Absolute Vertical Accuracy Flatness Rating / Openness Rating Flatness: Height difference ground point to control point (cm) Openness: % non- ground points within 10m of control point Poor>8>50 Weak6-825-50 Average4-615-25 Good2-45-15 Strong<2<5 Rate 'flatness' & 'openness' of area around each control point to determine suitability