4.15.1. Mosaics

The mosaic is an assembly of overlapping aerial photographs that have been matched to form a continuous photographic representation of a portion of the Earth's surface.

A mosaic in photogrammetry refers to a collection of aerial photographs that are overlapped and aligned to create one seamless image of the Earth's surface. This process is crucial for capturing large areas in detail and is particularly useful in mapping.

Imagine putting together a large jigsaw puzzle. Each piece represents an aerial photograph. When you fit these pieces together correctly, you create a complete picture of a landscape.

Photo-mosaics offer the best of both high resolution images and acquiring overlapping images over a larger area. The overlap portion allows the images to be merged to form a seamless mosaic that can be used for interpretation or map preparation.

Photo-mosaics provide high resolution images by utilizing overlapping parts of aerial photos. This means even while covering extensive areas, the details remain sharp and clear. The overlaps help in merging different images without significant distortions, leading to more accurate maps.

Think of a stitched photo album where each photo overlaps slightly. The overlapping areas make it easier to combine them neatly, thereby creating a large panoramic photo without gaps or distortions.

The creation of a mosaic may need less overlap than for DEM capture using photogrammetry techniques. However, larger overlaps allow the mosaic to be created from central part of the photographs which is considered to have less distortion.

When creating mosaics, the degree of overlap between images is important. While less overlap can be sufficient, a larger overlap typically results in enhanced accuracy. This is because the central portions of the photographs generally have fewer distortions, making for a clearer final image.

Consider the difference between a drawing done with a pencil and one done with a pen. The pencil allows for adjustments and corrections, similar to how larger overlaps help in correcting distortions during mosaic creation.

Mosaics fall into two broad categories:

1. Uncontrolled mosaic
   • Image details are matched in adjacent photos and photos are joined together manually.
   • No ground controls are used.
   • It is quick to prepare
   • It is not as accurate as controlled mosaics but for many purposes, such as reconnaissance surveys, they are acceptable.
2. Controlled mosaic
   • It is the most accurate.
   • Photo-rectification is carried out on the images.
   • Image features on adjacent photos are matched as closely as possible.
   • Ground control points and corresponding points on the images are used to create the mosaic.
   • Can be used for application where planimetric accuracy is important.

Mosaics come in two main types: uncontrolled and controlled. Uncontrolled mosaics are created quickly without precise alignment or control points, making them suitable for general surveys but less accurate. Controlled mosaics involve rigorous alignment using ground control points, resulting in higher accuracy crucial for detailed mapping efforts.

Think of an uncontrolled mosaic like a quick collage made from magazine cutouts – it might look good but isn’t perfect. In contrast, a controlled mosaic is like a carefully constructed architectural model, where every piece is measured and aligned for perfect accuracy.

Photomosaics are created by merging overlapping aerial photos. Additional information, such as place or road or river name can be taken form maps or ground survey. In this mosaic, there will be scale variation across the photomap, due to terrain undulations (scale of a photograph varies from point to point depending on the elevation of the point). In addition, towards the edges, there will be some relief displacement present.

Photomosaics involve the combination of aerial images to create an expansive view. However, they can have varying scales depending on the surface terrain. This means that features like roads may not uniformly represent their true positions throughout the mosaic due to elevation differences.

Imagine a map of a hilly region where some roads appear longer than they are because of elevation. If you look at a flat area versus a steep one, the way you measure their lengths will give different results, much like how scales vary in photomosaics.

An orthophoto is an aerial photograph that has been rectified so that it possesses the characteristics similar to a map. It is also known as the map substitute. An orthophoto is an image that shows objects in their true positions. They are geometrically the same as conventional maps, therefore they can also be used as basic map substitutes to take direct measurements without further adjustments.

Orthophotos are rectified photographs that correct for distortions and align features precisely like a map. They allow for accurate measurement directly from the image, unlike ordinary aerial photographs that can have scale variations based on terrain.

Think of an orthophoto as a high-resolution map where every detail is perfectly aligned. It's like using a clear, zoomable satellite image in Google Maps that allows you to measure distances accurately without worrying about distortions.

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