Using Targets
Last updated
Last updated
When capturing data of a site there must be known scale targets or survey point targets placed within the scene. These targets are used to set an accurate scale, reduce drift in the final output, and allow for alignment of the model. We provide both an origin plate to define a local coordinate system when accurately surveyed control points are not available, and a set of control point markers which should be used when three or more control points are available. We also provide a set of scale plates which should be used in either situation.
We recommend always using surveyed control points when attempting larger captures of over 1000 square metres.
Note: With all markers it is vital they do not move during a scan. They must be stationary to prevent geometric errors in the final model. Do not collect any of the targets until capture of the scan has finished.
Our standard set of scale plates includes a square origin plate which defines the centre of a local coordinate system for the scan, and eight unique rectangular scale markers which should be placed as evenly as possible around the site. We suggest trying to ensure the distances between targets are no more than 20 metres to minimise accumulation of drift in the scan. The square origin plate should be placed flat on a level surface. The rectangular scale pair markers may be placed in any convenient location. Leaning them against a wall can help maximise visibility.
For improved accuracy on larger captures and around repeating structures such as stairwells you may also use the control point targets without surveyed ground truth positions . These should be placed in locations with good visibility as shown below. It is important that these markers be secured if you are in a windy outdoor environment to prevent them moving during the capture.
These markers allow the captured geometry to be aligned with surveyed control points, providing accurate geo-referencing and drift correction. They should be placed around existing retro markers or bolts in the scene with known accurate positions. When uploading the captured video data for a scan, you will also need to provide a csv file listing the coordinates and target ID for each control point marker (described below). We recommend leaving these markers in place if possible so that successive scans can be captured without updating the marker coordinates.
You might need to secure the targets to the ground or other surface to prevent them moving in the wind for outdoor captures.
The origin target can be screwed into place to maintain a consistent coordinate system across multiple scans.
Using double sided tape is useful when fixing control point markers to walls, though if left too long this may rip off paint, so take care.
