The following points are made in favour of DTM - interpolated 
Cross sections:- 
1. They are readily interpolated for any number of alignments 
without adding to the photogrammetric task. 
2. At any time, additional cross sections can be generated at 
points of particular interest without recourse to photo- 
grammetry. 
3. The DTM is well suited to columnar calculation, which seems 
to be finding favour as a method of calculating volumes at 
junctions and interchanges. 0 e 
Clearly, at scheme stage the advantages of the DTM outweigh those 
of directly observed cross sections. It is only at design stage that opinions 
sometimes differ. Generally speaking, the DTM is of little value in urban 
areas, because 
a) spot heights at fixed centres cannot properly describe the 
cuttings, banks and steps of a man-made landscape; 
b) many DTM points would fall on buildings; 
c) other constraints will influence the alignment much more than 
the cost of earthworks, if indeed they do not override it 
altogether. o © 
On the other hand, the gentler landscapes of rural areas can usually 
be expressed quite adequately by a DTM, and lower land values with fewer 
constraints make it probable that several alignments will be tested. Here it 
seems the DTM is likely to be preferred. 
If nevertheless cross sections are to be observed directly, it is vital 
that two matters are first settled (in fact, the decisions should be taken 
much earlier, since they affect scheme stage computations). 
The first matter is the relationship of reference line to design. 
Normally the reference line defines the centre of a single carriageway road 
-9 .