What is Image Overlap?

Image overlap is the percentage of one photograph’s area that is also shown in a neighbouring photograph. Photogrammetry software needs this shared area to match the same ground points in several photos and to compute their positions.

Forward overlap and side overlap

A survey flight has two overlap values.

• Forward overlap, also called endlap, end lap or frontal overlap, is the common image area of consecutive photos along a flight line. Pix4D measures it “with respect to the flight direction”.
• Side overlap, also called sidelap, side lap or lateral overlap, is the common area of photos in adjacent flight lines. Pix4D measures it “between flying tracks”. It is there so that the 3D coverage of a multi-line project has no gaps.

The shared area of two successive photos gives the three-dimensional effect used for mapping. In older literature this pair is called a stereomodel. Forward overlap gives the stereo coverage, and side overlap closes the gaps between flight lines.

Why 60% forward and 30% side overlap are traditional

In crewed film mapping, a CRC textbook gives a normal endlap of 55 to 65%, with a nominal average of 60%. It gives a normal sidelap of 20 to 40%, with a nominal average of 30%. Natural Resources Canada designs its air photo surveys for 60% forward overlap and 30% lateral overlap.

Stereo coverage needs at least 50% of the area to show in both photos. University of Washington course slides state that overlap is usually more than 50% and that a minimum of 60% is most common. The slides add that overlap can be as much as 90% when triple coverage is needed.

How the overlap sets the flight plan

The distance between two exposure stations equals the ground footprint of one photo times (100 minus the overlap in percent) divided by 100. With a 100 m footprint along the flight line and 80% forward overlap, the camera must expose every 20 m. The spacing of the flight lines follows the same rule. Use the footprint across the flight line and the side overlap.

To set the camera trigger, divide the spacing by the ground speed of the drone. At 10 m/s and a spacing of 20 m, the camera must expose every 2 seconds.

The footprint depends on the flight height. The ground distance that one exposure covers is proportional to the flight height above the ground. It is inversely proportional to the focal length of the camera. If you fly higher with the same camera, each photo covers more ground. The same overlap then gives a larger spacing between exposures. If you raise the overlap from 80% to 90% at a constant height, the spacing halves. The flight then needs twice as many exposures along each line.

Typical values

For a corridor flown in a single line, Pix4D asks for at least 85% frontal overlap. For tall slender structures, it asks for 90% overlap between images taken at the same height and 60% between images taken at different heights. These values are one vendor’s guidance and no standard sets them. The Pix4D values apply to nadir flights. USGS notes that its own workflow also runs on drone imagery with greater overlap and a lower altitude.

What happens when the overlap is too low

Insufficient overlap may give poor reconstructions. USGS describes a distinct train-track pattern, with consistent gaps in the point cloud. Pix4D marks areas of the orthomosaic with too little overlap in red in its quality report, and says that they may give low-quality 3D reconstruction.

Terrain and flight height

Pix4D asks for a constant height over the terrain or the object. Photo scale is not constant over a project, because the distance between the camera and the ground differs from one exposure to another. Where the ground rises toward the camera, the footprint of a photo gets smaller. The real overlap on that ground is then lower than the planned value. Check the real overlap in the quality report when the relief is large.

Checking the overlap

The Pix4D quality report shows the number of overlapping images for each pixel of the orthomosaic. Red areas have too little overlap. Green areas are seen in more than 5 images. You can also compare two adjacent images by eye for the shared area.

Overlap for airfield surveys

The sources on this page set no overlap for an airfield pavement survey. A runway or a taxiway is a long, narrow area. The nearest case in the Pix4D guidance is corridor mapping. Use the guidance of your processing software, and check the quality report after the flight.