On the web, it can be defined as “Georeferenced orthorectified, maps.”
Input 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 not 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 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.
The projection is defined on the entire sphere, except at one point: the projection point. Where it is defined, the mapping is smooth and bijective. It is conformal, meaning that it preserves angles. It is neither isometric nor area-preserving: that is, it preserves neither distances nor the areas of figures.
The projection point would be in the mid-sky!
Some use of Orthomosaic work / mapping:
Commercial Software doing Orthomosaic / 3D Mapping
Pix4D Capability: 3D Point Cloud, Textured Model, Orthomosaic
There is a free Discovery version, with full features but no outputs and measurements. Nevertheless, you can generate video presentation and share to others.
There is a Swiss company SkyPlan using Pix4D as commercial solutions.
This is a powerful software to create 3D images out of plain still images, pretty cool!!
And there insteresting write-up about Oil Tank 3D mapping application.
Some Free Alternatives of UAV Mapping