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UAV flight missions • System solutions • Remote sensing • Aerial mapping • Geoinformation
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Orthoimage and Orthomosaic (= image map)

Inputs for the generation of an orthoimage are a georeferenced aerial image of central-perspective imaging geometry and its related digital terrain resp. surface model. Central perspective of the aerial image, terrain relief, and optical properties of the imaging sensor cause geometric displacements. Due to this, an aerial image cannot possess an uniform (homogeneous) scale, and true-scale measurement of distances, areas, or directions is not possible in an aerial image. Without some meta-information an aerial image is of no other (big) value to a geoinformation system (GIS) but being a nice picture.

To extract measurable, useful, spatial information from an aerial image e.g. for a GIS, it must get georeferenced and undergo an image transformation. Latter one is called a “rectification” and based on the digital terrain resp. surface model. The result of such a rectification process is a new and georeferenced image, which is called „orthoimage“ or „image map“.

 

 

The orthoimage possesses now a homogeneous scale and thus map-identical scale-properties. Many adjacent or partly overlapping orthoimages form a new, big, overall image, a so-called “orthomosaic”, which may cover very large areas of Earth's surface in true scale and projection.

The degree of detail-identification in an orthoimage resp. orthomosaic primarily depends on its ground sampling distance (GSD) per pixel. Flying height above ground level (AGL) and camera properties determine GSD. A color-orthomosaic with GSD 4 cm assigns each of its pixels the equivalent terrain area of 4x4 cm2 with one greyscale value in each of its red, green, and blue channels. Planimetric accuracy of UAV-Mapping derived orthomosaics often is in the magnitude of 0.5 GSD and thus enables the user to extract distances, directions, and areas with high precision.