Drone orthophotos

What an orthophoto is and what an orthomosaic is

In an orthophoto each pixel sits in its true position in a coordinate system, so distances, areas and angles can be measured directly on it. When it is built from hundreds or thousands of drone photographs, the result is called an orthomosaic.

The source photographs are georeferenced with the position of the drone at the moment each one was taken. Photogrammetric processing removes camera tilt and relief displacement, and projects the image onto the terrain model. The result is a continuous image, with no visible seams and with coordinates in ETRS89 UTM, the official reference system in Spain.

Orthophoto vs aerial photo: the difference

An aerial photo is a perspective view: objects away from the centre lean outwards, the scale changes with the relief and it cannot be measured reliably. An orthophoto corrects those effects. On an aerial photo you look; on an orthophoto you measure.

The difference matters as soon as the cadastre, an alignment or a setting-out has to be overlaid. On an uncorrected photo, a parcel can be shifted by several metres. The full explanation, with the geometry of perspective and relief displacement, is in the article what is an orthophoto and how it differs from an aerial photo.

How a drone orthophoto is produced

The flight is planned with high overlap between photographs, usually above 70% forward and 60% side overlap, at a height that sets the pixel size (GSD). Before the flight, ground control points (GCPs) are measured with GNSS RTK. These points tie the model to real coordinates and allow the accuracy to be checked at the end.

The metric RGB camera on the drone captures the images with their position and orientation. The photogrammetry software computes the geometry of all the photos, generates the terrain model and projects each image onto it. This produces the orthomosaic, which is reviewed, colour balanced and clipped to the project area.

For a true orthophoto (true ortho), in which buildings do not lean, the image is projected onto the digital surface model (DSM) instead of the digital terrain model (DTM).

Orthophoto resolution, accuracy and delivery formats

The usual GSD for this type of work is between 2 and 5 cm per pixel, depending on flight height and camera. It is the image resolution, not its accuracy.

Typical horizontal accuracy is 2 to 5 cm, expressed as root mean square error (RMSE) on open ground or well-defined surfaces, in the project reference system (ETRS89 / UTM). It is checked against independent checkpoints that are not used in the adjustment. It depends on the sensor, the GSD, the ground control and the vegetation. The accuracy for each project is agreed in the proposal and documented in the control report.

What fita delivers with an orthophoto

The exact content is set in the proposal. Usually:

• The orthomosaic as GeoTIFF with embedded georeferencing, as ECW for large files or as JPG with a world file.
• The true orthophoto, when the job requires it, projected onto the DSM.
• The orthophoto as tiles on our web GIS platform, to view without software.
• The control report, with the independent checkpoints and the RMSE obtained.

What an orthophoto is used for, by sector

The orthophoto is the background on which the rest of the project is drawn. Some frequent uses in the Valencia region (Comunitat Valenciana), across Spain and in Europe:

• Renewable energy, industrial estates and urban planning: base mapping for solar PV plants, estates and developments.
• Civil works: construction progress monitoring, with one orthophoto per visit to compare progress.
• Agriculture: parcel areas for the CAP (EU Common Agricultural Policy) and SIGPAC (Spain's agricultural parcel system), tree counts.
• Asset inventory on industrial estates, roofs and roads.
• Recording the condition of a site before works begin.
• Boundary checks and overlay with the cadastre. If the work continues with the documentation for the Cadastre and Land Registry, it is handled by Clotoides, fita's sister surveying company.

National PNOA orthophoto or a drone orthophoto

The PNOA (Spain's National Aerial Orthophoto Programme), run by the IGN (National Geographic Institute of Spain), offers free orthophotos of the whole country, with a pixel of 25 cm over much of the territory and 50 cm in other areas, with an update cycle that is currently three years. It is an excellent reference at a general scale and for checking what an area looked like in earlier flights.

Design work needs more. A project-specific drone orthophoto has a GSD of 2 to 5 cm/px, shows the site as it is on the flight date and is tied to control points measured in the field. It is the one that allows reliable measurement, setting-out and comparison. The guide PNOA LiDAR and IGN open data compares all the IGN products and explains when each is enough.