Design 
Also in the third stage of road-planning, viz., the setting up of the 
detailed plan of work on which the construction of the road is based, 
the air-pictures afford valuable help. The detailed planning of course 
makes great demands on the accuracy of the work through which the 
detail information is obtained. This applies both to the measurement of 
the longitudinal section of the road and its cross-sections and to the 
drawing of the detail maps with the help of which the special construc- 
tional work etc. on the road is planned in detail. In judging the possibi- 
lities of detail information about the terrain in the form of altitudes and 
the horizontal plane to be got from the aerial pictures, one should bear 
in mind that the desired information refers to the terrain considered as 
a surface and not considered by points, Even if in e. g. cross-sectioning 
in the field the differences in altitude are read off with an error of 
about one cm, this does not imply that the ground-surface is represented 
with an accuracy of 1 cm; rather, owing to variations in the terrain, 
should the accuracy be estimated as within about 1—2 dm. The methods 
applied for the calculation of masses imply a rectilinear interpolation of 
a surface that does not follow a rectilinear function. 
The drawing of a map with contours over a section of terrain in the 
conventional way also implies a form of rectilinear interpolation. 
On account of the factors mentioned above a point accuracy or stand- 
ard error in the horizontal and vertical planes of the order of magnitude 
0.1—0.2 m should be sufficient. 
To attain this accuracy in photogrammetric work the following de- 
mands must be made on the aerial photography and the treatment of 
the aerial pictures: 
1) The cruising altitude should not exceed 1.000 m. An altitude of 
800 m has been adjudged most suitable. 
2) The photographing must be undertaken in the spring before the 
appearance of the leaves and the vegetation period. 
3) The signalizing of certain points of reference should be carried 
out before the photographing. 
4) The pictures should be treated in precision instruments in order 
to extract the greatest possible accuracy. 
5) Extensive automatization of the recording of the numerical data 
is necessary to eliminate the human sources of error in the form 
of erroneous readings and mistakes in writing down figures, to be 
able in a convenient way to add to the numerical data the correc- 
tions that may be necessary for a completely valid result and, 
finally, to increase the capacity in connection with the treatment. 
With observation of the above points the following method has been 
worked out for the utilization of the aerial pictures at the stage of 
design. 
The road-line is staked out in the terrain in the light of the map ma- 
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