4.26. Graphical Radial Triangulation

Graphical radial triangulation is performed with simple instruments, such as mirror stereoscope, a ruler, a pencil and a tracing sheet.

Graphical radial triangulation is a method that uses basic tools to align and measure positions based on photographs. The instruments mentioned include a mirror stereoscope for viewing photographs in 3D, a ruler for measuring distances, a pencil for marking important points, and a tracing sheet for plotting and transferring data.

Think of it like putting together a jigsaw puzzle. You need specific tools (your hands, a table) to fit pieces together accurately and see the full picture clearly.

The photographs are laid out in strips and all GCPs are identified on the photographs and their numbers marked on the photographs.

In this step, photographs are organized in a sequence called strips. Ground Control Points (GCPs) are critical points on the ground whose locations are known. By marking these points on the photographs, the process ensures accurate interpretation of the terrain when analyzing the images.

Imagine preparing a treasure map where you have to mark locations of treasures accurately. Without clearly identifying where each treasure is on the map, it would be hard to find them!

The principal point is obtained for each photograph. The principal points are then stereoscopically transferred to the adjoining photographs as conjugate principal points.

The principal point is a central reference point on each photograph that is essential for accurate positioning. By transferring this point between adjoining photographs, it helps maintain consistency in measurements and relations between different images.

Think of it as a common meeting point for friends. If everyone knows where to meet, it’s easier for them to find each other regardless of where they are.

Now two points, called minor control points (MCPs), also called pass points or wing points, are selected on both sides of the principal point of photo...

MCPs are additional reference points chosen near the principal point to aid in accurate plotting. Several conditions ensure they are suitable for this role, including being at the same elevation as the principal point and positioned correctly in relation to adjoining photographs.

Consider a road with guideposts. The principal point is like the main junction, and the minor control points are like the guideposts along the road that help guide travelers and ensure they are on the right path.

The MCPs are selected and transferred stereoscopically to adjoining photographs.

This step involves using a stereoscope to ensure that the selected MCPs are accurately plotted on neighboring photographs, enhancing the spatial relationships depicted.

Similar to aligning pieces when building a LEGO structure. Ensuring each piece fits perfectly means that the whole structure will be stable and look correct.

The lateral control points (LCPs) are selected in the centre of lateral overlaps of adjacent strips...

LCPs serve as important reference points located at overlaps between strips of photographs, providing a connection and continuity across images. This helps in stitching together larger parts of the map accurately.

Consider the way dancers rehearse a routine. LCPs are like markers on the floor that guide where each dancer needs to be positioned to maintain formation and flow.

The radial directions from the principal point to all minor control, lateral control and ground control points appearing on the photograph are drawn, through the points.

In the final step, radial lines are drawn from the principal point to all identified control points on the photograph. This process visually indicates the relationships and directions to these key points, facilitating accurate mapping and measurements.

Imagine drawing lines on a treasure map from a 'You Are Here' marker to various landmarks (mountains, rivers) that will guide you. The lines help you to navigate efficiently to your destination.

Due to elevation differences of terrain and variations in the flying height of aircraft, the scale of photographs generally varies considerably...

In photogrammetry, variations in scale can lead to inaccuracies. Therefore, a common scale is established through graphical triangulation, ensuring all photographs fit together correctly.

Think of it like adjusting all your photos to the same filter on social media so they look consistent. This way, everything matches and appears orderly.

Each strip is plotted on a transparent sheet to facilitate the drawing. The photographs are laid out in their correct relative directions...

Plotting on transparent sheets allows for easy viewing and manipulation of the base photographs. It ensures that all elements can be aligned precisely as needed, maintaining the correct relationships between points.

Imagine laying a clear sheet over a picture to trace over key details. This ensures you capture all necessary elements without losing track of the original image.

The MCPs of different strips are at different scales. To bring all of them to the same scale, a projection is made...

When multiple strips are used, they often have varying scales due to different elevations or aircraft heights. By projecting and adjusting these scales, all strips can be aligned to a common standard for consistent analysis.

Similar to resizing images in a collage to ensure every picture is the same size, enhancing the overall aesthetic coherence of the layout.

After scaling all the points of minor control plots, these points are pricked through the tracing sheet...

After the points are scaled accurately, they are transferred carefully onto the plotting sheet, ensuring that all points align as expected. This step is essential in verifying the accuracy of the triangulation process.

Think of it like a final proofreading step on an essay. You're double-checking every detail to ensure everything is in its right place before submission.

--