330 
Discussion 
Discussion of the papers followed: 
Mr. Acker, of U. S. A.: I have a question on Kelsh plotter. In using the 3 point 
system for obtaining the dip slope, what is degree of accuracy when using photos 
at a scale 1: 20 000 for instance in the front range. 
Mr. John Davidson, U. S. A.: Accuracy is same as obtained for making topo 
graphic maps. We can read scale only so fine and place floating mark on bottom 
or top of the structure. Three readings are made and accuracy will depend on 
visual acuity of operator and »c» factor of the equipment. 
Mr. Acker: I was wondering whether you could cite field experience versus 
photo experience on measuring dip slopes. 
Mr. Davidson: I can in relation to stereo-slope comparator which is a less 
precise instrument than the Kelsh plotter. Slopes were read as 6, 7 and 8 
degrees. Field check gave them all as 8 degrees. Use of plotter work in geology 
could be applied in similar way in forestry. I mean for making measurements 
of crown diameters, tree heights or similar types of measurements. 
Mr. Losee, Canada: I have not made any use of Kelsh plotter though I have 
planned on so doing at favorable time. I believe it would be quite useful. I hope 
to achieve desired accuracy, using large scale mapping getting detailed estimates. 
At the present time we obtain desired accuracy in Canada using magnifying 
stereoscopes of 4 power which are crude compared to the Kelsh plotter. I be 
lieve Kelsh plotter should be tried. 
Mr. Rogers, U. S. A.: Studies are being considered for using the Kelsh plotter. 
We will use such precise plotting machines primarily to measure what we have 
on spot location like 1 acre or 1/4 of an acre. We will count trees, measure 
heights of trees and if possible identify them and see how far we can go in this 
direction. Also we wish to make studies of film and filter combinations on large 
scale photos. 
Mr. Kemp, England: When using Kelsh plotter, what ground and height con 
trol are required? What is relative accuracies with or without ground control? 
Air. Davidson: Reconnaissance geology studies have been made with approxi 
mate scales with the stereo model levelled relative to water bodies, and by 
prorating stream grades. The measurements are not absolute only relative. 
For some exploratory work this is of great assistance. If interest is for mining 
geology at every large scale the Kelsh plotter will give accuracies to 6 inches 
if the required control were available. 
Air. Kemp: Then without ground control this process is not of much use. I 
feel that you must have ground control to be of any real use. 
Air. Davidson: In Alaska operations are done with 250,000 scale maps as 
horizontal control and either stream gradients for vertical control or scattered 
readings of aneroid barometer. A reasonable good picture of geology is being 
done in this manner. 
Lt. Col. Ilauser, U. S. A.: I wish a description about techniques being used in 
preparation of soils analysis and Air. Heaths’ paper seems to indicate some 
approach to this. I should like more explanation on the use of these techniques. 
For instance, can’t we improve the accuracy for interpreting trafficability? 
Air. Coleman: In the absence of Air. Heath, I will make a few comments on 
this question. I cannot do it justice in a few minutes and I am not sure that I