Slope Length vs. Hypotenuse Variance?

Just wondering if anyone knows why the surface length is generally more than the length of the hypotenuse when measuring regular surfaces e.g. roofs? If I drop a line on a roof I find that the surface length is more than hypotenuse calculated either by myself or the surface/distance/slope app. I can understand if this was on the ground where the surface is irregular but it seems odd when it’s a man made structure like this.

Any thoughts welcome!


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I think you are correct that it is because of the irregularities of the surface, but it also has something to do with the accuracy of the drone. If you inspect the 3D model closely you will see that it is wavier than it appears when zoomed out. You will have to solve for the hypotenuse using the base (horizontal) and acute angle, but that is probably a good feature request as an added piece of information. in construction we use the surface length to calculate actual material on an irregular surface, but I can see that in roofing calculating the plywood width for resheeting or the length of a truss leg could be very valuable.

Yes I had thought that might be the case also. Almost a case of too much information as the roof measure will need the length of the hypotenuse as opposed to the actual surface (for valley details etc.). Below is a capture of the Surface Distance and Slope app from this project which is useful. When I calculate it myself from the length on plan and the height it is much closer to the hypotenuse length shown here so I will continue to use that. Cheers.

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