We use the term orthomosaic on our website and in our posts, but what does it actually mean? In this post, we’ll try to explain in simple terms what an orthomosaic is, how we create them and what makes them so useful.

Why is an orthomosaic different from an aerial photo?

Let’s get started with the following image; a small section of an abandoned airfield.

It looks just like any other aerial photograph, doesn’t it? In fact, it’s an orthomosaic, computer-generated from hundreds of drone photographs, showing an area of about 6.5 acres. To capture a similar image with a single aerial photograph you’d probably need to fly above the legal maximum height for a drone (120m / 400 ft).

We begin with a process called orthorectification. This corrects the photographs that will make up the orthomosaic, converting them into orthophotos. The prefix ‘ortho’ means upright, straight, regular or true; so orthorectification is the term we use when geometrically correcting a photograph to be ‘true’.

Why do aerial photographs need to be corrected?

Imagine flying above where you live or work and trying to take a photograph straight down. Looking at the resulting photograph you might notice that the camera lens has introduced some distortion. Also, if it’s not pointing perfectly downwards you’ll also see a degree of tilt in the photograph. Orthorectification compensates for both.

An example on a smaller scale

Look at the image of the chessboard shown below and you can see that both the top and base of the centre chess piece are clearly visible. As you move further away from the centre the pieces increasingly occlude themselves. In an orthorectified image, we would expect all the pieces to appear the same as the one in the centre.

Close up of a chess board showing the perspective of the pieces.

How the orthomosaic process works

Photogrammetry software takes many overlapping images and calculates height data for all the points in our scene. The software then uses this data to create a Digital Surface Model (DSM), which represents the topography of the scene. We then use the DSM to orthorectify the photographs and join them together to make the orthomosaic.

The advantage of an orthomosaic

As we mentioned at the beginning of this post, using orthomosaics allows you to generate an image showing a very large area; in some cases covering tens or hundreds of acres in minutes with our drones. Without orthomosaics this would require using an aeroplane or even a satellite to take the photographs.

The other benefit of this process is accuracy. We can create maps of an area which contain precise measurements of the distances and angles of the topography. We can then have confidence that we’re putting up the building in the right spot, or placing the property boundary in the right location, for example.

An example of orthorectification in action

The following image taken from the U.S. Geological Survey (USGS) is a great example of how orthorectification can show us more accurately what’s really happening on the ground. The two photos below show a pipeline running from top-left of each image to bottom-right. In the original aerial photograph on the left, the pipeline does not appear straight. A combination of camera tilting, lens distortion and ground topography conspired to show a distorted image. The orthorectified image on the right-hand side shows a more accurate representation of the straight pipeline.

Image showing comparison between an aerial photograph and a corrected orthophoto.

Orthomosaics are immensely valuable tools in a whole variety of situations. Consider crop mapping with multispectral cameras, precise measurements of archaeological dig sites or progress charting of large civil engineering projects. If you have a project that could benefit from using orthomosaics then we’d love to hear from you. Call us on 01353 655762 or send an email to contact@foxvolant.com and we’d be happy to discuss your needs.