4.14.1.1. Analog aerial triangulation

Graphical radial triangulation is performed with simple instruments, such as mirror stereoscope, a ruler, a pencil and a tracing sheet. The steps are given below:

Graphical radial triangulation is a method of capturing spatial data using readily available tools. It combines various instruments to identify and mark control points on photographs, which are crucial for creating accurate maps. The key tools used include a mirror stereoscope, which allows for 3D visualization, and simple drawing instruments for marking and measuring.

Imagine you are creating a treasure map using photos of landscapes. You use a special pair of glasses (the stereoscope) to see the depth in the images. Then, you mark important locations with a pencil as you look through these glasses, making the map more accurate and easier to understand.

1. 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, several aerial photographs are organized in a sequence called strips. Ground Control Points (GCPs) are crucial reference points on the photos that correspond to specific known locations on the ground. They enable the accurate scaling and alignment of the photographs, ensuring that the final map will be correctly proportioned and oriented in relation to the actual terrain.

Think of GCPs as landmarks in a neighborhood. If you want to draw a map, knowing where the library or the park is allows you to create a more precise representation of the area, helping others to find their way based on your map.

2. 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 the center of the photograph and serves as a focal reference for triangulation. Once established, this point on one photograph is transferred to adjacent photographs in a stereoscopic manner, ensuring consistency in orientation and scale across the entire collection of photographs. This step is vital for establishing a coherent relationship between photographs.

Imagine you are trying to assemble a jigsaw puzzle. The principal points are like the corner pieces; they provide a starting point for fitting the rest of the pieces together accurately. By making sure each piece connects properly, the whole picture comes together seamlessly.

3. 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, at about 2 cm from the upper and lower edge of the photograph, fulfilling the following conditions: a. The two points should be as nearly at the same elevation as the principal point. b. The points should be at a distance from the principal point, which is equal to twice the mean base of the adjoining photographs. c. The points should lie approximately on the bisector of the base angle on either side, and d. The point should serve as lateral point as well.

Minor Control Points (MCPs) are carefully selected based on specific conditions to ensure they can help maintain scale and orientation across the photographs. They need to be positioned symmetrically around the principal point, as this helps balance the triangulation process. Each selected point must also meet certain elevation and distance requirements to ensure accuracy in the mapping process.

Think of MCPs as the middle supports in a bridge. They need to be evenly spaced and strong enough to support the entire structure. If they are placed incorrectly, the balance of the bridge could be off, leading to a weaker and less accurate representation of the terrain beneath.

4. The MCPs are selected and transferred stereoscopically to adjoining photographs.

Once MCPs are selected, they must be stereoscopically transferred to neighboring photographs. This transfer maintains the spatial relationship between different photographic strips, enabling accurate triangulation and ensuring that all points correspond correctly across images.

Consider mixing colors in painting. If you have a specific shade on one palette and want to replicate it on another canvas, you need to ensure that the color matches perfectly. Transferring MCPs is similar to ensuring that you keep the same color consistency across different canvases.

5. The lateral control points (LCPs) are selected in the centre of lateral overlaps of adjacent strips to serve as connecting points between different strips. These LCPs are selected at least at the beginning and the end of the strip and on every third photo of the strip. These are then stereoscopically transferred to photographs of adjoining strips.

Lateral Control Points (LCPs) function as essential linkages between photo strips. They are chosen at specific intervals to ensure that adjoining photographs maintain alignment and accuracy throughout the overlapping areas. LCPs help correct any errors that may have arisen during the aerial photography or previous mapping stages.

Think of LCPs as the stitches in a quilt. If you want all the pieces to fit together seamlessly, you need to ensure that the stitches (LCPs) connect the patches (photographs) properly, allowing for a unified and beautifully finished result.

6. 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.

This step involves drawing lines from the principal point to all control points on the photograph. These radial lines are crucial for visualizing the spatial relationships and triangulations required to create accurate maps. They help in determining where each point lies in three-dimensional space.

Imagine you're casting fishing lines from a boat. Each line represents a connection to a different fishing spot (control point). Just as you cast to ensure you reach different areas of the lake, drawing the radial lines ensures you cover all spatial data points needed for accurate mapping.

Due to elevation differences of terrain and variations in the flying height of aircraft, the scale of photographs generally varies considerably. The photographs of a strip are required to be brought to a common scale through graphical triangulation.

Variances in elevation and aircraft altitude can lead to inconsistent photo scales, complicating the mapping process. Therefore, the photographs must be standardized to a common scale, ensuring accurate comparisons and measurements across all images.

Think of viewing a mountain range from various heights; the view changes dramatically. To get an accurate picture of the entire range, you need to standardize your point of view, akin to adjusting the scales in aerial photography to match the terrain across different heights.

It is preferable to start selecting MCPs somewhere in the middle of the strip to avoid accumulation of azimuthal errors. Each strip is plotted on a transparent sheet to facilitate the drawing.

Selecting minor control points from the center of the strip minimizes errors that can accumulate from incorrect angles or orientations. Using a transparent sheet allows for overlaid viewing and adjustment of the photographs, facilitating precise plotting and ensuring that calculations remain accurate.

Imagine trying to pin down a piece of fabric for sewing. If you start pinning from the edges, the fabric can bunch or distort. By starting in the middle and working your way out, you keep the fabric smooth and properly aligned, which is similar to avoiding errors in scaling when selecting MCPs.

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

Since the MCPs across various strips may be at different scales, projections are necessary to align them to a uniform scale. This is done by recalculating their measurements so that every photograph can be compared and utilized effectively in the triangulation process.

Think of a collage made of different sized photographs. To ensure that all pictures fit nicely together on a single page, you need to resize them, just as projections adjust the scales of MCPs before they are integrated into the final map.

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