What is an Orthomosaic? The Drone Survey Output Explained

An orthomosaic is a single, geometrically corrected image created by stitching together hundreds or thousands of overlapping drone photographs. Unlike a regular aerial photo, an orthomosaic has been corrected for lens distortion, camera tilt, and terrain elevation — meaning every pixel is positioned at its true geographic location and the image can be used for accurate measurement. It is one of the most common outputs of a drone survey and is typically delivered as a GeoTIFF file.

If you’ve ever zoomed into Google Maps satellite view and noticed the imagery looks flat, consistent, and seamlessly stitched — that’s an orthomosaic, just at a much larger scale. The ones you produce from a drone survey are the same thing, but at centimetre-level resolution instead of metre-level.

How an orthomosaic is created

The process starts with a drone flight. The drone flies a grid pattern over the survey area, capturing overlapping photographs — typically 70-80% frontal overlap and 60-70% side overlap. A single site might produce anywhere from 200 to 5,000 individual images depending on the area and the flight altitude.

These images are then processed using photogrammetry software. The most common tools are:

• DJI Terra — included with DJI enterprise drones
• Pix4Dmapper — industry standard for drone photogrammetry
• Agisoft Metashape — widely used in surveying and research
• OpenDroneMap — open source alternative
• DroneDeploy — cloud-based processing

The photogrammetry workflow follows these steps:

1. Alignment and tie points

The software identifies matching features across overlapping photos — a rock that appears in photo 47, 48, 49, and 50, for example. These matching features (tie points) allow the software to determine the exact camera position and orientation for every photo.

2. Dense point cloud generation

Using the camera positions and tie points, the software calculates 3D coordinates for millions of points across the survey area. This produces a dense point cloud — a 3D representation of the terrain and features.

3. Digital Surface Model (DSM)

The point cloud is interpolated into a continuous surface — the DSM. This is a height map where every pixel has an elevation value. The DSM represents the top surface of everything visible: ground, buildings, vegetation, vehicles.

4. Orthorectification

Each source photograph is reprojected onto the DSM surface, correcting for:

• Lens distortion — the barrel and pincushion effects of the camera lens
• Camera tilt — the drone is rarely perfectly level when each photo is taken
• Terrain relief — objects at different elevations appear displaced in a perspective photograph

After correction, each pixel in the output image is positioned at its true planimetric (map) position.

5. Mosaicking

The corrected images are blended together into a single, seamless mosaic. The software uses colour balancing and seam line optimisation to minimise visible boundaries between source images.

The result is the orthomosaic: one large image, fully georeferenced, with consistent scale across the entire area.

How orthomosaics differ from regular photos

The key distinction: you can measure from an orthomosaic. A distance between two points in the image corresponds to a real-world distance, within the accuracy of the survey. A regular photo doesn’t give you that.

Resolution and accuracy

Two terms matter here:

Ground Sample Distance (GSD) is the real-world size of one pixel in the orthomosaic. A GSD of 2 cm/pixel means each pixel represents a 2 cm square area on the ground. GSD is determined by the flight altitude and camera sensor — lower flights produce smaller (better) GSD values.

Absolute accuracy is how close a measured coordinate in the orthomosaic is to the true coordinate on the ground. This depends on whether ground control points (GCPs) were used:

• Without GCPs (RTK/PPK only): 3-5 cm horizontal, 5-10 cm vertical
• With GCPs: 1-3 cm horizontal, 2-5 cm vertical
• Without GCPs or RTK: 1-3 metres (GPS-only positioning)

For construction and engineering projects, GCPs or RTK/PPK are standard practice. For agriculture and environmental monitoring, GPS-only accuracy may be sufficient.

Common uses of orthomosaics

Construction and earthworks

Orthomosaics provide a current, accurate aerial view of a construction site. Project managers use them to track progress, verify earthwork volumes (when combined with elevation models), and compare as-built conditions to design drawings.

Regular orthomosaic captures — weekly or monthly — create a time series showing how a site evolves. This is valuable for progress reporting, dispute resolution, and compliance documentation.

Agriculture

In precision agriculture, orthomosaics from multispectral drone cameras show crop health indicators invisible to the naked eye. NDVI (Normalised Difference Vegetation Index) maps derived from multispectral orthomosaics highlight areas of stress, disease, or nutrient deficiency.

Even standard RGB orthomosaics are useful for crop counting, irrigation monitoring, and field boundary verification.

Mining and quarries

Volumetric surveys of stockpiles use orthomosaics and DSMs together. The orthomosaic provides visual context; the DSM provides the elevation data needed for volume calculations.

Environmental monitoring

Coastal erosion, riverbank changes, vegetation encroachment, and wetland boundaries — all can be tracked with time-series orthomosaics. The georeferencing means you can overlay orthomosaics from different dates and directly measure change.

Solar and rooftop surveys

Orthomosaics of roof surfaces are used for solar panel layout design. The accurate scale allows designers to calculate available roof area, identify obstructions, and plan panel placement.

File formats for orthomosaics

The standard format for orthomosaics is GeoTIFF — a TIFF image with embedded geospatial metadata (coordinate reference system, spatial extent, resolution). GeoTIFF is supported by every GIS tool and is the de facto standard for geospatial raster data.

Other formats you may encounter:

For client delivery, GeoTIFF is the right choice. If you’re sharing via a web viewer, Cloud Optimized GeoTIFF (COG) is preferred because it allows the viewer to stream tiles on demand rather than loading the entire file.

How to share orthomosaics with clients

This is where most surveyors hit a wall. The orthomosaic is a multi-gigabyte GeoTIFF. You can’t email it. You can’t expect the client to install QGIS. And a Dropbox download link, while functional, loses all the geographic context that makes the orthomosaic useful.

Your options, from basic to professional:

Export a JPEG

Strip out the georeferencing, export a compressed JPEG, and email or message it. The client can see what the area looks like. They cannot measure from it, zoom to full resolution, or see coordinate data.

This works for a quick preview. Not for a deliverable.

Upload to Google Earth (KMZ)

Export a KMZ from your processing software. The client opens it in Google Earth and sees your orthomosaic overlaid on Google’s satellite imagery. Gives good geographic context but quality is limited by KMZ compression.

Share via a browser-based GeoTIFF viewer

Upload the GeoTIFF to a platform that renders it on a web map. The client opens a link, sees the orthomosaic overlaid on a basemap, and can pan, zoom, and (depending on the platform) read coordinates and measure distances.

Swyvl does this automatically — upload a GeoTIFF, and it’s rendered on a Leaflet web map with the full resolution preserved. Share the link with your client, and they can explore the orthomosaic in their browser. I wrote a detailed guide on this: How to Share a GeoTIFF Online.

This is the approach I recommend for professional delivery. The client gets an interactive, zoomable view of the orthomosaic at full resolution, positioned correctly on a map, with no software to install.

Orthomosaic quality checklist

Before delivering an orthomosaic to a client, check for these common issues:

• Blurred areas: Usually caused by insufficient photo overlap in that region. Re-fly if critical.
• Colour banding: Visible boundaries between source images. Adjust colour correction settings in processing.
• Warped edges: The orthomosaic edges often have warping artefacts where coverage drops off. Crop to the reliable coverage area.
• Shadow inconsistency: Different lighting conditions during the flight (clouds passing over) cause visible brightness changes. Fly in consistent lighting.
• Moving objects: Cars, people, and machinery that moved during the flight appear duplicated or ghosted. Flag these to clients.
• Coordinate system: Verify the CRS matches what the client expects. Construction projects in the UK typically use OSGB36; Australian projects use MGA zones.

What clients actually need to know

When you deliver an orthomosaic, most clients don’t need a technical explanation of photogrammetry. They need to know:

1. What they’re looking at: An accurate aerial photograph of their site, stitched from drone imagery
2. That they can measure from it: Distances and areas measured from the orthomosaic are accurate to within a few centimetres
3. When it was captured: The survey date and time
4. How to view it: Ideally, a link they can click — not a file they need to download and figure out

If you’re delivering via Swyvl, all of this is handled: the orthomosaic loads in the browser, the capture session shows the date, and the client can pan and zoom without installing anything.

For more on professional delivery workflows, see How to Deliver Drone Survey Data to Clients Professionally.