4.14.1.2. Semi-analytical aerial triangulation

Semi-analytical aerial triangulation or independent model aerial triangulation is partly analog and partly analytical procedure. Here, each stereo-pair of a strip is relatively oriented in the plotter.

Semi-analytical aerial triangulation combines features from both analog and analytical methods. In this technique, each stereo-pair, which is a pair of images taken from slightly different angles, is adjusted to fit together in the plotter. This way, the coordinates for each model are kept independent from one another during the initial orientation step.

Imagine trying to assemble pieces of a puzzle where each piece must fit into the correct spot independently, but when combined, the pieces form a complete picture. Each stereo-pair works similarly to the puzzle pieces, being oriented first before connecting them into a cohesive model.

The coordinate system of each model being independent from the others; model coordinates of all control points and pass points have to be read and recorded for each stereo model.

In this step, every model needs to record specific points called 'control points' and 'pass points'. Control points are known locations that help in accurate mapping, while pass points act as bridges connecting adjacent models. Each model must maintain its own coordinate system until they are linked through these pass points to ensure accuracy and continuity.

Think of a road trip where you have a map for each city but need to note down landmarks (control points) along the way. These landmarks help you navigate and ensure you can find your way from one city to another without losing your path.

By means of pass points common to adjacent models, a 3D coordinate transformation is used to tie each successive model to the previous one to form a continuous strip.

Using the common pass points, 3D coordinate transformation is applied, allowing different models to align with each other. This transformation is crucial for creating a continuous and encompassing strip of the area being mapped, facilitating a seamless integration of data.

Imagine building a long train track where each segment must align perfectly for the train to run smoothly. The pass points are like the connectors that ensure each segment of the track fits seamlessly into the next.

This strip may then be brought to ground coordinated system again through 3D coordinate transformation and adjusted numerically utilizing the polynomial equations.

After aligning all the models into a continuous strip, there's a need to convert the data from the model's coordinate system to the ground coordinate system. This process may involve using polynomial equations to accurately reflect the real-world geography on the mapping outputs.

Think of adjusting a camera shot to ensure that the visual content matches the real perspective; just as a filmmaker ensures that the scene fits with reality, the mapping must ensure that the aerial views accurately correspond to the land they depict.

Relative orientation of each individual model is performed using a precision plotter. Strong imaging geometry plus a minimum of three ground control points are needed to solve the six unknowns per bundle.

Precision plotters conduct final adjustments for each model to guarantee everything fits together correctly. To solve for unknown variables, a minimum of three well-known ground control points is required to ensure accuracy in positioning each part of the aerial photograph.

Consider baking a multi-layer cake; you need to ensure each layer is properly aligned and supported by the layer beneath it. In mapping, those well-known control points provide support and guidance to ensure every part fits perfectly together.

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