Most of the speakers' presentations at the March BIM breakfast were devoted to successful cases of laser scanning and creation of point clouds for objects of varying complexity.
In contrast, Sergey Bakunov's report from SGSCAN contains advice and recommendations for customers on choosing a contractor to perform 3D measurements. In it, the speaker explained in detail why it is so important to write out the technical specifications for scanning in detail and what exactly needs to be specified in the requirements.
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Sergey Bakunov decided to explain the concept of laser scanning from the point of view usa email list of the basic operating principles of the equipment used to perform it:
"We have a ground-based stationary scanner that rotates 360 degrees vertically and horizontally and emits a beam. This beam hits an object, returns to the source and records the distance to the object as a dot. This dot contains information about the position along the X, Y, Z axes and color."
The equipment scanning speed varies from 500 thousand to 3 million points per second, which allows collecting a large amount of information about an object in a short period of time. At the same time, the scanner measures only what is directly in its visibility zone. Therefore, it has to be moved from station to station to obtain complete information about the object. Based on the measurement results, you will receive a digital model from point clouds for performing, for example, measurement drawings.
Criteria for selecting a performer
Sergey Bakunov presented three main criteria for choosing a contractor: quality, deadlines and cost.
"Naturally, the third criterion has become very important in the market recently. Basically, everyone wants to do everything cheaper, faster. And they start to forget about the quality of work," the expert complained, adding that in his speech he would focus on the quality criterion.
Quality of work
What is high-quality laser scanning work? Sergey Bakunov gave a fairly simple explanation: a high-quality result of work implies the fulfillment of the customer's requirements, which must be fixed in the technical task. There are three stages in the formation of the technical task:
Data collection
They include BTI plans, executive drawings, photographs, topographic surveys, general plans, situational plans, etc. All these materials will allow contractors to more accurately estimate the time frame and cost of work.
Project goal
The written goal of the project will significantly help the contractors to carry out work on measuring the object. For example, if the customer writes the definition of load-bearing structures as a goal, then a competent contractor will immediately understand what to do and will start asking the necessary water questions to understand the scope of work.
If the goals are not specified, the laser scanning contractor will be overworked “in the field”, which will increase the overall cost of his services.
Point Cloud Requirements
Sergey Bakunov described this criterion in much more detail than the previous points, talking about local and absolute accuracy, combined shooting, methods of combating noise, requirements for cloud density and color, etc.
Cloud Accuracy
This requirement is divided into two categories: local and absolute accuracy. A cloud with local accuracy is obtained using equipment with one scanning station. It depends on the technical characteristics of the device, laid down by the manufacturer.
A point cloud with absolute accuracy is a processed, stitched and registered cloud, the elements of which were collected from all scanning stations. It is this array of data that is transferred to the customer, according to which he subsequently develops a 3D model or measurement drawings.
How to achieve maximum accuracy of the point cloud? One option is a combined survey with electronic tachymeters. Their measurement speed is almost 5-10 times slower than that of a scanner: 40 thousand points per second compared to the minimum scanner characteristic of 500 thousand per second. However, tachymeters compensate for this disadvantage with much higher accuracy. When using a scanning tachymeter, it is also not necessary to follow the rule on overlapping between stations.
To achieve maximum accuracy, it will be enough to create a closed loop of tacheometers around the object, take measurements and “sew” into the resulting frame a cloud of points obtained using terrestrial laser scanning.