Any land (or elevations) with contours depict the ‘undulations’ on the surface of the site or area in question and these are normally shown at quarter or half metre intervals related to the site or Ordnance Survey Grid System and height datum. On elevations these can show irregularities with the contours depicted at 5 – 10mm and could show ‘sags or bulges’ in the façade or structure. Data supplied by CD Surveys or by a third party can be processed, then presented as a fully edited three dimensional Digital Terrain Model (DTM).