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Accuracy & limitations
This tool is a visual aid, not a calibrated instrument. We publish limits, not inflated ±0.0° claims. The math (arccos(dot/|A||B|)) is exact for the points you place — real error comes from how you place them and how the photo was taken.
Updated Online Protractor
Core principle: the tool measures the 2D projection you see on screen, not the physical 3D object. Photo angle ≠ real-world angle without controlled setup.
For safety-critical tasks, verify with a physical calibrated protractor.
1. Perspective distortion
A tilted photo skews angles: a rectangle becomes a trapezoid, a right angle can look acute. Keep the lens parallel to the surface and centered. If you must use a tilted shot, note it is the apparent angle.
2. Vertex placement
Error grows near the vertex. Use crosshair, grid and the endpoint magnifier that appears while dragging. Zoom the view (not just the image) to place the glowing center exactly on the two-line intersection.
Keyboard: Tab to focus center/A/B, arrows nudge 1px (Shift 10px), Shift while dragging snaps angle.
3. Ray length
Short rays amplify error: 1px near the vertex swings the angle. Drag handles outward along the edges to lengthen rays. Longer rays + magnifier = stable reading.
4. Line thickness
Thick lines create ambiguity: center vs edge of the stroke can shift 1–2°. Align to the centerline of each edge. For thin blueprint lines, high-contrast mode helps.
5. Image resolution
Low-resolution or compressed images blur edges. Use the highest-resolution crop, avoid recompression, and fit image to view. Export PNG from the tool preserves vectors, not photo pixels.
6. Browser zoom & display scaling
Page zoom (Ctrl +/−) and OS display scaling change on-screen pixels but not the angle calculation itself. They do affect touch target size and placement. Reset to 100% for repeatability, use grid for reference.
7. Decimal precision vs real accuracy
Precision 0–2 decimals controls display only. Showing 45.12° does not mean ±0.01° real. Use decimals to track trends (e.g., 89.8° → 90.1°) while describing real accuracy as “within a few degrees given photo/placement”.
- Good practice: quote as “~90° (photo, uncalibrated)” not “90.00° ±0.01°”.
- Toggle degrees/radians in the toolbar; supplement/complement helpers live on angle relationships.
How we keep it honest
FAQ
Is the decimal precision the real accuracy?
No. The tool can display 0.01°, but your placement, photo perspective, line thickness and lens distortion dominate error. Use decimals for trend, not proof.
Does zooming affect the angle?
Browser zoom or page zoom does not change the angle math, but it changes on-screen placement and touch targeting. Use grid, crosshair and magnifier to re-center.