| Note |
Ground Control Points (GCPs) should be at least 3 times (5 times recommended) more precise than the target specification for the ortho, e.g. in the case of a target 2.5m Root Mean Square Error (RMSE), the GCPs should have a specification of 0.8m RMSE or better.
GCPs should ideally be determined from field survey, using Differential Global Positioning System (DGPS) supported with geodetic control points or a Global Positioning System (GPS) reference station network, though direct measurement survey methods for precise ground control are also acceptable. However in exceptional cases if this is not possible they may be scaled from maps of sufficiently high precision, or taken from an oriented flight of an appropriate scale measuring in stereoscopic mode or from orthoimages and associated Digital Surface Models/Digital Terrain Models (DSMs/DTMs). Where ground control is obtained from topographic mapping, a digitization error (0.02mm at the map scale) must be allowed for, thus an accuracy improvement factor of at least five is recommended when estimating a suitable map scale for planimetric ground control points. |