Published · 2026-08-06
Light: diffuse, never raking
Raking light emphasises surface relief, which often helps documentary photography; for vectorisation it is the main problem. As far as the signal is concerned, the boundary of a hard shadow is indistinguishable from an arris: it is recognised and traced as geometry. On coursed masonry that means doubled lines — one polyline on the stone, one on its cast shadow.
The best condition is diffuse light: an overcast day, or a surface in full shade lit evenly by the sky. Indoors, indirect light or bounced flash reduces the contrast of self-shadows. Check the histogram on site: if highlights are blown or shadows fully crushed, the detail in those areas no longer exists and cannot appear in the drawing.
Position: frontal, centred, no tilt
Shoot as perpendicular to the surface as you can, with the sensor parallel to the plane of the elevation and the centre of the frame at mid-height of the element. Every degree of tilt introduces a scale gradient: the farther part comes out smaller, and two-point calibration stays dependable mainly along the direction you measured.
When you cannot step back far enough, split the elevation into separate frontal portions and convert them one by one, rather than forcing a single strongly oblique or strongly wide-angle shot. On narrow streets, a raised floor or an external stair often allows a frontal view of upper levels.
Lens and focus
Use a normal or short telephoto focal length — the equivalent of 35–70 mm on full frame — and avoid extreme wide angles, where edge distortion bends straight lines and falsifies profiles exactly in the peripheral areas. If a wide angle is all you have, fill the centre of the frame with the subject and discard the edges.
Stop down enough to keep the whole surface in focus (typically f/5.6–f/8), keep ISO low to limit noise and, in poor light, use a tripod instead of raising sensitivity: blur and noise erase the fine edges that the engine has to recognise.
Metric reference: one well-chosen measurement
Calibration needs a single real distance between two points you can recognise in the photograph. Measure it on site with a tape or laser meter, as long as possible and lying on the same plane as the element you intend to dimension: the width of an opening between jambs, the spacing between two stone arrises, the rise of a repeated step.
A levelling rod or scale bar resting against the surface, in the plane of the elevation, makes calibration easier and stays documented in the image. Always record the measurement and the two points chosen: that record is what makes the deliverable verifiable later.
What to keep out of the frame
Anything in front of the surface gets vectorised, because it genuinely is in the image: scaffolding, site netting, cables, signage, branches and shrubs. Move removable obstacles where you can; where you cannot, budget the clean-up time in CAD.
Avoid specular reflections on polished slabs, glass and honed marble too: they produce highlight patches with no stable edge, which generate incoherent paths. A small change of angle, while keeping the view frontal, usually removes them.
Delivery format and next step
Export high-quality JPG, PNG or TIFF, within 40 MB per image, with no artificial sharpening and no aggressive contrast correction: boosting local contrast creates halos that the engine reads as lines. If you shot RAW, develop it neutrally.
With the frame ready, follow the photo to CAD workflow: upload, generate, calibrate on two points in the vector preview, and download SVG and DXF already in scale. If your deliverable must be metrically verifiable across the whole surface, prepare a rectified photoplan or orthophoto instead and use that dedicated workflow.