Metric drawings from point clouds : the basics

In Recap point clouds are impressive. To get to something you can draw from they need slicing.

Remember line of sight from the scanner will result in lost data in occluded areas, the ‘dark’ shadows are holes in the data.

A slice attached to CAD in WCS will determine the orientation for the CAD work. Local UCSs will be needed to thread ploylines into the 3D space prior to extrusion into solid components.

The point cloud in CAD is frustrating: getting a tight zoom to fix details makes them dissapear. Setting 3Dobject snap on is a must to position UCSs and polyline traces. In this view there are 4 pegs through the collar brace tennon but only 3 are visible in the cloud.

Snapping to the points needs care, the point cloud node snap is very sensitive.

In Recap it is possible to interrogate the bubble views as scaleable data. The dimension tools edge detection is a bit clunky and needs repetition to be confident with the results.

The model scale is no beter than 1: 100 given the confidence over the dimensions from Recap.

Overall scaling (the positioning of the centres principal components) is better than that for detailing: if the node snap to the points is unambiguous the scale is equivallent to the scan scale. Roughly 1: 50 ( +/-10mm) for these scans.

Having built a model from the point cloud as a route to the perspective drawing a fly around is a useful way to share the spatial sense of the structure.

From scanner to Recap and thence to AutoCAD a path is possible for the draughtsman.


About billboyheritagesurvey

Heritage worker
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