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Lecture 17 Content

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Lecture 17 Content

Aerotriangulation (AO)

- Defined as the procedure of establishing the geometric relationships among overlapping and side lapping photographs for determining the positions of supplemental horizontal control points
- Reduces substantially the control required by field methods

- Triangulation calculates:
- The position and rotation of the camera each time an image was exposed (Exterior Orientation Parameters)

- The X, Y and Z coordinates of tie points

30% overlap between

runs or swaths

Run #1

Run #2

- 1. Photographs or images

- 2. Camera information

X,Y,Z

- 3. Control

- A BLOCK is all the information needed to triangulate a set of air photographs in one process

- This must include:

- 4. Projection, Spheroid, Datum Information

60% Overlap

- A Block created in Stereo Analyst contains two overlapping images (a Stereo Pair).

- Aeotriangulation adopts photogrammteric triangulation which establishes the geometric relationships among overlapping and sidelapping photographs to determine supplemental horizontal and vertical control points
- Aeotriangulation is performed using one of two methods:
- Semianalytical (or independent model triangulation)
- Analytical (or bundle block)

- Semianalytical
- passes control from one model to the next
- Model’s relative orientation is done instrumentally on the plotter or analytically using a computer program, but the scale transfer between successive models is accomplished analytically
- Hence each model are independently generated
- Each model will have at least 3 tie points in common

- Relative orientation is done for each model and absolute orientation is done for the first model
- Use is made of the coordinates of the perspective centers of both photos (using the same coordinate system) as tie points

- Coordinates for the second overlap is transformed mathematically by making coordinates of the left-hand perspective center of the first overlap and then rotating the second coordinate system about its three axes to make the points coincide with the first overlap

- The perspective center coordinate provides a strong geometric tie between the models
- Tie points are computed using computer program that implements a transformation involving 7 parameters:
- 3 translations
- 3 rotations
- One scale change

- A TIE point is the image coordinate position of an object appearing on 2 or more images

- The X, Y and Z coordinates of a tie point are unknown and are determined by OrthoBASE during the aerial triangulation procedure

- Analytical
- This is the second method of aerotriangulation
- Image point-measuring instruments (comparator, analytical plotter, workstation) are used to measure the x-y plate coordinates of each tie point and control point in each photograph
- Sequence of mathematical models are formed from which strip coordinates are derived
- Mathematical model represent the geometric relations between object space, perspective centers, and photographic images

- Image points are represented by their photographic coordinates measured
- Input are photographic coordinates, camera parameters, and ground control points
- Use is made of the collinearity and coplanarity equations to manipulate the coordinates in a high speed computer
- Output of the computations are ground coordinates and elevations of the tie points
- Ideal tie points are the same points used in relative orientation
- Ability to correct for all possible systematic errors, such as film shrinkage, lens distortion, atmospheric refraction, and so on

- Block of eight images…

- Image & ground measurements

Least Squares Adjustment calculates new points based on distributing and minimizing residuals throughout the ENTIRE block

- There are RESIDUALS for:
- Each ground point
- Each image point
- Each perspective center

Predicted Location

Standard Deviation

Measured

Location

- The adjustment distributes error throughout the block trying to minimize all the residuals
- You can control the adjustment process with the quality estimates

- Least Squares is an iterative process. So, how does the process know when to stop adjusting the points and recalculating residuals?
- We define a threshold value in meters
Convergence Value

- Once the process reaches convergence it stops
- What is convergence?…

#2

0.044

0.024

0.436

0.087

0.021

0.111

0.434

0.432

0.153

0.321

0.654

0.543

- After each iteration residuals calculated for each measurement

If every difference between these values is less than the Convergence value, the iterations will STOP

Iteration

#1

0.054

0.049

0.386

0.195

0.054

0.054

0.674

0.912

0.282

Indicates that the triangulation has met the required accuracy

0.513

0.589

0.766

Self Calibrating and Bundle Adjustment

- Focal length

- Principal Point

- More GCPs an advantage (6 per overlap)

- SCBA will estimate the Interior Geometry of the sensor

- Cameras not designed for photogrammetry:
- Non-metric camera, digital camera or videography

- Cameras with outdated or no calibration reports
- Will estimate the interior orientation parameters of the camera/sensor
- Focal length
- Principal Point in the x direction
- Principal Point in the y direction

… The End …