PHOTOGRAPHIC SURVEYING, 
129 
Work in the Office. 
The first thing to do is to plot the station points on the skeleton plan 
if they have not been already plotted in the field. As with all other 
methods of surveying it is a matter of the greatest possible importance 
to be sure about the correct plotting of the stations, because any errors 
in the positions of the station points will cause errors in the plotting of 
nearly all points viewed from those stations. The most thoroughly 
reliable results are obtained when the stations have been fixed trigono¬ 
metrically. If many construction lines are necessary for fixing the exact 
positions of the station points, the sheet on which the stations are 
originally plotted can be laid over a clean sheet and the station points 
pricked through so as to avoid a superabundance of construction lines on 
the actual plan. 
If no preliminary or concurrent triangulation of the area to be plotted 
has been effected it may be necessary to fall back on the photographs for 
fixing the stations like other points. Before using the photographs for 
actual plotting it is best to have them enlarged several diameters; three 
or four will generally suffice, but much depends upon the scale of the 
map, and, generally assuming absence of distortion, the greater the 
magnification the more accurate should be the results of plotting. 
Let us assume now that all the photographs have been enlarged three 
or four diameters or more so as to have an equivalent focus or distance 
line of from 1| to 2 feet or more; it is then necessary to determine the 
exact equivalent focal distances for each picture, which can be easily done 
by multiplying the length of any straight line measured from zero along 
the tangent scale on the picture by the numerical value for the cotangent 
of the angle corresponding on the scale to that length. Note the value 
thus obtained on the back of the print. Then, assuming any two points 
at a convenient distance apart to be station points, as we may do if we 
are starting with a blank sheet of paper or taking any two stations 
previously fixed, if we have a skeleton plan to start with, the next 
practical step is to select views from those stations which will yield 
fairly good intersections for most of the points which they have in 
common. An inspection of the pictures will show what those points are, 
and a glance at the compass bearings will afford a ready indication of 
VOL. I. ‘ K 
