Orthomosaic maps explained for site managers

What an orthomosaic actually is

An orthomosaic is a single large image of a site, built from many overlapping drone photographs, corrected so that scale is consistent across the whole picture. Corrected is the important word. In a normal photograph, objects lean away from the centre of the frame and the ground further from the camera appears at a different scale from the ground directly below it. An orthomosaic removes that distortion using the geometry of the flight and a surface model of the ground. The result behaves like a plan view: you can measure on it.

It is produced by flying a grid pattern over the site at a set height, taking hundreds of photos with heavy overlap, then processing them together. The software works out where each photo was taken from, builds a three dimensional surface, and drapes the imagery onto it. What you get back is one file covering the whole site at a consistent resolution.

How it differs from a single aerial photo

A single aerial photo is useful for showing someone what a site looks like. It is not useful for measuring. Take a photo from 80 metres over the middle of a compound and the fence at the far edge of the frame is shown at an angle, partly from the side. A stack of pipes near the edge appears to lean outwards. Distances across the frame are not comparable.

An orthomosaic fixes that. Every part of the image is shown as if viewed from directly above. A metre at the top left is the same as a metre at the bottom right. That is the difference between a picture and a drawing you can work from.

The trade off: an orthomosaic shows roofs and tops of things, not faces. Vertical surfaces are compressed to nothing. If you need to see the state of a wall or a parapet, you need obliques or an elevation set, not a map. We cover that under commercial inspections.

How it differs from a satellite image

Satellite and aerial basemaps are free and convenient, and they are usually the wrong tool for a live site. Three reasons.

Age. You rarely know when the imagery was captured, and on a construction site anything more than a few weeks old is fiction. A drone orthomosaic is dated to the day it was flown.
Resolution. Ground sample distance on a drone survey is typically a small fraction of what a public basemap gives you. You can read kerb lines, drainage grates, setting out pegs and individual stored materials.
Accuracy and control. A basemap is positioned well enough for navigation. It is not positioned well enough to scale a haul road off, and you have no record of how it was georeferenced.

Satellite imagery is fine for a location plan in a tender document. It is not fine for quantities, setting out checks or disputes.

What you can measure on one

Once the image is georeferenced and scaled, the obvious measurements are horizontal:

Distances between any two points on the ground.
Areas: hardstanding, stripped topsoil, surfaced car park, laydown areas, vegetation to be cleared.
Positions: where a feature sits relative to the boundary, the building line or the kerb.
Counts: bays, units, plant, stored materials, trees.

Vertical information comes from the elevation model produced alongside the imagery, not from the orthomosaic itself. With that you can get spot heights, contours and volumes of stockpiles and excavations. That sits closer to a topographic survey, and it is worth being clear at the quoting stage which of the two you need, because it changes how the flight is controlled and what gets delivered.

Be honest about accuracy. Precision depends on flight height, camera, overlap and, above all, ground control. A survey flown with surveyed control points on the ground is a different proposition from one positioned by the drone's own GNSS alone. Tell us what tolerance you need to work to and we will tell you whether a drone will meet it.

How it is georeferenced

Georeferencing means tying the image to real world coordinates so it drops into the right place in your software. In the UK that normally means British National Grid. Our drone mapping service delivers georeferenced orthomosaics in British National Grid as GeoTIFF, KMZ or PDF, plus point clouds.

What each format is for:

GeoTIFF. The working file. Loads into GIS and CAD in the right position, at full resolution. This is what your technical team wants.
KMZ. For viewing in common earth browsers. Easy to send to someone who has no GIS and just needs to look.
PDF. For printing, marking up and issuing with a report or a drawing pack.
Point cloud. The three dimensional data behind the surface, for sections, volumes and modelling.

Ground control improves absolute accuracy. Targets are placed and surveyed before the flight, then identified in processing. Without them you still get an internally consistent image, but its position on the grid can drift.

Practical uses on a live site

Setting out checks. Overlay the orthomosaic on the design drawing and see immediately whether the as built position matches the plan. Differences show up visually before anyone measures anything.

Progress and record keeping. Fly the same grid at intervals and you have a dated sequence you can slide between. That underpins construction progress monitoring and gives you something concrete for valuations and site meetings.

Logistics planning. Compound layout, crane positions, access routes, storage. Easier to argue about on a current scaled image than on a drawing that predates the groundworks.

Earthworks. Stockpile volumes and cut and fill progress, using the elevation data captured on the same flight.

Evidence. A dated, scaled image of site boundaries, neighbouring land and the condition of access routes is useful when a claim arrives months later. Related work is covered under dilapidation surveys.

Stakeholder updates. A clean plan view of the whole site communicates better to a client or a planning officer than ten photographs.

What it will not do

It will not see under canopy, under scaffold sheeting or inside buildings. It will not give you reliable ground levels where vegetation is dense. It will not replace a measured survey signed off by a chartered surveyor where contract or statute demands one. And it will not show you the state of vertical faces. Where you need any of those, commission the right thing instead, or alongside.

Next step

Tell us the site boundary, the output format you work in and the accuracy you need, and we will quote. See drone mapping services or get in touch.