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pal point; c) and the coordinates of the principal point. 
a) We define principal distance as the distance from the inter- 
nal perspective center (nodal point) to the plane of the reticle 
measured along the perpendicular to the plane of the reticle: the 
perpendicular has to be coincident with the optical axes of the 
lense. This distance is variable in discrete Steps according to. 
the possibility of the metric camera the projector works with. 
b) We define principal point as the foot of the perpendicular 
from the interior perspective center to the plane of the reticle. 
The reticle, that is coated on an optical glass, is mounted on a 
frame: the frame is put in situ always in the same position. On 
the frame there are reference points which, when preperiy connec 
ted, determine the X,Y axes: these reference points have to be 
coincident with particular ones on the edges of the reticle. 
c) We define coordinates of principal point as those determined 
with reference to the X,Y reticle axes. The reticle as mentioned 
before is made up of two series of parallel lines intersecting at 
90° in point called nodes: the lines are bright on a black back- 
ground. 
Actually in raster-photogrammetry the reticle or pseudo-photogram 
has the same function of the second program in normal methodolo- 
gy: for this reason we must have a reference system X,Y, inorder 
to read the coordinates of the nodes. 
Besides, it would be advisable that: 
1)the principal distance of the camera and of the projectoe were 
the same, 
2)the size of the reticle to be projected were approximately e- 
qual to the plate of the camera. 
When. the above conditions 1), 2) are satisfied, we can take, by 
converging the optical axes of the camera and of the projector, 
all the illuminated area onto which the reticle is projected. 
Actually as we will not employ a plotting device with a human 
observer we have not to limit the angle of convergence to a few 
degree as generally requested. 
Moreover camera and projector can be mounted at the ends of a ho 
rizontal base supported by a special tripod: the base lenght can 
be varied. from 700 mm to 2000 mm. (see fig. 2). 
This equipment is very usefull for close-range standard survey: 
when the survey is done in almost the same conditions and it re- 
gards objects of the same size and morphology. 
The base can be oriented with azimuthal direction according to 
viewing conditions: moreover the device can rotate around the ba