What front and side overlap mean
Front overlap, also called forward or longitudinal overlap, is the share of each photo that also appears in the next photo along the same flight line. Side overlap, also called lateral overlap, is the share of each photo that also appears in the photos of the neighbouring flight line.
Both are expressed as a percentage of the image footprint on the ground. A front overlap of 75 percent means the aircraft moves forward by one quarter of the footprint length between two photos. A side overlap of 70 percent means neighbouring flight lines are separated by 30 percent of the footprint width.
Front overlap is controlled by the time or distance between shots and the speed of the aircraft. Side overlap is controlled by the spacing between flight lines. Most flight apps let you type in both percentages and calculate the rest for you, but it pays to understand what they calculate.
Why photogrammetry needs so much overlap
Photogrammetry reconstructs a surface from photos taken at different positions. To place a point on the ground in three dimensions, the software has to find that same point in several images and intersect the rays from each camera position. The more images see a point, and the wider the angle between them, the better the solution.
Overlap also feeds the step before that. During alignment, the software finds thousands of tie points, which are small features that appear in more than one photo, and uses them to solve the position and orientation of every camera. Photos that share too few tie points with their neighbours fail to align, and whatever they covered drops out of the map.
A common rule of thumb is that every point on the ground should appear in at least five to nine images. With front and side overlap in the usual range, a point in the middle of the site is seen many times more than that. Points near the edges of the flight area are seen far fewer times, which is why edges are the first place quality drops.
Common starting values for drone mapping overlap
There is no single correct setting, but widely published guidance gives a good starting point. Treat these as defaults to adjust, not rules.
- Open ground, construction sites, roads and quarries with good texture: 75 percent front and 65 to 70 percent side overlap
- Crops, grass, dense forest canopy, sand, snow and other uniform or moving surfaces: 80 to 85 percent front and 70 to 80 percent side overlap
- Urban areas and tall structures: higher front and side overlap, often combined with oblique photos from a crosshatch pattern
- Multispectral flights for NDVI or NDRE: follow the camera maker's guidance, which is usually at the high end because multispectral sensors have fewer pixels and fields have little texture
Uniform surfaces need more overlap because they offer fewer distinct features to match. A wheat field in wind looks different from one photo to the next, so the software needs more chances to find stable tie points.
How overlap, altitude and ground sampling distance fit together
Overlap is defined on the ground, so it depends on the image footprint, and the footprint depends on altitude and camera. Ground sampling distance, or GSD, is the size of one pixel on the ground. It is calculated from the sensor width, the focal length, the image width in pixels and the flying height.
Take a common one inch sensor camera as a worked example: sensor width 13.2 mm, focal length 8.8 mm, image size 5,472 by 3,648 pixels, flown at 100 m above ground. GSD is 13.2 times 100 divided by 8.8 times 5,472, which gives about 2.74 cm per pixel. The footprint is then about 150 m across the wide side of the image and about 100 m along the short side.
Most mapping flights fly with the short side of the image along the flight line. With 75 percent front overlap, the aircraft moves 25 percent of 100 m, which is 25 m, between photos. At a ground speed of 10 m per second, that is one photo every 2.5 seconds. With 70 percent side overlap, flight lines are 30 percent of 150 m apart, which is 45 m.
You can run the same numbers for your own aircraft with our ground sampling distance calculator. It gives GSD, footprint and the spacing that follows from your overlap settings.
Terrain changes the overlap you actually get
The overlap you type into the flight app is only true at the height the app assumes. Most apps assume flat ground at the takeoff elevation. When the ground rises toward the aircraft, the camera is closer to it, the footprint shrinks and the real overlap on that high ground drops. When the ground falls away, the footprint grows and overlap increases.
On a site with a large hill, a spoil heap or a quarry face, the top of the feature can end up with far less overlap than the rest of the site. That is exactly where stockpile volumes and cut and fill are measured.
- Use terrain follow where your flight app supports it, based on a reliable elevation model
- If you fly at a fixed altitude, set the overlap for the highest ground on the site, not the takeoff point
- Take off from high ground when it is safe and allowed, so altitude above the tallest features is known
- Add extra overlap on sites with tall structures, stockpiles or steep slopes
Speed, shutter and motion blur
Overlap and speed are linked. Higher front overlap at the same speed means shorter intervals between photos. Every camera has a minimum interval it can sustain, and if the flight asks for photos faster than the camera can take them, the app either slows the aircraft or skips photos.
Motion blur is the other limit. The ground moves under the camera during the exposure. Blur in centimetres is roughly ground speed multiplied by exposure time. At 10 m per second and 1/1000 of a second, the camera moves 1 cm during the exposure. With a GSD of 2.74 cm, that is well under half a pixel, which is generally acceptable. At 1/250 of a second, the same flight smears 4 cm, more than a pixel, and sharpness suffers.
Cameras with a mechanical or global shutter avoid the distortion that a rolling shutter causes at speed. If your aircraft has a rolling shutter, slow down or enable any rolling shutter correction your processing supports.
Overlap and image count: what higher settings cost
More overlap means more photos, and more photos mean longer flights, more batteries, longer uploads and more of your processing quota. The relationship is not linear, so it is worth doing the math before raising both values.
Using the example above, 75 percent front and 70 percent side overlap gives 25 m between photos and 45 m between lines, or one photo for every 1,125 square metres of ground. Raising the settings to 80 percent front and 75 percent side gives 20 m between photos and 37.5 m between lines, or one photo for every 750 square metres. That is 50 percent more photos for a five point increase on each setting.
On many sites that increase is worth it. On a large, well textured earthworks site, it may only add flight time without improving the result. Fly the higher values where the surface demands them and stay with the defaults where it does not. Our drone mapping pricing page lists the monthly image quota for each plan, which helps you see what a change in overlap means for a busy month.
Extend the flight beyond the site boundary
Edges always have less overlap than the middle, because there are no photos beyond them. If the site boundary is also the flight boundary, the outer strip of your deliverable is the weakest part of the map.
- Extend the flight area by at least one line spacing beyond the boundary on each side
- Add one or two extra photos at the start and end of each flight line
- For corridors such as roads and pipelines, fly at least two or three parallel lines rather than one
- Check that ground control points are inside the well covered area, not on the outer edge
Signs of too little overlap in the results
If you process a dataset and see any of the following, suspect overlap first.
- Holes or blurry smears in the orthomosaic, usually over vegetation, water or roofs
- Photos that fail to align and are listed as uncalibrated or dropped
- A surface model that bends or droops near the edges of the site
- Stockpile tops or building edges that look melted in the elevation model
- Duplicate or ghosted features where two strips were stitched with weak tie points
Some of these problems can be reduced by reprocessing with different settings. Most can only be fixed by flying again with more overlap, which is why checking in the field is worth the few minutes it takes.
How to check overlap before you leave the site
The best time to discover a gap is while the aircraft is still on site. A quick review of the photos and their positions catches most problems.
- Check that the number of photos matches what the flight app promised
- Plot the photo positions on a map and look for missing lines or gaps along a line
- Confirm that the altitude in the metadata is consistent across the flight
- Open a few photos from the high ground and the site edges and check sharpness
You can do the position and spacing check on our drone image processing software page. It reads the EXIF metadata of your own flight photos in the browser, plots their positions, estimates GSD and footprint, and estimates overlap from the spacing between shots. Nothing is uploaded.
A short overlap checklist
- Start from 75 percent front and 65 to 70 percent side on textured sites
- Go to 80 percent front and 70 to 80 percent side on crops, canopy and uniform surfaces
- Set overlap for the highest ground or use terrain follow
- Keep motion blur under half a pixel by matching speed and shutter
- Extend the flight beyond the site boundary
- Check photo positions and count before you pack up