band of interest. This is particularly true of course if the maps are drawn 
from a variety of sources, or if they are ungridded, as for example in the case 
of the old 1/2,500 scale County series of the Ordnance Survey. Remarkable 
though it may seem, the dangers of error arising from paper distortion, re- 
compilation procedures and tracing are sometimes overlooked by engineers, 
and in the past much abortive work must have been caused by the use of in- 
accurate maps at the scheme stage. To appreciate this one has only to consider 
the difficulty that soil surveyors will have in recording their findings on in- 
accurate maps, and the difficulty which the engineer will have when trying to 
relate these findings to the design. Worse still, he may be faced with the 
need to alter the route after it has been legally adopted - a situation not e e 
condusive to good relations with the administrative authority concerned. 
Unless the terrain is very flat, a digital terrain model (DTM) at 
scheme stage also has much to commend it. It will enable the engineer to 
evaluate very rapidly the earthwork quantities for any number of alternative 
routes. Of course the cost of earthworks is only one of many factors affect- 
ing route selection, and roads have been designed when aesthetic, land use, 
military or political considerations overrode all others, but the occasion 
must be rare when earthwork costs are too inconsequential for inclusion in 
the cost/benefit analysis. If a preliminary air survey has been commissioned, 
the additional cost of preparing an open-mesh DTM need not be prohibitive, : 
and it is well worth pointing out to the engineer the benefits to be obtained. e e 
However, a firm line must be taken in securing acceptance of a realistic 
accuracy specification, or there may be a wasteful imbalance of scale and 
heighting requirements. Often half-metre heighting accuracy will serve the 
engineer's needs perfectly well at this stage. 
There are several types of DTM, the best-known and simplest being 
a square grid pattern of spot heights. Other systems employ a triangular 
pattern, parallel cross sections or random spot heights. Triangular patterns 
may demand care in the positioning of spot heights, the intention being that each 
triangle should define a plane on the ground. This can add considerably to the 
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