4. It is generally accepted that aerial photos must be retaken in Central 
Sweden at intervals of 10 years. The same applies to Southern Sweden, 
because of the minor róle played by the photos in forestry; otherwise an 
interval of 4—8 years would be justified. In Upper Norrland an interval 
of 10—15 years should suffice. In 1955 the report of an official inquiry 
into “photogrammetric operations” recommended that practically every 
part of the country, with the exception of the mountain districts, be photo- 
graphed every 10th. year at public expense, for the production of photos 
on the scale of 1:20 000. If this recommendation is followed, the use of 
photographs for forest mapping will be promoted and methods can be 
adapted to suit a 10 year rotation. This would not prevent intensively ma- 
naged enterprises from contracting for intermediate photographing, say 
after an interval of 5 years, in order to keep their photos up-to-date, avoid 
the addition of drawing and recordings and possibly to develop a control 
system based on the photos. Possibly photos might be taken even more 
frequently, but restricted to the operational divisions on which forestry 
in the Norrland is being more and more concentrated. 
5. Mapping on aerial photos as regards the State Forest Service has been 
done by its own personnel and the companies mainly employ the Com- 
mittee of Forest Photogrammetry. The procedure has consisted of making 
distinction between stands, swamps, hills and other separate areas, and 
making some streams, trails etc. stand out more clearly. This sketching is 
done indoors, using colored pencils. The sketching is done on standard 
material (contact prints of vertical photos, mostly 1:20 000), by stereo- 
vision and sometimes with the aid of forest maps, for example to facilitate 
distinction between clearings and swamps. Mirror stereoscopes and Delft 
stereoscopes have been used, but the main part of the work is done with 
pocket stereoscopes of 2 X magnifying power. The personnel has been 
specially trained for mapping in the field as well as for interpretation. 
Most projects have comprised forests in Norrland, which are usually 
sketched somewhat quicker and with better results than forests in Southern 
Sweden. 
6. The choice of suitable mapping method is governed both by the forest 
conditions and by the technical facilities. Cooperation, especially between 
the Geographical Survey Office and those engaged with forestry, has lately 
developed in an exemplary fashion. Much remains to be done, however, 
which may be expected to be reflected in future modifications of the 
methods employed. Looking well ahead, for example, one may count on 
better photos, including color photos. In forestry circles efforts, which will 
certainly be made, to carry out cruisings mainly on the basis of aerial 
photos will certainly impose fresh and greater demands on the quality of 
the photos, which in turn may affect mapping methods in the future. The 
drawn maps have remained rather unchanged in type in Sweden for a 
century. It will probably be a long time before the new methods attain 
anything approaching that permanence.