# How Photo Measurement Works

Question: A photo only has two degrees, so how do you pick a 3D {X, Y, Z} point?

Answer: If you pick the same point from two different perspectives (two different photos) then Cupix can calculate the 3D point.

In practice, this means you have to select each point two times.

### Stereo Photo Point Selection

When we know two pixels from two photos were projected from the same 3D point, we can define the 3D point by reversely projecting them in the space and triangulate the crossing point. This works because Cupix maps the position and orientation of each photo within 3D space of your project coordinate system.

### Accuracy of measurement

The measurement, inevitably, will have a degree of accuracy, which mainly depends on the accuracy of the reference plan, the quality of the capture, and the precision of point selection. For more information, refer to the article on Measurement Accuracy.

Note: you can also learn the basics of taking a measurement in a SiteView if interested.

M
Matt is the author of this solution article.

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