The selection of a laser scanner can be a difficult decision. Lets face it, they all are cool and it would be nice to have one of each! But ultimately, it is a significant investment and you want to make sure that you get the optimal value out of your purchase. Laser scanners may have specific purposes, environments, or applications for which they are best suited. Caution must be used when comparing scanner specifications because there are no consistent standards for testing and reporting, although the ASTM E57 committee is working to address this issue. For example, the accuracies of scanners vary non-linearly with distance and some manufacturers publish accuracies at 50 m and others at 100 m. Overall accuracies are influenced by the surface being scanned, geometric configuration of the scanner and target, environmental conditions, laser properties, and many other factors. Point of Beginning offers a very helpful list of specifications (usually updated yearly) for a variety of scanners and should be consulted by anyone looking to purchase a scanner.
As I have discussed in previous articles, scan geo-referencing can also induce error. For example, the accuracy of a survey grade GPS unit is typically 1.0 cm in the horizontal and 2.0 cm in the vertical. Thus, data acquired from a scanner with a millimeter level accuracy will no longer be of millimeter accuracy if GPS is used for the scan geo-referencing.
Some criteria that should be weighted in the selection of a scanner include (although not limited to):
1. Type of laser (wavelength) How well does the laser reflect off the type of surfaces to be scanned? Are you scanning metal objects, topography, wet surfaces, dark surfaces?
2. Spot size (pulse width, beam divergence) What will be the size of the laser spot on the target? Do features smaller than this need to be modeled?
3. Speed The most common question/metric is: How many points does it collect per second? Most scanners are fast so this is not the most crucial ifnormation. A more important question is: How much setup time (e.g. mount the instrument, boot up, select scan window, acquire imagerytargets, and scan) is involved? The latter can dominate a lot of the actual field time required. Another question is how much processing is required and what workflow optimizations are presented by the scan manufacturer.
4. Field of View How wide is the scan view? (Many scanners can scan 360 degrees horizontal, but have a limited vertical window).
5. Accuracy What level of instrument measurement error is acceptable to fit into your overall error budget? This will depend on the intended applications.
6. Resolution What point density is achievable/required?
7. Range How much coverage is needed in a single scan?
8. Geo-referencing Does the scanner have a calibrated GPS mount for geo-referencing? Or are external targets required?
9. Level compensatorsInclination sensors Can the scanner correct for out of level setups, improving data quality?
10. Mobility, durability Is the scanner designed for field operation? Or is it meant to be an indoor scanner?
11. Photography Do you need photographic information? Does the scanner use an internal or external camera? What steps need to be taken to map the photograph to the point cloud if it is external? Is the photograph taken simultaneously or as an additional data collection procedure? How long does this take?
12. Software What software package is used for aligning and meshing the scans? What features are available? How does this package integrate with the other packages you are currently using?
13. Effects on humans Is it eye safe? Will it set off security alarms (e.g., in cultural heritage applications, you may be scanning in a museum with an alarm system (I have had that happen before!))?
14. Maintenance What are the costs associated with maintaining the scanner? Many manufacturers offer maintenance packages.
15. Upgrades Scanner technology evolves quickly. Would you be able to upgrade your scanner (or obtain a trade-in credit) when it is time to upgrade?
Obviously you would also want to do a cost/benefit analysis when comparing different systems. Ultimately, invite the manufacturers out and have them demo the scanner in the environment you will be using it in most frequently. Also, ask them to let you demo the software so you can get a feel for the workflow and how that will integrate into your current operations.
If your agency is looking to purchase multiple scanners, a few questions arise. Do you want to go with various types of scanners so that you have a versatile amount of equipment for any job? Or is it better to standardize your equipment so that all personnel are using the same scanner, reducing training costs?
Good luck in your new scanner purchase!