3.5.5 Proximity of controls
Controls on different courses placed too close to one another can mislead runners who have navigated correctly to the control site. According to Rule 19.4, controls shall not be sited within 30 metres of each other (15 metres for map scales 1:5000 or 1:4000). Only when the control features are distinctly different in the terrain as well as on the map, should controls be placed closer than 60 metres (30 metres for map scales 1:5000 or 1:4000). The distance between the controls is measured in a straight line.
3.5.6 The control description
The position of the control with respect to the feature shown on the map is defined by the control description.
The exact control feature on the ground, and the point marked on the map, must be indisputable. Controls which cannot be clearly and easily defined by the IOF control symbols are usually not suitable and should be avoided.
Le reste est de l'affaire de Robert...