Orthomosaic vs point cloud: which one actually documents condition?

If you've had a photogrammetric survey quoted for a listed structure, you've probably seen both terms on the deliverables list and wondered whether you need one, the other, or both. They come from the same flight and the same set of overlapping photos, but they answer different questions on a condition record.

What each output actually gives you

An orthomosaic is a single flat image, corrected for lens distortion and terrain so that every part of it sits at true scale. It's built by stitching the overlapping photos from the survey flight into one continuous, measurable picture. You can draw a line across it and get a real-world distance. You can lay this year's orthomosaic over last year's and see, to the pixel, where a crack line has crept or where a lime render patch has started to fail. It's close to a photograph, except you can measure it.

A point cloud is a different kind of output entirely. It's a set of XYZ points, usually millions of them, built from the same photoset but describing the three-dimensional shape of the structure rather than its surface appearance. The orthomosaic is flat; the point cloud has depth. That's what you need if the question is about geometry rather than finish: has a parapet bulged outward, is a buttress still plumb, has a roofline dropped at one corner since the last inspection.

Most condition records end up needing both, because a monument degrades in both ways at once. Spalling, staining, mortar loss and render failure show up on the orthomosaic. Movement, settlement and deformation show up in the point cloud. A quinquennial inspection that only has one or the other is missing half the picture.

Why informal photo comparisons fall short

The usual fallback is a set of elevation photographs taken from roughly the same spot each time, eyeballed against the last set. It works for obvious changes but it struggles with anything gradual. A photo taken at a slightly different distance, angle or time of day makes a genuine 3mm crack movement look identical to a lighting difference. There's no scale in the frame, so you're comparing impressions, not measurements.

An orthomosaic replaces that guesswork with something you can actually overlay and measure, survey to survey. A point cloud goes further and lets you check the structure's shape hasn't shifted, not just its surface. Neither depends on the surveyor happening to stand in the same footprint as last time, because the model is georeferenced and can be sliced, overlaid or measured from any angle on the next visit.

Choosing between them for a condition record

If the brief is mainly about weathering, render and masonry condition on visible elevations, an orthomosaic set at sub-10cm resolution from RGB imagery covers most of what a conservation officer needs to flag and track. If there's a known or suspected structural concern, bulging walls, leaning towers, roof spread, a point cloud from the same flight gives you the geometry to check against.

In practice the two aren't really a choice. They come off the same drone or crewed aircraft survey, from the same set of RGB photos, so specifying "orthomosaic only" or "point cloud only" usually just means someone decided not to process half of what the flight already captured. The sensible approach is to treat the first survey as a baseline covering both outputs, then repeat the same flight pattern at each future review so the comparison is like for like.

Heritage Documentation flies that baseline survey and returns it as a measurable record rather than a photo set, then flies the same monument again at each condition review so you're comparing model to model. If you're weighing up what a baseline survey like this would look like for a specific site, the offer is laid out here.