146 
THE CANADIAN SURVEYOR 
light to the left plate from the fluorescent tubes at the rear of the instrument. By using heat 
absorbing glass and the mirror, we are able to keep the left plate near room temperature. Other 
wise, this plate would expand and a warm-up period of at least half an hour would be required 
each morning before the instrument could be used. 
The main feature of the S.L.P.-l is that it places a plane of lines in a stereo model. A 
topographer can deform and control this plane of lines and, guided by the intersection with 
the model, he can contour the model at set intervals. The instrument is equipped with a number 
of sets of plates for the left side and for the right camera. The set of plates with a wide line 
separation would be used for rough terrain, with large differences in .r-parallax, and a different 
set of plates with lines closer together would be used for flat terrain. 
The instrument is not designed to replace existing equipment, but rather to do a particular 
job with a minimum of effort and time. It could have special uses, such as setting the plane 
of lines to observe and measure the heights of obstructions in a glide path at the end of a 
runway. With a few known elevations it would be very easy to find the highest hill in an area or 
obstructions between hills as would be required in setting routes for micro-wave and hydro 
electric lines. The use of two broken lines instead of a plane of lines would make it possible to 
measure the relative dip and strike of exposed rock outcrops. In the early stage of road design, 
this instrument could rapidly produce contours or form lines that would guide the engineer in 
making preliminary route selections. 
In short, the S.L.P.-l is an inexpensive photogrammetric instrument, one that does not 
require precise calibration or expert knowledge to operate, but that will produce spot heights 
and contours on paper prints for preliminary engineering studies, for planning, for geology 
and for mapping.