4.8.20. Overlap

The common region (expressed as a percentage) between two photographs is called overlap. The overlap between two successive aerial photographs in the same flight line is called longitudinal overlap or forward overlap or end lap, and the overlap between photographs in adjacent parallel flight lines is called the lateral overlap or side lap (Figure 4.7).

Overlap refers to the area that is shared between two aerial photographs. It is measured as a percentage and is crucial for photogrammetry. There are two types of overlap: longitudinal, which occurs between two photographs taken in the same flight path, and lateral, which occurs between photographs taken in adjacent flight paths. Recognizing these overlaps is important for accurately stitching images and creating 3D models.

Think of overlap like the way puzzle pieces fit together. When you put two pieces together, there are sections where both pieces cover the same picture. If you have just enough overlap, it allows you to create a bigger, more complete image, similar to how overlapping photographs help to create an accurate map.

The amount of end lap is kept a minimum of 60%, which is useful to generate 3D view of the common area between the photographs.

In aerial photography, maintaining a minimum longitudinal overlap of 60% is essential for creating three-dimensional models from the photographs. This overlap ensures that there is sufficient common ground captured in the images, allowing for accurate measurements and depiction of the terrain features. If the overlap is too small, important details may be missed, resulting in incorrect or incomplete 3D models.

Imagine taking a series of photographs of a beautiful landscape but not positioning yourself correctly. If you only catch parts of the view without overlapping shots, you might end up with many gaps in your photo album. By ensuring that each photo overlaps significantly, you can recreate the entire view in stunning detail, just like how 60% overlap in aerial photography allows for comprehensive mapping.

The lateral overlap or sidelap is kept between 25-40%, which is used to create a mosaic of the area.

Lateral overlap is the percentage of overlap between adjacent flight paths. This overlap is typically maintained between 25% to 40%. It is crucial for creating mosaics, which are composite images made by stitching together several photographs. Good lateral overlap helps in connecting different images smoothly, ensuring that the entire area is represented without gaps or discontinuities.

Consider how a quilt is made from different fabric patches. Each patch overlaps slightly with the next, allowing them to be stitched together seamlessly. Similarly, when taking aerial photographs, having a solid lateral overlap ensures that all parts of the area are covered comprehensively, leading to a smooth and clear final ‘quilt’ of images.

Superlap: The common overlap between three successive photographs is called superlap. It means that a photograph with 70% overlap will have 40% superlap region.

Superlap is a concept that describes the overlap area shared by three consecutive photographs. If each of the three photographs overlaps by 70%, the overlapping portion between them is called superlap and is calculated to determine the extent of common area. Understanding superlap is vital for ensuring accurate data collection and 3D modeling since it reinforces the spatial relationships captured in the photographs.

Think of superlap as a layered cake. Each layer of the cake represents a photograph, and the frosting in between layers that holds the cake together represents the overlap. The more you have frosting in common between the layers, the more integrated and stable the cake becomes. This is similar to how superlap provides a robust connection between photographic layers, leading to clear and detailed maps.

Mosaic: The process of seamlessly joining a series of overlapping air photos together to form one large image is called a mosaic. It is created to view and analyse the large area.

Creating a mosaic involves combining multiple overlapping aerial photographs to produce a single, larger image. This is essential in photogrammetry for visualizing extensive areas, enabling effective analysis and mapping. By carefully stitching these overlapping images, cartographers can depict the terrain with better detail and continuity. A precise mosaic can provide insights into landscapes or urban areas without having to view each photograph separately.

Imagine creating a giant mural from many smaller pieces of art. Each piece has some overlap with its neighbors, allowing you to see the complete picture when viewed together. By making sure each piece fits well with the others, your mural becomes an impressive cohesive work. Similarly, a well-executed mosaic from aerial photographs provides a unified view that showcases the subject area beautifully.

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