Last reviewed 16 Sept 2026 · 9 min read
Photogrammetry
Photogrammetry is the science and technology of obtaining reliable measurements, maps and 3D information of objects and terrain from photographs (and digital images).
Types
| Type | Description |
|---|---|
| Terrestrial photogrammetry | Photographs taken from fixed ground stations with phototheodolites — cliffs, quarries, buildings |
| Aerial photogrammetry | Photographs taken from aircraft — topographic mapping of large areas |
| Close-range photogrammetry | Objects at short distances — architecture, heritage, industrial measurement, deformation |
| UAV (drone) photogrammetry | Low-altitude images from unmanned aerial vehicles processed by structure-from-motion software — rapid mapping of sites, mines, corridors |
| Satellite photogrammetry | Stereo satellite images (e.g. Cartosat) for DEMs and maps |
Aerial photographs
| Type | Tilt of camera axis | Features |
|---|---|---|
| Vertical photograph | Camera axis vertical (tilt less than about 3° — "near vertical") | Scale nearly uniform on flat ground; used for mapping |
| Tilted photograph | Unintentional tilt of a few degrees | Scale varies across the photo |
| Low oblique | Camera axis inclined; horizon not visible | Pictorial views |
| High oblique | Camera axis highly inclined; horizon visible | Reconnaissance, large coverage |
Terms
- Aerial camera — high-quality lens (e.g. focal length of about 150 mm for wide-angle mapping cameras), traditional film format 23 cm × 23 cm, now digital aerial cameras.
- Fiducial marks — marks on the edges or corners of the photograph whose lines join at the principal point.
- Principal point (p) — foot of the perpendicular from the lens centre to the photo plane.
- Nadir point (plumb point, n) — point vertically below the lens centre on the photograph.
- Isocentre (i) — point on the photograph bisecting the angle between the principal point and nadir; relevant for tilt displacement.
- In a truly vertical photograph, principal point, nadir and isocentre coincide.
- Flying height — altitude of the camera above datum (MSL).
Scale of a vertical photograph
= focal length; = flying height above datum; = elevation of the ground point above datum.
Average scale:
Scale can also be found as photo distance ÷ corresponding ground distance, or map distance comparison.
Because ground elevations vary, the scale of a vertical photograph of hilly terrain varies from point to point (larger scale for higher ground).
Relief displacement
On a vertical photograph, objects above or below the datum are displaced radially from the nadir (principal) point — tops of tall objects appear displaced outward relative to their bases.
= relief displacement on the photo; = radial distance from the principal point to the displaced (top) image point; = height of the object above the datum (or its base); = flying height above the same datum.
Height of an object:
- Displacement is zero at the principal point and increases towards the edges.
- It increases with object height and decreases with flying height.
- Relief displacement enables height measurement from single photographs but distorts planimetric positions, which must be corrected (orthophotos).
Tilt displacement — occurs in tilted photographs, radial from the isocentre; points on the upper side of the photo are displaced inward and on the lower side outward.
Stereoscopy
Overlap
- Forward overlap (end lap) — overlap between successive photographs along a flight strip — commonly about 60% — enables stereoscopic viewing (each ground point appears in at least two photos).
- Side lap — overlap between adjacent strips — commonly about 25–30% — ensures no gaps and allows strip connection.
- Increased overlaps are used in mountainous terrain and for digital photogrammetry.
Stereoscopic vision and stereoscopes
A pair of overlapping photographs (stereo pair) viewed so that each eye sees one photograph produces a 3D (stereoscopic) model.
- Lens (pocket) stereoscope — simple, portable, small photo separation.
- Mirror stereoscope — mirrors allow full photographs to be viewed with magnification.
- Digital photogrammetric workstations use polarised or shutter glasses.
Parallax
Absolute stereoscopic parallax of a point = the difference in x-coordinates (along the flight line) of its images on the two photos: . Parallax increases with ground elevation (points closer to the camera).
Height (elevation) from parallax:
( = air base — ground distance between exposure stations.)
Difference in elevation between points A and B using measured parallax difference (e.g. with a parallax bar):
When A is on the datum and ≈ photo base : .