An orthomosaic, explained.

How it differs from an aerial photo.

A single aerial photograph is a perspective view. Objects near the edge lean away from the camera, tall things hide what is behind them, and distances across the frame are not consistent. An orthomosaic removes that perspective. Each part of the image is taken from directly above and corrected for the lens and the terrain, so a metre on the ground is the same length wherever it falls in the image.

How it is made.

The aircraft flies a planned grid over the area at a set height, taking photographs with a high overlap between neighbouring frames. Photogrammetry software finds the same points in many photographs, works out where the camera was for each one, and builds a dense 3D model of the surface. The orthomosaic is that model viewed from directly above, with the photographs draped over it.

What georeferenced means.

Each image carries the position where it was taken, so the finished map is placed on real-world coordinates and can be opened over other mapping or site plans. Where tighter accuracy is needed, surveyed ground control points are marked on the site before the flight and used to pin the map to known positions.

What resolution means in practice.

Resolution is expressed as the ground distance covered by one pixel. A lower flight gives a finer resolution and more detail, over a smaller area per flight. The resolution is chosen for the purpose: a site overview needs less, a record of surface condition needs more.

Elevation and 3D models.

The same flight produces a digital elevation model, which records the height of the surface across the site, and where required a textured 3D model that can be turned and measured on screen. Volumes of stockpiles and cut and fill between visits are measured from these models.

What it is used for.

Site planning and layout, progress monitoring on construction sites, earthworks and stockpile records, rural and acreage planning, vegetation and erosion monitoring, asset records and the before-and-after comparison of a site over time.

What it is not.

An orthomosaic is a planning and monitoring tool. It is not a cadastral or registered survey, which defines legal boundaries and can only be produced by a registered surveyor. Where a survey is needed, the orthomosaic is supplied in a form that can be read alongside it.

The files you receive.

A georeferenced orthomosaic in common image and GIS formats, an elevation model, a 3D model where it was part of the job, measurements where requested, and a short note of the resolution and the accuracy the job was flown to.