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photogrammetric task, and calls for a higher grade operator than the grid 
method. Some (but not all) of the so-called random systems are merely vari- 
ations of the triangular method flying false colours. From the photogrammetric 
standpoint much of the most attractive system at present is the simple square 
grid. It is usual for the engineer to specify grid spacing, so that this method 
relieves the photogrammetrist of all responsibility for the positioning of spot 
heights, and incidentally it simplifies cost estimating. 
The DTM may be prepared by interpolation from contours followed by 
manual punching of the data, which is a method economical in instrument time 
but open to human error at both stages. More favoured is the method of 
observing spot heights directly in the sterec-model, with automatic recording 
on to paper tape. Naturally, all photogrammetrists will at first be attracted 
to the idea of a grid that is parallel to the XY axes of the autograph, but every 
flight line would then generate a different grid, none of which would be accept- 
able in the design process because they are not recoverable on the ground. 
Whatever grid is chosen, it should be suitable for every stage of the project - 
the inconvenience of working in a variety of grid systems needs no elaboration. 
It can be avoided if a national or otherwise standard grid is adopted at the outset 
and used for all design processes. 
This point may need stressing because few engineers (and for that matter 
not all photogrammetrists) really understand the technicalities of the map 
projection used for their country's National Grid. Rather there is a tendency to 
regard the scale factor introduced by the projection as an unnecessary compli- 
cation, but it need not be so if all distances and levels are measured on the 
spheroid and all coordinates in the grid. Engineers may also be loth to adopt 
the National Grid if its unit of measurement is not one to which they are 
accustomed. Surveyors in Britain have long been reconciled to the existence 
of English and metric units side by side, and it has been found that the engineer 
who adopts one unit for coordinates and another for distances and levels causes 
no problem. In fact the unit conversion factor and National Grid scale factor 
can be replaced by one composite factor for computational purposes.