Since the adopted system will seldom if ever be parallel to the autograph 
axes, the only practical means available to most photogrammetrists for position- 
ing the reference mark on the grid is the use of a profiloscope - type instrument 
in conjunction with a gridded sheet on the autograph plotting table. This method 
is effective if not very elegant. Automatic guidance mechanisms like the PR 
Profilometer would be of value here, enabling the operator to move directly 
from grid point to grid point without constant reference to the plotting table, 
save for the purpose of identification. Perhaps in time equipment such as this 
will come to be generally used. 
The grid-style DTM is usually divided into a number of blocks, some- > 
times rectangular but more frequently square. Now it is unlikely that each e e 
block will be matched by a stereogram having identical boundaries - in fact one 
stereogram may well include parts of 2, 3 or 4 different blocks, and one block 
parts of several stereograms - thus if DTM points are recorded directly onto 
paper tape they will almost certainly be in some disorder. Moreover the XY 
coordinates registered on tape or card will of course be referred to the auto- 
graph axes, not to the National Grid. These difficulties need not arise if 
observations are entered on data sheets by hand, and although it might seem 
foolish to abandon the advantages of automatic registration it is sometimes 
cheaper to do so. 
It is pertinent at this point to consider who will undertake the format 
conversion, by which is meant the reconstitution of photogrammetric data into e 9 
the separate, ordered blocks required by the design programs. The conversion 
may well involve not only a re-ordering of the data but also a change of style, 
code and medium, and the only efficient way to make it is by computer. If the 
reconstituted data is found to be incompatible with the structure of the design 
programs, responsibility for any consequent wastage of design effort must rest 
with the organisation which made the conversion. Since the conversion is some - 
times quite complicated, acceptance by the engineer of the original photo- 
grammetric output can be a valuable concession. 
B. 
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