In this article we want to talk about a laser scanner survey performed with the target acquisition technique directly in the field, a work methodology that is certainly more “onerous” in terms of time in the “on site” survey but extremely faster and more precise in the subsequent work joining the clouds.
Abbiamo utilizzato questa metodologia di lavoro durante il rilievo generale della Chiesa della Pieve di Corsignano eseguito da La Sia a supporto del rilievo del quadro fessurativo.
in particular 13 external stations and 12 internal stations for a total of 25 stations completed in about 7 hours of work. The instrumentation used was a Leica P30 Laser Scanner and 6 4.5” black and white targets (4 mounted on revolving supports and 2 fixed to the wall); Leica Cyclone software was used to merge the clouds.
The layout of the church is rectangular with a bell tower in the lower left corner and annexed sacristy in the upper right corner; in the external part of the church, behind the altar, there are the remains of what must have been the bases of the apse of the church.
The survey first involved the exterior: starting from the north-east facade, it turned counterclockwise on the 4 sides up to the south-east part where the outcropping remains of the original wall system were also found (subject of recent archaeological excavations which brought them to light). We then moved inside, positioning the stations in the three naves, in the bell tower and in the sacristy (the underground crypt was not observed) trying not to leave blind corners and giving more detail to some areas indicated by the structural engineer.
In the surveys, especially of the exteriors, we almost always use the targets to have a better result in the subsequent phase of merging the clouds, this time, also given the greater precision required, we also opted for the acquisition of the targets directly at the end of the scan through a function dedicated to the scanner.
Each scan was linked to the next with at least 3 targets positioned at different distances and at different heights, always trying to have the fixed points not closely grouped but “scattered” in the scene to be acquired. The acquisition of the targets, with the laser scanner used (model Leica P30), takes place after the scan of the scene: obviously without moving the instrument, the visible targets are selected one by one from the scan preview, assigning each one a unique ID (in the blotter of the survey, where we usually print the plan of the building to be surveyed, together with the approximate position and the scan number, we also write down the ID of the various targets positioned).
During the survey, the correct assignment of the right ID to each target is essential in order not to find yourself in the studio with clouds that do not hook up (obviously the assigned ID can always be corrected afterwards in the subsequent cloud processing phase). Once the targets have been identified and registered, the actual acquisition is carried out by the instrument which is reactivated and searches, in the previously selected points, for the black and white pattern of the circular target.
If the acquisition takes place correctly (the instrument manages to identify and scan the target) a positive check is obtained from the instrument on the identified targets, and it is possible to proceed to register them in the scene.
But what is the difference between the acquisition of targets in the field and the one obtained by doing the same operation in the studio on a single point cloud? Operationally speaking none, in both cases we obtain fixed points in the scene to be used for the union of the various clouds. Technically, however, there is a huge difference, mainly in the precision of the survey of the center of the target and we can already understand this from how the instrument ” calibrates ” itself when it has to acquire the target.
The Leica P30 has several pre-set resolutions for scanning (from a maximum of 0.8mm to a minimum of 50mm calculated at 10m), when acquiring targets “on site”, already knowing the distance at which the targets are positioned (a scan has already been performed) it “calibrates” to obtain the right detail on each target; standing close to the instrument you can see (and hear…) the prism which has a different rotation speed (the slower it is, the higher the scanning resolution) depending on the distance from the target to acquire.
This allows, for example, to make a complete scan of the scene at a low resolution (for example 50mm) and, subsequently, an acquisition of a target even 50m away with a resolution of 0.8mm. If instead in the post-processing phase of the clouds we tried to acquire a target 50m away in a scene scanned at a good resolution (for example 3.2mm) we would have a possible error of the “detected” center of the target of 8mm (a bit too much for perform a cloud merge where a lot of precision is required).
This fact of the possible error is linked to an aspect to be taken into consideration, namely that the survey of the “on site” target, after the scan, generates a new point in the scene based on the “second” scan of the target only (the “real” center ” is identified through an algorithm that takes into consideration the black and white portions of the target); instead in the target recognition procedure in the post-processing phase of the clouds, the algorithm always generates a new point in the scene (the center of the target) but based on the points already existing in it with all that follows in terms of precision.
We have tried to carry out, on a target only 11m away from the instrument, the two procedures, namely on-site recognition, and recognition in the post-processing phase; the scan resolution was 3.1mm@10m. As you can see in the image on the side, the error between the two centers detected (“01” is the center of the target detected on site and “C” is the center of the target detected in the cloud post-processing phase) was 1mm (0.98mm to be precise).
Lastly, there is the aspect linked to the points actually recorded in the scene and therefore to the weight in terms of Gigabytes of the files to be managed. Scanning at high or very high resolution to record everything that “could” be useful (including targets) in the subsequent post-processing phase makes little sense other than wasting a lot of time on site. Each scan or survey campaign must be “calibrated” according to the purpose of the survey itself and the subsequent return phase.
In summary, if you want to obtain greater precision in the union of the stations of your surveys, we advise you to try the on-site acquisition technique of the targets.