22,
Another problem consists of the difficulties of taking into
account the influence of terrain break lines as chanels, exis-
ting roads and other constructed elements on the interpolation.
Those systems using linear interpolation based upon terrain
elements as breaklines or contours have overcome this problem.
In some other systems a combination of measured single spots’
and breaklines has been introduced in order to improve the quali-
ty of interpolation. Systems measuring only single spots in the
form of grids or otherwise introduce computation problem in rough
terrain or in terrain with many breaklines as an increased den-
sity of measured points means increased time and cost of measure-
ment and interpolation.
A further problem is the lack of quantitative information avail-
able about the existing systems. Although it is possible to esti-
mate the probable accuracy of a given system there has been little
factual evidence in the past to support subjective judgements.
In order to help filling this lack of information the British
Road Research Laboratory is investigating among other facts the
accuracy-density-terrain type relationships for different exis-
ting DTM:systems. The result of this work is nearing completion
and will be published by the Road Research Laboratory.
One of the essential problems of the various DTM:s from photo-
grammetric point of view has been the registration of the model
coordinates. This problem has, however, now been solved by the
development and introduction of more efficient equipments, as
was mentioned above on page 15. The geometrical quality of the
photogrammetric data can also be improved by introducing correc-
tions for known systematic errors before the transformation into
the ground system. Reference is made to Ternryd (26). One remaining
problem is, however, the completeness of the measured data and
the amount of gross errors. Some kind of plotting seems to be re-
quired in order to check the completeness of the data. An inte-
resting possibility for solving this problem is offered by the
Zeiss Planimat (see fig 10), which has an internal plotting
table which enables the efficient making of a control pencilled
manuscript simultaneously with the registration. Reference is
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