I had to measure a roof for a quote. Since the geometry was anything but a rectangle and the tape measure couldn't reach every corner, I figured I'd do it from the aerial image. The problem was that almost every online tool I found for this was paid, required a sign-up, or didn't allow working with arbitrary polygons.
So I built one.
The result is Trazado, a lightweight web tool to measure irregular areas right from the browser. It works like graph paper: you click to drop vertices, and the polygon closes automatically once you place the third one. Any vertex can then be dragged around to fine-tune the outline.
The interesting part is the scale calibration: since aerial images come without measurements, you just pick a side of known length (a wall, a fence, a parked car) and enter the actual meters. From that point on, the rest of the polygon is measured in meters automatically.
You can also lock the length of any side with a padlock, so it keeps its value while other vertices are being moved. If you lock two adjacent sides, the vertex between them gets anchored at the intersection of the two corresponding circles, which is very handy when reconstructing a plan from real-world distances but no angles.
Area is computed with the good old Shoelace formula, applied to pixel coordinates and scaled by the calibration factor. It runs in any modern browser, with no server, no external libraries and no data collection.
Available here: Trazado, irregular area calculator.