Published · 2026-09-14
Where scale lives in an orthophoto
The scale of an orthophoto is a single piece of information: the real-world size of the pixel, the GSD (Ground Sample Distance). If you know that one pixel equals 3 mm, the image is measurable; if you do not, it is not. Everything else — coordinates, reference system, orientation — serves georeferencing, not measurement.
That number can be found in three places, in decreasing order of reliability.
In the file header
A GeoTIFF carries the pixel size and the reference system in its internal tags. This is the only case in which the information travels with the image and cannot be separated from it by moving the file.
In an accompanying world file
It is a six-line text file with the same name as the image and a derived extension: `.tfw` for TIFF, `.jgw` for JPG, `.pgw` for PNG. The six lines are, in order: pixel size in X, Y rotation, X rotation, pixel size in Y (negative), X coordinate of the centre of the top-left pixel, Y coordinate of the same. The first line is the one that matters.
0.003000000000
0.000000000000
0.000000000000
-0.003000000000
2291455.847000
5034120.219000
The world file only works if it stays in the same folder as the image and keeps the same name. It is not embedded: it is a piece of paper taped to the image, and it can come unstuck.
In the processing report
Metashape, RealityCapture and 3DF Zephyr produce a processing report in which the GSD is stated explicitly, together with the error on ground control points. Even if the image has lost all metadata, that number can be recovered from there — reason enough to always archive the report next to the deliverables.
Five points where scale breaks
JPG export without world file
The most common case. JPG has no georeferenced header: if the software does not write the accompanying `.jgw`, or if the option is not checked, the information ceases to exist at the very moment of export.
Moving the file without the world file
The image is sent by email, uploaded to a shared drive, dragged into another folder. The `.tfw` is left behind, because nobody sees it and nobody thinks it matters.
Resampling
Reducing the image to make it lighter — from 8000 to 4000 px — doubles the real-world pixel size. If the world file is not updated accordingly, it contains a number that no longer matches the image, and this is worse than no information at all: it is wrong data that looks valid.
Cropping
Here the consequence is less serious than feared, and it is worth understanding why. A crop does not change the pixel size: it only changes which portion of the image you are looking at. The world file will have wrong origin coordinates, so georeferencing breaks, but the scale survives. To bring a drawing into CAD you need the pixel size, not the absolute position: a cropped orthophoto remains fully measurable.
Screenshots
It sounds obvious, yet it happens constantly: the view in QGIS or the processing software is captured from the screen instead of exporting. The result is an image at monitor resolution, with scale lost and resolution degraded in one move.
How to check whether the image still has scale
The quickest check is from the command line, with GDAL:
gdalinfo image.tif
The lines that matter are `Pixel Size`, which reports the real-world pixel size in the units of the reference system, and `Coordinate System`, which tells you which units are being used. If `Pixel Size` is there, the data is there.
Without the command line, the alternative is to drag the image into QGIS: if it positions itself in space and the distance tool gives plausible values, it is georeferenced. If it lands in a corner with sides equal to the number of pixels, it is not.
The check that really matters, however, is none of these. It is measuring a distance on the image whose real value you know — taken on site, not derived from a drawing — and checking that it matches within the tolerance of the scale at which you must deliver. A metadata entry that is present but wrong is the most insidious risk in the whole chain, and this is the only way to catch it.
Reconstructing scale when it is lost
If the GSD is known from the processing report, the real width of the image is obtained directly:
real width = GSD × image width in pixels
A 4000 px orthophoto with a GSD of 3 mm covers 12 m. From there the drawing scale is determined, and nothing else is needed.
One condition applies: this only works if the image was not resampled after export. If you are not sure, check that the pixel count matches the value declared in the report.
If the GSD cannot be recovered, the site measurement remains. It is the only always-valid reference, and the criterion used to choose it determines the precision of the result:
- base as long as possible. The relative scale error is the ratio between measurement uncertainty and length: 5 mm of uncertainty over 1 m gives 0.5% error; over 10 m it gives 0.05%.
- materially sharp ends. Live arrises, not frayed plaster edges.
- laser distance meter, not measuring wheel.
- second measurement on a different axis, used for verification and not for calibration. If after applying the scale the second measurement diverges beyond tolerance, the problem is not calibration: it is the upstream processing.
Moving to CAD
A correctly scaled orthophoto can still produce an out-of-scale DXF, for two reasons that are both trivial and both common.
The first is the `$INSUNITS` variable in the DXF. If it is not set, the file may be rescaled by a factor of 10, 100 or 1000 when imported into another environment, with no warning. Set the drawing unit before exporting and state it in the deliverables.
The second is that most vectorisation tools, whether generic or specialised, do not read the georeferenced metadata of the image. They receive a raster and return vectors in pixel coordinates, so the scale has to be restored downstream anyway — even starting from a perfectly referenced GeoTIFF.
This is where CVCAD calibration fits: after conversion, before download, you select two snap points on the preview and assign the real distance in millimetres, centimetres or metres. The DXF and SVG you download are already scaled. If you are coming from a photogrammetric workflow and you know the GSD, you do not even have to measure the distance: you calculate it from the pixels between the two chosen points.
Recommended procedure
- Export the orthophoto as GeoTIFF, not JPG. If JPG is needed for file-size reasons, check that `.jgw` is written and keep it with the image.
- Record the GSD from the processing report and keep it in the same folder. It is the only datum that no file transfer can corrupt.
- Do not resample. If you need to make the file lighter, crop: cropping preserves pixel size, resizing does not.
- Take at least two control distances on site on elements with sharp ends, using a laser distance meter.
- After CAD conversion, verify one of the two distances on the drawing before starting work on it.
- State on the drawing the measurement used for calibration, the point where it was taken and the instrument. This is what makes the survey verifiable by third parties.
Related: From orthophoto to CAD · Degradation mapping UNI 11182