Remote Sensing Geometry of Aerial Photographs

1. Geometry of Vertical Aerial Photograph Oblique photographs - Cameras oriented toward the side of the aircraft Vertical photographs  - camera aimed directly at the ground surface from above  - difficult to recognize ground features but measurements can be made

Geometry of Aerial Photo

Black and White Aerial Photographs

Color Aerial Photographs

NIR Aerial Photographs Photo taken as part of the NAPP mission in color IR at a scale of 1:40,000. (1”=3,333’). This image has been enlarged by x factor.

1. Geometry of Vertical Aerial Photograph Photogrammetry  - science of making accurate measurements from aerial photographs

2. Basic Elements Fiducial marks Principle point Ground nadir Photographic nadir Isocenter

Frame Number Date of Photography Mission ID Nadir Point Principal Point Fiducial Marks

2. Basic elements Fiducial marks  - at the edges and corners recorded during exposure  Principle point  - intersection of lines connecting opposite pairs of fiducial marks  

Basic Elements Ground nadir  - point on the ground vertically beneath the center of the camera lens during exposure  Photographic nadir  - intersection of the photograph and the vertical line that intersects the ground nadir and the center of the lens

Basic Elements Isocenter    - the focus of tilt    - on a true vertical photo, the isocenter, the principle point, and the photographic nadir coincide

Vertical Aerial Photography

Vertical Aerial Photography Most common type aerial photography Camera axis as vertical as possible Generally a tilt or error introduced in photography of 1 – 3 degrees Small amount can be easily adjusted

Geometric Errors of Vertical Aerial Photography Optical distortion   - caused by camera problems Tilt - caused by displacement of the focal plane from a truly horizontal position by aircraft motion (attitude)   - image areas on the upper side of the tilt depict ground features in smaller than the normal scale   - Roll, Crab, Pitch

Tilt Roll distortion - about its flight axis - roll compensation Crab distortion  - caused by deflection of aircraft due to crosswind - corrections: on the plane or by computer Pitch distortion  - result in local scale change  - can be ignored in most analyses

Tilt

3. Relief Displacement The direction of relief displacement is radial from nadir It increases with (1) increasing height of the feature and (2) the distance from nadir

Relief Displacement

4. Multiple Photographs Forward overlap: 50-60% along flight line; sidelap: 5-15%

Stereoscopic Pairs Flight line # 2 Flight line # 1

Pre-Marked Points Panels (targets) are placed at control point locations prior to the flight so that they will appear on the photography Panels size depends on the scale of the photography Painted on hard surfaces Made of plastic sheeting, cloth, or wood on grass or soil surfaces

Multiple Photos Stereoscopic parallax  - difference in appearances of objects due to change in perspectives  - it can be measured to compute the elevations of terrain

Photogrammetry

Measurement

Orthophotographs

5. Orthophotographs Aerial photographs without geometric errors introduced by tilt or relief displacement  Orthophotomaps and digital orthophoto Quarter Quad (DOQQ)  - show correct planimetric position and consistent scale

Readings Chapter 3

Multiple Photos Conjugate principle - the image centers of preceding and succeeding photographs - lines connecting the conjugate principle points define flight axis - parallax occurs parallel to the line of flight