What is an Orthophoto (Orthomosaic)?
An orthophoto is an aerial photograph that has been geometrically corrected to a uniform scale, so every point appears as if seen from directly overhead — which means real distances and areas can be measured directly from it, unlike an ordinary aerial photo. This short guide explains what makes an orthophoto different, and how it pairs with a DEM.
Corrected, not just captured
A raw aerial photo has perspective distortion — objects lean, and scale changes across the frame. An orthophoto is geometrically corrected so that every point is shown as if viewed straight down, giving a uniform scale across the whole image.
Because the scale is uniform and the image is georeferenced, real distances and areas can be measured directly on it, like a map.
Orthophoto vs orthomosaic
The terms are often used interchangeably. An orthomosaic is simply an orthophoto stitched together from many corrected images to cover a large area, which is what a drone survey typically produces.
Why pair it with a DEM
An orthophoto gives visual context — what is actually on the ground — while a DEM gives the elevation. Together they let you interpret a site and measure it at the same time.
In practice you drape the orthophoto over the terrain and use it as a precise basemap to digitize vectors and read the site alongside slope, contours and volumes.
Frequently asked questions
What is an orthophoto?
An orthophoto is an aerial photograph that has undergone geometric correction so every location appears as if captured from directly overhead, giving it uniform scale throughout — which means real distances and areas can be measured directly from it, functioning like a map.
What is the difference between an orthophoto and an orthomosaic?
The two terms are frequently used interchangeably. An orthomosaic specifically refers to multiple corrected (orthorectified) images stitched together to cover a larger area, which is the typical output of a drone survey.
Why can't you measure accurately from a raw aerial photograph?
Raw aerial photographs contain perspective distortion — objects appear tilted and scale varies across the image — so accurate direct measurement isn't possible until the image is geometrically corrected into an orthophoto.
Why pair an orthophoto with a DEM?
An orthophoto supplies visual information about ground conditions while a DEM supplies elevation data; combined, they let you interpret a site and measure it at the same time, typically by overlaying the orthophoto on the terrain as a basemap for digitizing vector data and analyzing slope, contours, and volumes.
Sources
View and measure on large orthophotos in STREAM
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