Photogrammetry

What is photogrammetry?

Photogrammetry is a method of deriving 3D geometry from overlapping photographs of the same object or building. The Slovak term is fotogrametria. The usual output is a dense bodový oblak (point cloud) and a textured 3D mesh. The photographs can be taken handheld from the ground or from a drone, and the geometry comes from measurement rather than from drawing the building by hand.

The principle is old. What has changed is the software: modern pipelines can turn a large set of ordinary photographs into a model without the plate cameras and manual plotting that earlier surveyors needed. That lowered the entry cost, which is a large part of why the method is now common on small projects.

How does photogrammetry turn photographs into 3D geometry?

The software first finds points that appear in several photographs, then calculates where each camera stood and which way it pointed. This step is often called structure from motion. Dense matching then estimates depth for a large share of the pixels, and the result is meshed and textured.

Because the reconstruction is built from image correspondence alone, the model has no absolute scale or position until it is tied to reference points. These control points are measured independently, usually with a total station or GNSS, and they fix the model in the local coordinate system. Without them, a model can look right and still be the wrong size or sit in the wrong place.

When does photogrammetry beat a laser scanner?

Photogrammetry is strongest on textured exterior surfaces such as brick, render, stone, timber cladding and weathered roofs, where the software has plenty of detail to match. It also wins on equipment cost: a camera, a drone and processing software cost far less than a terrestrial scanner. Colour comes straight from the photographs, so the textured model suits facade documentation and heritage recording.

Situation	Photogrammetry	Laser scanning	Practical note
Textured facade (brick, render, stone)	Strong	Strong	Both work well; photogrammetry usually needs less equipment.
Roof seen from above	Strong with a drone	Limited without a drone-mounted unit	Depends on the flight plan and image overlap.
Dark interior	Weak	Usually suitable	Low light gives noisy matches and gaps.
Plain white wall	Weak	Suitable	Too little texture for the software to match.
Glass or polished metal	Weak	Weak	Both need a separate check or a treatment step.
Tight budget, exterior or roof	Often a good fit	Higher equipment cost	Compare itemised quotes before choosing.

When does photogrammetry fail?

Photogrammetry fails where the software cannot find enough matching detail. Plain white or uniform render, new asphalt, snow, water and poorly lit interiors all produce gaps or smeared surfaces. Glass and mirrors are a problem because the surface shows the reflected scene rather than its own geometry. Moving objects such as foliage in wind, people and parked cars also corrupt the result.

Repetitive patterns, such as identical windows or regular roof tiles, can confuse the matching, so the shots need to vary in angle and distance. Overlap and sharpness matter as much as the camera. A blurred photograph adds little, and a sharp one taken from a steady position adds a lot. The result is only as good as the photographs behind it, so the capture plan matters as much as the software.

Does a textured model mean an accurate model?

No. Texture makes the model look right, but it does not prove the geometry. Accuracy depends on the control points, the camera calibration, the image quality and the processing, and it has to be checked against measurements made independently. A clean-looking mesh of a facade can still hide a distorted section.

How is photogrammetry different from laser scanning?

Laser scanning measures distance directly with a laser pulse, so its geometry does not depend on surface texture or on the light in the room. Photogrammetry infers geometry from how points shift between photographs, so it depends on image quality and surface detail. Both produce point clouds, and both can feed a Scan-to-BIM workflow. The laser scanning entry covers the scanner side of that choice.

Capture method	Equipment	Best use	Main limitation
Handheld photographs	Camera on a pole or tripod	Facades within reach, small objects, well-lit rooms	Reach and height depend on the operator
Drone photographs	Drone with a camera	Roofs, full elevations, site context	Flight planning, aviation rules, wind and weather
Terrestrial laser scan	Scanner on a tripod	Interiors, uniform surfaces, dense records	Equipment cost and slower setup; colour depends on an attached camera

Where does photogrammetry fit in a Slovak project?

In Slovakia the official survey documents for a building project are normally prepared by a licensed surveyor, and a photogrammetric model supports that work rather than replacing it. It is useful for early design, heritage recording and checking an existing facade or roof before a reconstruction. In those cases a topographic survey or an as-built survey is usually ordered as well.

Under the building act 25/2025 Z. z., in force since 1 April 2025, the permit file still depends on the documents the authority requires, so check those requirements before relying on a model alone. Keep the original photographs and the processing settings with the model, because a later reviewer will want to see how it was produced. Drone flights are subject to national aviation rules, and some sites lie in restricted airspace, so the pilot and the flight permission should be arranged before the site visit.