Introduction to the Principles Introduction to the Principles of Aerial Photographyof Aerial Photography

Some content is from Remote Sensing Core Curriculum module:

Introduction to Photo Interpretation and Photogrammetry

by John Estes and Jeff Hemphill

Camera componentsCamera components

Similar to an eyeballSimilar to an eyeball Lens focuses light rays onto a Lens focuses light rays onto a

recording mediumrecording medium Recording medium is usually film or a Recording medium is usually film or a

digital imaging sensor (2-D array of digital imaging sensor (2-D array of detectors)detectors)

Focal length is distance between focal Focal length is distance between focal plane (film plane) and center of the plane (film plane) and center of the lenslens

Camera typesCamera types

Many different types of cameras Many different types of cameras developed, especially for military usesdeveloped, especially for military uses

Metric cameras are produced for high Metric cameras are produced for high precision applicationsprecision applications

Large format cameras (> 100mm film size) Large format cameras (> 100mm film size) are common for aerial surveys are common for aerial surveys

Medium format cameras (>35mm – Medium format cameras (>35mm – 100mm) are common for mapping small 100mm) are common for mapping small areas and reconnaissanceareas and reconnaissance

Small and medium format cameras are Small and medium format cameras are common for do it yourself applicationscommon for do it yourself applications

Special considerations for Special considerations for aerial camera systemsaerial camera systems

Hold large amounts of film or storage Hold large amounts of film or storage space for digital camerasspace for digital cameras

Film cameras use vacuum film plates to Film cameras use vacuum film plates to hold film flat against the film platehold film flat against the film plate

Image motion compensation moves Image motion compensation moves camera to compensate for forward aircraft camera to compensate for forward aircraft motionmotion

Stabilized mounts compensate for roll, Stabilized mounts compensate for roll, pitch, and yaw of the aircraftpitch, and yaw of the aircraft

Navigation instrumentation records Navigation instrumentation records position and orientation of aircraftposition and orientation of aircraft

Require aircraft with the appropriate ports Require aircraft with the appropriate ports in the belly of the planein the belly of the plane

RC30 Aerial Camera System

Image from Remote Sensing Core Curriculum Introduction to Photo Interpretation and Photogrammetry by John Estes and Jeff Hemphill

Recording mediaRecording media

FilmFilm Black and whiteBlack and white True colorTrue color Infrared (color and black and white)Infrared (color and black and white)

Sensitivity to different wavelengths varies with Sensitivity to different wavelengths varies with different types of filmdifferent types of film

Digital cameras record images using a 2-Digital cameras record images using a 2-dimensional array of detectorsdimensional array of detectors

Aerial digital cameras are slowly replacing film Aerial digital cameras are slowly replacing film camerascameras

All digital systems improve efficiency of All digital systems improve efficiency of processing since most manual tasks can be processing since most manual tasks can be automatedautomated

PlatformsPlatforms

Aircraft are most commonAircraft are most common Small fixed wing aircraft are relatively Small fixed wing aircraft are relatively

inexpensive but not very stableinexpensive but not very stable High altitude jets are stable but very costly to High altitude jets are stable but very costly to

flyfly Unmanned aerial vehicles (UAVs) are Unmanned aerial vehicles (UAVs) are

growing in popularitygrowing in popularity Remote controlledRemote controlled AutonomousAutonomous

Terrestrial photos acquired from mountains Terrestrial photos acquired from mountains or other features with a high vantage pointor other features with a high vantage point

Terms related to aerial photosTerms related to aerial photos

Fiducial marks: Reference marks on the corners Fiducial marks: Reference marks on the corners and/or sides of aerial photos used to define and/or sides of aerial photos used to define coordinate axes and to locate the geometric coordinate axes and to locate the geometric center of the photocenter of the photo

Principal point: The geometric center of the Principal point: The geometric center of the photophoto

Conjugate principal point: The point Conjugate principal point: The point representing the principal point of an adjacent representing the principal point of an adjacent photophoto

Photo base: The distance between the principal Photo base: The distance between the principal point and the conjugate principal pointpoint and the conjugate principal point

More termsMore terms Nadir: The point on the photo vertically beneath Nadir: The point on the photo vertically beneath

the camera lens at the time of exposurethe camera lens at the time of exposure Isocenter: A point roughly halfway between the Isocenter: A point roughly halfway between the

principal point and nadir from which tilt principal point and nadir from which tilt displacement radiatesdisplacement radiates

Vertical viewing geometryVertical viewing geometry

Tilt less than ~3°Tilt less than ~3° Scale relatively constant if terrain is Scale relatively constant if terrain is

not severenot severe Easier to use for stereo visualizationEasier to use for stereo visualization Direction (angles) can be more Direction (angles) can be more

accurately measuredaccurately measured

Oblique Oblique viewingviewing geometry geometry

Oblique: Tilted off of vertical axis Oblique: Tilted off of vertical axis High oblique: horizon is visibleHigh oblique: horizon is visible Low oblique: horizon not in imageLow oblique: horizon not in image

Covers a larger areaCovers a larger area Sides of objects can be seenSides of objects can be seen Scale decreases toward horizonScale decreases toward horizon Can be difficult comparing to maps or Can be difficult comparing to maps or

orthorectified imagery due to different orthorectified imagery due to different perspectiveperspective

Orthogonal vs. central Orthogonal vs. central projectionprojection

Orthogonal projection Orthogonal projection Each point is placed as if it is viewed from aboveEach point is placed as if it is viewed from above Also called a map planeAlso called a map plane

Central projectionCentral projection Projection of features in an aerial photographProjection of features in an aerial photograph Each point on a photo is imaged after reflected Each point on a photo is imaged after reflected

light passes through a single point (lens)light passes through a single point (lens) Causes features with a height above a base Causes features with a height above a base

elevation to radiate outward from the center elevation to radiate outward from the center

Central projection: aerial photo

Orthogonal projection: map

Image from Remote Sensing Core Curriculum Introduction to Photo Interpretation and Photogrammetry by John Estes and Jeff Hemphill

Image distortion and Image distortion and displacementdisplacement

DistortionsDistortions Film and paper shrinkageFilm and paper shrinkage Atmospheric distortionsAtmospheric distortions Image motionImage motion Lens distortionLens distortion

DisplacementDisplacement Caused by:Caused by:

curvature of the Earthcurvature of the Earth tilttilt topographic relief and height of featurestopographic relief and height of features

Makes stereo viewing possibleMakes stereo viewing possible Allows us to measure heights and make topographic Allows us to measure heights and make topographic

mapsmaps

Shadows: Caused by a feature blocking the Shadows: Caused by a feature blocking the sun’s illumination from another feature or sun’s illumination from another feature or the Earth’s surfacethe Earth’s surface

Hotspot: The area on an aerial photograph Hotspot: The area on an aerial photograph where shadows are hidden by the feature where shadows are hidden by the feature casting the shadow thereby causing a bright casting the shadow thereby causing a bright spotspot

Specular reflection area: An area on an Specular reflection area: An area on an aerial photograph where the sun’s rays are aerial photograph where the sun’s rays are reflected directly into the camera lens reflected directly into the camera lens causing a white spot if a highly reflective causing a white spot if a highly reflective feature is in this areafeature is in this area

Illumination features in aerial Illumination features in aerial photosphotos

Image from Canada Centre for Remote Sensing Glossary of Remote Sensing Terms: http://ccrs.nrcan.gc.ca/glossary/index_e.php?

id=3133

Diagram of specular bright spot and hot spot on an airphotoA = specular reflection bright spot; B = hot spot; C = camera; N = nadir; I = illumination source; theta = angle of incidence = angle of reflection

What is photogrammetryWhat is photogrammetry

The art, science, and technology of The art, science, and technology of obtaining reliable information about obtaining reliable information about physical objects and the physical objects and the environment, through the process of environment, through the process of recording, measuring, and recording, measuring, and interpreting images and patterns of interpreting images and patterns of electromagnetic radiant energy and electromagnetic radiant energy and other phenomena.other phenomena.