Flying lower to the ground results in less ground coverage captured at a higher resolution, while flying higher results in more ground coverage at a lower resolution. This assumes that the same camera and lens is used.
Photomosaics offer the best of both by flying at lower altitudes and acquiring overlapping imagery over a larger area allows the images to be merged to form a seamless mosaic that covers a larger area but at high resolution.
The overlaps ensure that there are no gaps in the resulting imagery. Less overlap is required for producing aerial mosaics than for DEM capture using photogrammetry techniques. However larger overlaps allow the most central part of the image to be used (the further from the centre of the image the greater the distortion caused by the lens).
Types of photomosaic:
Photomosaics are created by merging overlapping aerial images. They can also be used as basic map substitutes.
Additional information such as place or road names can be superimposed on the images (either directly to the image file or if the images are georeferenced as overlaid GIS or CAD files).
Unless the terrain is perfectly flat there will be scale variation across the extent of the photomap. Towards the edge of the images there will be some element of relief displacement, the effects of this can be limited by flying with suitable overlaps so the edges of the image are not used.
Orthophotos and Orthophoto Mosaic
An orthophoto is an image that shows objects in their true positions. They are geometrically the same as conventional maps. Unlike regular aerial images they can be used as maps from which you can take direct measurements without further adjustments. The processing to create orthophotos removes the effects of relief displacement and photographic tilt.
Orthophotomaps are produced using either one or multiple overlapping orthorectified aerial images which have the benefits of both aerial photos and line maps.
Types of aerial mosaic
Mosaics fall into three categories:
- The most accurate of the three classes
- Photo rectification carried out on imagery
- Image feature on adjacent photos matched as closely as possible
- Ground control points used and correspond to points on the images
- Image details matched in adjacent photos
- No ground control
- Quick to prepare; while not as accurate as controlled mosaics for many purposes they are acceptable.
- Use some combination of controlled and uncontrolled techniques
- Compromise between accuracy and cost