The Bottom Line: Reflectorless Total Stations & Their Applications

 

Last month I wrote about modern layout techniques and focused on field controllers and application programs that enhance and speed up construction layout

This time I want to address a recent advancement in total station technology: the reflectorless measuring technique.  Manufacturers refer to this technology as “reflectorless”, “prismless” or “direct reflex”.   This is a feature that surveyors had dreamed of for decades before it became available. It is a fantastic tool for topographic work and avoids the many vicious, barking dogs that wait just across the fence where you need to locate an existing structure, among other things. The applications for construction layout and as-built’s are plentiful as well.

Without going into the technical explanations of how this works, the technology allows for measurements to objects or points without the need of placing a prism at those points. This is a great time-saver when points are difficult or impossible to reach.  It also improves safety when you need to determine positions on elevated structures, falsework, or any point that is dangerous and time-consuming to reach.  Some examples are: measuring to structural steel, checking the clearance heights of overhead power lines, measuring objects that are inaccessible or across obstructions, checking the position of forms prior to a pour, etc.

At first the technology was limited by range and measuring beam-width issues, but today most of those limitations have been overcome.  Reflectorless range has been extended to up to 2000 meters (over 6500 feet).  Accuracy of reflectorless total station measurements are as good as +/- (1 mm + 1ppm).  Of course, when no person is required to place the reflector at the point being measured, measurements become a one-person operation, saving labor costs.  When clever support application software compliments this capability the applications and benefits increase even more. These programs allow measurement into corners, measuring to objects that have obstructions between them and the total station and automated measurement of a series of points, to name a few.

When combining reflectorless distance measurement capability to a motorized total station even more possibilities are realized. One of these applications is generally referred to as “scanning”.  With clever application software it is possible to measure surfaces, such as stockpiles, without placing a person and reflector on them.  These routines allow the operator to define the area to be scanned, set intervals and other parameters and then walk away from the instrument.  The instrument will then automatically take measurements in the pre-defined area and at the pre-set intervals.  The total stations motor-drive will position the instrument for every shot until the entire area has been measured. As stockpiles are time-consuming to scramble up and two people are needed, using conventional instruments, this is a time and labor saver.  Scanning surfaces for quantity determination is not the only application of these instruments. Profiles can be automatically scanned; subgrade and other material layers can be as-built.  The time-saving benefit of this technique is considerable. Some estimate the productivity increase of surface scans can be up to 40%. Points can be measured at a rate of 20 points per second or more, and there is no waiting for a rodperson to arrive at the point.

When using the scanning technique there are some limitations as well.  One situation that limits the use of the reflectorless capability has to do with the angle of incidence. For the measurement beam to return to the total station (which is how the distance is determined) the object being measured must be within a certain range of angles relative to it.  A good example is a long roadway.  At a certain distance from the total station the angle to the surface will become too shallow and the measurement beam can no longer return to its source.  There are tricks to mitigating this problem; placing the instrument much higher than the surface being measured, or making multiple instrument setups.  As these limitations on apply only to long or very wide horizontal surfaces they are not encountered very frequently by users.

Almost every manufacturer of total stations has reflectorless models.  The range, accuracy and other specifications may vary slightly, but all are sufficient for use on the job site. When combined with motorized and robotic total stations the benefits are significant, greatly increasing productivity while reducing manpower requirements.  The application software that delivers the functionality of these units does vary from manufacturer to manufacturer, so do your homework and investigate several models and arrange for some demonstrations from multiple suppliers to find the best fit for your needs. The return on investment will be substantial, if you are not already using this technology.

Reflectorless measurements can be further complimented by video capture, enhanced scanning capability and other documentation and applications tools, but that is the topic of next month’s article.

 

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