Air Photo Interpretation, Stereoscopy, and Photogrammetry

Vocabulary flashcards covering stereoscopy, photogrammetry, aerial photo interpretation factors, scale calculation methods, and digital area determination.

Stereoscopy

Viewing two overlapping photographs from different perspectives to create a 3D image.

Stereo Pair

Two overlapping aerial photographs showing the same area from slightly different positions.

Stereoscopic Parallax

The apparent shift in an object's position between two photographs, allowing depth and elevation to be measured.

Forward Overlap

The overlapping area between consecutive photographs, typically $60\%$ for stereoscopic viewing.

Exposure Station

The location of the camera when an aerial photograph is taken.

Flightline

The path followed by an aircraft while taking aerial photographs.

Stereoscope

An instrument used to view overlapping photographs in 3D.

Principal Point (PP)

The point where the camera's optical axis intersects the photograph.

Conjugate Principal Point (CPP)

The location of an adjacent photograph's principal point on the current photograph.

Pseudoscopic View

An incorrect 3D view where high features appear low and low features appear high.

Photogrammetry

Using overlapping photographs to measure objects, distances, and elevations.

Digital Photogrammetry

Using computer software to extract measurements and 3D information from photographs.

Digital Elevation Model (DEM)

A digital representation of terrain elevation using a grid of elevation values.

Orthorectification

The process of correcting geometric distortions in an aerial photograph using elevation information.

Orthophoto

A geometrically corrected aerial photograph that can be used for more accurate measurements.

Air Photo Interpretation

Identifying and analyzing features in aerial photographs.

Interpretation Key

A reference tool used to help identify objects in aerial photographs.

Selective Key

An interpretation key that identifies a feature by comparing it with reference examples.

Elimination Key

An interpretation key that identifies a feature by eliminating possibilities step by step.

Time-Series Air Photos

Photographs of the same area taken at different times to identify landscape changes.

Shape

The outline or form of an object; geometric shapes often indicate human-made features.

Size

The dimensions of an object, either relative to other objects or measured using scale.

Tone/Colour

The brightness, darkness, or colour of a feature in a photograph.

Texture

The smooth or rough appearance caused by variations in tone or colour.

Pattern

The spatial arrangement or repetition of objects.

Shadow

An interpretation factor that provides clues about an object's height, shape, and orientation.

Association/Site

Identifying features based on nearby objects or their geographic setting.

Photo Scale

The ratio between a distance on an aerial photograph and the corresponding ground distance.

Representative Fraction (RF)

Scale expressed as a ratio, such as $1:10{,}000$.

Large-Scale Photo

A photo with a smaller denominator (e.g., $1:5{,}000$) that shows a smaller area with more detail.

Small-Scale Photo

A photo with a larger denominator (e.g., $1:100{,}000$) that shows a larger area with less detail.

Nominal Scale

The approximate average scale of an aerial photograph.

Focal Length ($f$)

The distance between the camera lens and image plane when focused at infinity.

Flying Height ($H$)

The height of the camera above the terrain, used to calculate photo scale.

Method 1 — Focal Length & Altitude

A scale calculation method used when camera focal length and flying height above ground are known: $RF=\frac{f}{H}$.

Method 2 — Photo-to-Map Distance

A scale calculation method used when distances between matching points can be measured on a photograph and reference map: $RF=\frac{\text{PD}}{\text{MD}\times\text{MS}}$.

Method 3 — Photo-to-Ground Distance

A scale calculation method used when the actual ground distance between two photographed points is known: $RF=\frac{\text{PD}}{\text{GD}}$.

Digitization

Tracing features in a digital image to identify their boundaries and measure them.

Pixel

The smallest individual image element in a digital image.

Spatial Resolution

The ground dimensions represented by an image pixel.

Pixel Area

The ground area represented by one pixel.

Area Calculation (Pixel-Based)

A method to determine ground area by multiplying the number of pixels covering a feature by the ground area of each pixel: $\text{Area}=\text{Number of pixels}\times\text{Pixel area}$.