An orthophoto is an aerial or satellite image geometrically corrected so that scale is uniform and features sit at their true map positions — an image you can measure on. Standard orthos correct terrain displacement using an elevation model; true orthophotos go further, correcting building lean so rooftops sit exactly on their footprints. Orthoimagery is the visual foundation of modern basemaps, cadastre and asset mapping.
Why raw images lie
A photograph is a perspective projection: anything above or below the reference plane is displaced radially from the image centre. Hills shift, valleys shift oppositely, and buildings lean outward, hiding streets behind their own facades. Distances and areas measured on raw imagery are wrong by varying, position-dependent amounts.
Orthorectification re-projects every pixel to its correct planimetric position using the sensor model and a terrain model — converting picture into map.
Ortho versus true ortho
Conventional orthos use a bare-earth DTM, so terrain is corrected but buildings still lean — acceptable in open landscapes, problematic in cities where lean occludes streets and displaces rooftops off their parcels. True orthophotos rectify with a surface model including buildings, using overlapping images to fill the areas each facade hides.
The cost is significant extra imagery overlap and processing, which is why true ortho is specified for dense urban programmes and premium basemaps rather than everywhere.
Mosaic craft and quality
Project deliverables are mosaics of many rectified images: seamlines routed to avoid slicing through structures, radiometry balanced so no join is visible, and consistency maintained across thousands of tiles. Accuracy is certified against ground checkpoints; completeness and visual quality reviewed systematically.
Formats have modernised — Cloud-Optimised GeoTIFF and tile services let a national orthomosaic stream instantly into any GIS or web map.
Frequently asked questions
What is the difference between an orthomosaic and an orthophoto?
An orthophoto is one rectified image; an orthomosaic stitches many into a seamless, colour-balanced carpet over a project area. Most delivered “orthophotos” are technically mosaics.
When is true ortho worth the cost?
In dense urban areas where building lean would hide streets and misplace rooftops — cadastral overlays, city basemaps, rooftop analytics. Rural and open-terrain projects rarely need it.
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