3" or 1/4"
3D point cloud data (nothing to do with storage or computing clouds) recorded by high-speed laser measuring devices are the new norm for data collection, inside and out. They are used to produce large-size scans of roads, buildings, and manufacturing plants. Versions are available also for small size scans, such as of just-manufactured parts and car bodies.
The problem of raster has dogged it since it was invented: the higher the resolution (ie, more accuracy), the bigger the file size. For 2D images, like photograph, the increased file size is by the square: twice the resolution make the file size 4x bigger. Compression schemes, such as ZIP and JPEG, reduced sizes by about 10x.
For 3D laser scans, the problem is cubed: doubling the resolution makes the file size 8x bigger (2x high by 2x wide by 2x deep). While storage space is dirt cheap these days, and new algorithms tackle the problem of billion-point files, there still is the time it takes to collect points. So there is an interest in balancing resolution against file size.
The Massachusetts Association of Land Surveyors & Civil Engineers (MALSCE) has taken a stab at the problem, last month releasing a brief Best Practice Guidelines. In brief, the guidelines recommend lower resolutions for feasibility studies, and higher resolutions for construction documents.
Different stages of a project gain different levels of accuracy:
Level Purpose Precision Accuracy
D Feasibility Undefined n/a
C Planning 3" 0.25'
B Design 1/2" 0.10'
A Documentation 1/4" 0.04' (half-inch)
The Guidelines note that surveyors should be aware of insufficient control and target checks, vibration during scanning, wind, and shadows from blocked areas.
Download the PDF from http://files.engineers.org/file/MALSCE-Point-Cloud-Standards-V1-9.pdf