Introduction to Soft Copy Photogrammetry

1. Introduction to Soft Copy Photogrammetry Working in a totally digital environment

2. Advances in photogrammetry • Plane table photogrammetry (~1850) • Used techniques common to surveyors • Analog photogrammetry (~1900) • Uses large optical instruments for making maps • Analytical photogrammetry (~1960) • Integrated digital processing with analog equipment • Digital or soft copy photogrammetry • Processing of aerial photos that are in a digital format

3. Analog and analytical plotters Wild A-10 Analog Stereoplotter Wild BC-2 Analytical Plotter

4. Getting photos in a digital format • Acquire them with a digital camera • Large format metric cameras are very expensive • Technology is advancing relatively fast • Scan the photographs • Important to include fiducial marks in the scanned image • Need large scanner to handle 23cm photos • Geometric distortions can be problematic with office-grade scanners • Select appropriate resolution • Best to scan film products rather than paper

5. Why use a photogrammetric approach? • Conventional geometric correction methods work by warping the image based on control points and work best in level terrain • Photogrammetric methods work by modeling the image acquisition process and can re-project each pixel to the “proper” location so accurate measurement can be made • With photogrammetric methods it is possible to: • Derive elevation/height values • Process large blocks of photos at a time

6. Working with single or multiple images • Working with photos one-at-a-time is very time consuming if many photos will be processed • Each photo needs at least 3 ground control points (GCPs) • Using a process called block (or areal) triangulation it is possible to more easily process large blocks of photos • Determines the position and orientation of each photo when it was acquired • Only a few (minimum of 3 but generally more) accurate GCPs are needed for a large block • Tie points (GCPs derived from the photos) are generated automatically using feature correlation techniques • Facilitates creation of mosaics

8. Coordinate systems • Pixel coordinate system • Origin in upper-left • Image (imaging plane) coordinate system • Origin (principal point) in the center based on fiducial mark intersection [units usually mm] • Image space coordinate system • Same as “Image coordinate” with addition of a “Z” optical axis with the “Z” origin being the lens which is a distance equal to the focal length from the imaging plane • Ground coordinate system • X and Y are same as projected map coordinates and Z is elevation

9. Determining the relationship between the image plane and the photo • Interior orientation • Associates the image pixels to the image space coordinate system • In other words, it transforms the pixel coordinates into the image space coordinates • Uses a camera model that defines: • Fiducial mark locations in image space coordinates • Principal point location in image space coordinates • Focal length • Lens distortion parameters

10. Determining the relationship between the image plane and the terrain • Exterior orientation • Defines the position and orientation of the camera when the photo was acquired by associating GCP coordinates (X,Y,Z) with the image space coordinates • The position of the photo center is defined by X, Y, and Z (flying height) coordinates • The camera orientation is described using angular and rotational parameters • roll (omega [ω]) • pitch (phi [ϕ]) • yaw (kappa [κ])

11. What can you get out of all this? • Create orthorectified image mosaics • Removal of camera and relief displacement to create an image map (convert from central perspective to orthogonal perspective) • Create oriented images that can be used in stereo analyst • Derive spot elevations, contours, or digital elevation models • Measure heights of features • Draw planimetric features to create or update maps