Introduction to Aerial Photos

Introduction to Aerial Photos

Remote Sensing – detailed description

- observation/ measurement of object(s) without touching

- use of EMR to record images of the environment (surface of the earth)

- images are used for interpretation (ID and significance), measurement, map making & orienteering/navigation

Platforms - aircraft or satellite

Signal Source

passive - record naturally generated energy (light/heat) - e.g. a camera
active - transmits and receives "bounced" energy (e.g. RADAR)

Image Types

air photos (oblique or vertical) - broadband (many wavelengths into one image)
multi-spectral satellite images - record images in separate wavelength bands (RGB, & near, mid and far IR) then combine into one image
narrow band - RADAR

Oblique vs. Vertical Air Photos

Uses of Air Photos (and satellite images)

· Orienteering - In orienteering you use a map and compass to find your way across unfamiliar terrain.

· Photo Interpretation - The process of studying and gathering the information required to identify the various cultural and natural features is called photo interpretation.

· Photogrammetry (measures & map making) - The science or art of obtaining measurements or producing contour maps directly from aerial photographs or other remotely sensed imagery. Also, the determination of surface coordinates in a 3-dimensional coordinate system.

Basics of Stereo Viewing

· Vision: Monocular vs. Binocular (2nd binocular)

· Stereoscopic Vision (stereoscopy)

o Definition - use of stereoscopic vision to view a stereo-pair of air photos to form a single 3D image in our brain

o Conditions - stereo pair of photos (i.e. one photo for each eye), properly oriented, stereoscope (to ensure each eye sees only one photo)

· Stereoscopes - mirror & pocket

Lights, Action, Camera!

Lights
- EMR spectrum & atmospheric (optical) window
- Travel & reactions (transmittance, reflectance, refraction, absorption) (refracted pencil)
Action (a.k.a. take the pictures )
Camera: wow, that’s big

Orienting Photos for Stereo Viewing

Select adjacent photos
Locate PP’s & CPP’s (hey, what’s a “PP”?)
(Orient shadows)
Align photos with a straight edge
Set stereoscope along flight line
Adjust photos for (inter-pupil) distance

Relief Displacement

can measure heights of features
"tops" (including hilltops) are in the wrong spot!
orthophoto

Some Terms

ENDLAP: The overlap of aerial or space photographs or images along the flightline - about 60% - allows for stereo viewing

SIDELAP: The area common to two photos in adjacent flight strips - about 20-30% - ensures complete coverage (no gaps)

DRIFT: The horizontal displacement of the aircraft, caused by the wind, from the intended path - caused by side wind

CRAB: Turning of an airplane (into the wind to counteract drift) which causes its longitudinal axis to vary from the track of the plane - thus plane is "angled" but flies along the intended path

FIDUCIAL MARKS: Index marks (usually 4) found on most optical imagery. The marks are adjusted so that the intersection of lines drawn between opposite fiducial marks defines the principal point.

PRINCIPAL POINT (PP): This is the geometric center of a photograph.

CONJUGATE PRINCIPAL POINT (CPP): The location of the PP from an adjacent photo.

Photo lines (from BC Airphoto Inventory)

Air Photo Index (with photo centers) - from "BC Airphoto Inventory" Ministry of Sustainable Resource Management

Air Photo Index (with photo foot prints) from “Make a Map” Ministry of Sustainable Resource Management