 
As the scale in the cylindrical projection of Mercator may vary very 
much, it would be a practical proceeding to have the frames of the 
working map arranged beforehand with a division in kilometres, side 
by side with the division in minutes of latitude. 
The levellings. 
A fairly reliable determination of the heights of mountains is 
one of the most difficult tasks in the execution of a geographical 
survey, and in many cases the difficulties are of such a kind that 
they cannot be surmounted at all — in any case not in a fairly 
satisfactory manner — under the conditions in which the observations 
take place. As far as the reliability of the map is concerned, it is, 
therefore, of great importance for one to keep one’s critical faculties 
alive, in particular as regards the results of the levellings. As to the 
altitudes of the stations, from which the observations are performed, 
it will generally not be difficult to get a fair estimate of the accuracy, 
but as to the points determined by bearings and levellings from the 
1) K. V. Ravn: Lærebog i Opmaaling I, pp. 46—40. 
