- 17«- 
over the entire project area, 
In order to reduce the density of vertical control, 
the mapping photography should be flown in conjunction with 
Statoscope and horizon camera. Either of these auxiliary 
instruments will be helpful in carrying out aerial triangu- 
lation, between the control established by APR, 
The use of laser APR is frequently mentioned as a 
possibility in forested areas. It appears that it may be 
possible for the laser, due to its very narrow beams, to "see" 
the ground between the trees. However, the laser APR has not 
been tested as yet over a tropical rain forest and judgment 
as to its suitability cannot be made at this time. When 
considering the use of laser APR, it should be remembered 
that the laser APR gives accurate terrain clearance information 
but the accuracy of determining elevations is limited, as is 
the case in conventional APR system, by the height corrector 
which senses the difference in pressure corresponding to the 
variation in aircraft altitude, relative to a barometric 
surface. Another characteristic of the laser APR is that, 
since it is a light beam, it "sees" through clear water. 
Consequently, elevations of waterbodies are unreliable as 
they do not refer to the surfaces but to a point below the