and the middle of the central reserve of a dual carriageway, but where single 
and dual carriageways meet, or where dual carriageways become separated, 
to follow this rule would cause discontinuities in the reference line, thus 
complicating the horizontal alignment calculations. Various solutions have 
been proposed, such as adopting a reference line in the offside verge or 
employing a separate reference line for each dual carriageway, but all have 
their drawbacks, some causing serious problems in volume calculation. The 
engineer must consider this matter very carefully before coming to a decision. 
The second matter arises from the fact that any alteration to an 
alignment will almost certainly affect its length. This makes necessary: - 
a) The acceptance of a chainage discontinuity at the point where the 
amended route rejoins the original, or 
b) the acceptance of non-standard chainages for all stations from 
this point through to the end of the route, or 
c) the re-positioning of all stations and the re-observation of all 
cross sections, from this point through to the end of the route. 
The first of these alternatives is usually accepted, since it causes the 
least immediate work. But if the alignment is altered two or three times 
confusion will result, as those who have tried it will know. 
It seems sensible therefore to break down the route into several parts 
each with a zero starting chainage, and to observe new cross sections for the 
whole of any part affected by realignment. Ideally each part should contain 
only 1 or at most 2 curves. A similar plan with chainage reset to zero at 
selected block boundaries would of course save computing costs of the DTM 
method also. 
There is a third matter to be considered and that is the siting of cross 
sections. The engineer will specify cross sections at regular intervals with 
perhaps one increment for straights and wide curves and a smaller increment 
for short radius curves. But he will also expect cross sections where the 
ground slope changes sharply, e.g. at ravines, and at points where the 
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