Construction of white horses
White horses and other hill figures are usually intended to be seen from below. The steeper the slope on which the figure is cut the better, but whatever the degree of slope there will be a foreshortening effect, and the design needs to compensate for this if the figure is going to be seen in its intended proportions.
The first step in construction is to mark out the hill figure. This is not as easy as it may sound, as the figures are generally quite large, and can not easily be seen as what they are by someone standing on the site. Several methods have been used. One involves a team of people on the hill, each with a flag on a pole, or a large lump of chalk, and someone at a suitable viewing point shouting directions, or calling them through a megaphone, until the markers are correctly positioned to form the outline of the figure. Another method involves using a plan with measurements to various points around the outline from fixed points, and marking out on the ground first the fixed points and then the points around the outline. Yet another way is to mark out a grid on both the plan and the hill, and transfer the design step by step.
Cutting and filling the figure
Once the outline of the hill figure is established, the next step is to cut a trench along that line, then, if the figure is to be solid rather than in outline only, the turf within the outline is removed. The chalk bed is sometimes only a matter of inches below the surface, but the depth of the soil can vary, and usually it is necessary to cut into the chalk bed to get a reasonably level surface. At this point, there is a hill figure plainly visible from afar, but left like this it would be only a short time before the turf around the edges crumbled, and the outline would soon become blurred. So the shallow excavation is then filled with loose chalk, and often banked up above the surrounding turf. Sometimes shuttering is used to reinforce the edges.
Even with the excavation filled, there is still erosion and encroachment by the soil. Hill figures thus need to be periodically scoured to restore their appearance, and some figures have been lost through lack of maintenance. Scouring involves removing weeds and soil from the surface of the figure, and sometimes removing the top layer of chalk itself. The edges need to be redefined, and any shuttering may need to be renewed, then the figure is coated with new chalk. With the white horses this was often done at roughly seven year intervals, and at some of the horses this undertaking was something of a local holiday. Food and drink would be carried up to the site, and the work would be an excuse for a huge open air party.
The cutting of white horses can be called Leucippotomy. The term was coined by Morris Marples in his book "White Horses and Other Hill Figures", published in 1949.