On the question of the relation between map and photo scale used for revision, the USA reported a significant 
departure, in that, whereas previously the photo and map scales were equal, experiments are presently being 
conducted employing 1:140,000 scale photography for their 1:24,000 map revision. 
Whether the field checks should be carried out before or after plotting was discussed next, the main consensus 
being that since the up-to-dateness is decisive, the field checks should be as close as possible to the publication 
date of the map. This is, however, not always feasible, particularly in countries employing rectified photos in 
the checking phase. 
The ratio between the time spent on photogrammetric operations versus field checks appeared to depend on 
the procedures used. In Switzerland, 15 man days are spent per map sheet on field checks, compared to 15 to 
20 days for the plotting phase. The use of rectified photographs has considerably reduced the time spent on 
field checking in West Germany, nowadays only 3-4 days being required per sheet. 
On the question of instruments and methods employed, both Belgium and Switzerland reported that classical 
restitution procedures are used; the former, since rectification procedures, including orthophotography, do not 
fulfill their accuracy standards for detail not at ground level. Great Britain uses rectified enlargements for most 
of their work; stereoplotting and, recently, orthophotos only being used in the hillier areas. The USA reported 
that whilst they agreed that stereoplotting is the most accurate procedure, they have also to consider the 
economy, and will therefore adopt cheaper systems if these still fulfill their requirements. 
Questions raised from the floor included items such as the information density of maps revised and revision 
rates. The panel replied that this is largely dependent on the type of area involved, and on the number of 
changes that have occurred. Switzerland reported that, on the average, one topographer processes 14 sq. km. 
per day. 
Before closing the session, the panel Chairman thanked the panel, and hoped that their contributions would 
prove to be of assistance to countries yet to embark on map revision programmes. 
Tuesday August 1 1972, 13:30 
Orthophotography Session 
(Joint Session - Commissions II and IV) 
Chairman : H.G. Jerie 
Panel Chairman :G. Konecny 
Panel + T.J. Blachut 
T. Jornstad 
G. Krauss 
R. Mullen 
J.E. Steakley 
J. Visser 
M.C. van Wijk 
J. Zarzycki 
In opening this joint session of Commissions II and IV, the Chairman announced that both the 
instrumentational aspects as well as the applications of orthophotography would be treated. 
T.J. Blachut first presented his Invited Paper, “Methods and Instruments for Production and 
Processing of Orthophotos", after which three of the four authors of the Invited 
Paper "Performance and Applications of Orthophotomaps’ summarized their contributions, 
namely: J. Visser, “Height Information on and for Orthophotomaps™’; 
M.C. van Wijk, “Experimental Results from the Stereo-orthophoto Technique" and 
R. Mullen, “Application of Orthophotomap Products”. 
The second half of this double session was reserved for the discussion, in which selected 
questions were first put to the panel, and then to the floor for a general discussion. 
The panel Chairman proposed to first deal with instrumentation, and then applications, and 
read the presented papers on instrumentation, by title, namely: 
D. Hobbie, “New accessories for the GZ-1 orthoprojector’