Tilt without guilt: No-calibration tilt compensation is now standard

Prior to the advent of tilt compensation for surveying and construction GNSS rovers, there were incremental approaches to tilt, with limited success. However, five years ago, “no-calibration tilt compensation” was first incorporated as a standard option for rovers. Some users remain skeptical or exercise the same caution as they did when such innovations as EDMs were first introduced. Nevertheless, the adoption of tilt compensation — for appropriate tasks — has spread rapidly. How did we get to this point?

For centuries, plumb bobs and bubbles were the only viable options to level an instrument or pole about a point. Early references to spirit levels appeared in the 15th century; however, siphon style water levels may have been in use in ancient Greece, China, and elsewhere for much longer. In more recent centuries, various types of level vials became a standard feature for surveying transits, theodolites and levels. Vials with a slight upward curve position a bubble between defined center marks when level.

Circular, convex glass bubbles appeared for industrial applications in the 19th century and were soon incorporated into surveying instruments and survey poles. In recent decades, electronic bubbles, or “e-bubbles” emerged, using microelectromechanical (MEMS) tilt sensors along with various methods to apply an orientation to compute the position of the pole tip relative to the phase center of the GNSS antenna. This is in contrast to relying on a bubble alone to orient the phase center directly above the pole tip.

There are both pitfalls and potential productivity losses if the pole has to be leveled solely with a bubble for each measurement; we’ll examine these later. If freed from the bubble — as electronic bubbles, tilt sensors, and various methods for orientation enable — how much productivity gain can be realized? For which tasks do the users find tilt compensation most useful? For which do they not? We talked with manufacturers, dealers and field users to find out.

Early adopters

Lee Landman owns a firm in the Cape Town area of South Africa that provides construction layout, civil engineering layout and related topographic mapping services. Landman obtained a Trimble R12i GNSS rover, with no-calibration tilt, shortly after its release in 2020.

“Tilt is my go-to tool for almost all tasks now, except layout that needs better than 15 mm to 20 mm tolerance,” Landman said. “For topographic mapping, I will get everything possible with tilt, and then use a total station to get the points I can’t with the rover.”

Landman reports productivity gains of 30% to 50% on certain jobs. His crews will try to leverage a tilt rover for as much of a job as they can, provided it meets precision needs. For checking work, such as grade checking or layout verification, they will try to use tilt for everything first. Then in any areas that look suspect, they will set up a total station to confirm. He said this saves a lot of time up front.

After several years of use, Landman said there are specific tasks where they will not use tilt, and we find this echoed by other users interviewed (and from my own tests). For instance, construction layout of such structures as walls and columns where a consistent 5 mm to 10 mm tolerance is required. He said the same applies for tasks where precise elevation is key, such as on road curbs and final road levels. However, he noted: “That’s a GNSS precision thing, not a tilt issue.”

Landman provides other caveats,“I am nervous using tilt on long rods or when you constantly change rod height, as the results of using a wrong rod height would be disastrous, and the deflection on long rods could also degrade the results.”

Summarizing the overall impact on his operations, Landman explained, “We have become more competitive. Not by sharpening our price, but by the fact that using tilt is less fatiguing and faster to do layout and data collection. That gives us an edge over firms that are not using it.” He provided the example of a foundation layout that needed 300-400 points laid out and chalked in an hour or two so that the excavators that were standing by could start digging as soon as possible. “It normally takes two to three moves of the pole and bubble checks to get a point on position without tilt,” Landman said. “Now, when you are doing 300 points, that is 600-900 times that I don’t have to look at the bubble and adjust the rod. The amount of energy and fatigue that saves is just outstanding. No sore lat muscles and eye fatigue.”

Then there are shots that you cannot get with a bubble plumbed pole, Landman said. For instance, in checking rebar layouts prior to construction, as well as marking out on or below the steel rebar cages for plumbing points, voids and slab penetrations.

“Previously we could not easily do this, as you cannot get the pole plumb for total station shots or GNSS to place or check a point,” Landman said. “You just have to check positions on the steel for a column or wall to see if it has enough concrete cover or is in the right position prior to pouring concrete.”

James Richards is the survey manager for Benchmark Surveys, a family-owned and operated firm in the southwest of the UK that has steadily grown its portfolio of services. In part, this growth has resulted from their willingness to embrace new and emerging technologies. This included adding tilt compensated R12i rovers to their instrument inventory shortly after they became commercially available.

“We use tilt on every surveying task where we can use GNSS,” Richards said. “Tilt has enabled us to complete numerous jobs where we would have otherwise only been able to use a total station.” Apart from control, there are not many tasks for which Richards would not recommend using tilt, “It has helped improve our surveys. We can capture data quicker and easier than before and with greater accuracy.” Examples of daily challenges his crews face include getting shots in and around ditches, field boundaries, and boundary fences in foliage, and walls with foliage overhang. These are now easily captured using tilt.

“Tilt has had an enormous impact on our business. We complete work to a higher standard, capturing data quicker and easier,” said Richards. “It helps us capture data that was not possible without offsets. We’ve seen a rise in profitability since using tilt. Surveyors seem to be happier with day-to-day work, knowing that they can capture the data required to meet our high standards, and clients are also happier when receiving more data than expected from surveys.”

Stages of adoption

CHC Navigation is a GNSS developer and manufacturer that has sold hundreds of thousands of units over the past 15 years. They were quick to develop and implement tilt compensation technology, which has now become standard on all of their current models.

Rachel Wang, product manager of CHC Navigation’s Surveying and Engineering division explained the four stages they undertook in developing tilt.

“In the first stage,” said Wang, “users had to rely on the survey pole’s bubble to maintain a centered state, which had significant limitations in terms of measurement accuracy and accessibility.”

In addition to any GNSS error, there could be additional error due to a poorly calibrated bubble, a pole that is not straight, misalignments with each joint of telescoping rods, and user error in trying to keep the bubble lined up while simultaneously operating the field collector (if not using a bipod). Often it seemed that a surveyor would need extra hands and an extra set of eyes.

“The second stage introduced the first generation of tilt compensation using an electronic compass,” said Wang. “Although this technology enabled the first tilt measurements, it was hampered by problems such as low accuracy, tedious calibration, poor reliability, and susceptibility to interference from electrical currents or magnetic fields.”

Such magnetic oriented tilt compensation had been implemented on rovers several years prior to no-calibration methods, by manufacturers that included Javad, Trimble, Topcon, and others. The calibration step often involved rotating the rover vertically in eight or more horizontal positions. This was cumbersome, and the orientation quality changed over time, mostly unbeknownst to the user. It was no surprise that “mag tilt” never really caught on, and unfortunately it made some users wary of tilt in general, even when no-calibration solutions came along.

“The third step was the development of the second generation of tilt compensation, using hybrid positioning based on GNSS + IMU,” Wang said. “This technology was less affected by magnetic interference, but still required initialization of the IMU by shaking the survey pole.” I had tried several models from manufacturers of early “minimal calibration tilt” enabled rovers. For each, a certain amount of movement had to be induced on the pole, by walking around a bit, swinging the pole back and forth, or in a circular sweep. It often did not take more than a minute or so, and then normal moving around on the site would usually keep it calibrated. This was a tremendous step up compared to the old “mag tilt.”

“More recently, we are proud to announce the fourth step in our tilt measurement technology integrated into our new i93 GNSS RTK rover,” Wang said. “Our Auto-IMU technology further simplifies the IMU initialization process by observing acceleration at some point between startup and RTK operation. This replaces the previous repeated shaking of the survey pole for initialization. In fact, users can initialize the IMU while walking or moving normally. In addition, once initialized, the IMU feature is not easily lost even if the pole is carried on the shoulder, held horizontally, or even upside down.”

Wang said that all current CHC survey rovers are equipped with their newest tilt compensation technology. Since the international launch of their first CHCNAV GNSS RTK with IMU, the i90 GNSS in 2019, they have continued to incorporate this feature into all subsequent GNSS rovers. “Based on feedback from users, we know how valuable this feature is,” said Wang. “That is why we have made tilt compensation a standard feature on our current i73, i83, i90 and i93 models.”

Uptake

For the past year, I’ve been talking to other GNSS manufacturers, their dealers, and customers, together with monitoring the subject in surveying and construction groups and forums online. Manufacturers have reported overwhelmingly positive feedback from dealers and customers about the tilt function. Typical feedback focuses on convenience and time savings of not having to level the pole manually.

“Based on our research, we have found this feature to be extremely useful for surveying and staking out on construction sites,” Wang said. “It has increased speed and efficiency by up to 30%.” I have heard similar statistics from each of the manufacturers contacted, as well as their dealers, and most of their customers that have used tilt.

There are common threads to much of the feedback from various sources: the tilt function is now an indispensable tool for many surveying applications.

As Wang noted, “While some users still prefer to use the traditional bubble to plumb the pole, we have seen a clear trend toward adoption of the tilt function in the field. The benefits of tilt, such as faster and easier surveying, are becoming more apparent to our users. As we continue to improve the accuracy of our tilt-compensation, we expect that more and more users will choose this convenient feature over other traditional GNSS rovers in the future.”

Another common observation (no pun intended): even if the pole-tilt feature offers significant convenience and time-saving benefits, it may not be the best option for tasks that require very high accuracy, such as surveying control points. For such tasks, manufacturers, dealers, and users recommend using the traditional bubble on a pole with a bi-pod mount for more accurate measurement.

As something completely new, the uptake across the surveying profession and for construction took time to grow. “It took quite a while to catch on here,” said Keith

Belsham, branch manager for Spatial Technologies, a measurement solutions dealer in the Vancouver region of British Columbia Canada. They specialize in solutions from Leica Geosystems and were an early provider of the Leica GS18 T, widely recognized as the first GNSS rover with no-calibration tilt.

“My perception is that in the United States and some other countries, there are more companies trying to stay at the top of technology,” Belsham said. “However, in our region surveyors are very cautious and need to do a lot of checks and look to see how others respond before they consider it. It was that way with other new technologies; I have memories of numerous total station demonstrations, when prism-less EDM’s were first coming out, where surveyors would pull out a tape measure to check whether the instrument was giving the correct distance.”

About three years after the introduction of no-calibration tilt is when Belsham said it really took off in his region, and it is now quite popular. He gave an example of a customer buying a Leica GS16 (no tilt), saying that they did not see a need for tilt, considering the extra expense. They then upgraded a week later once they were in the field and recognized many instances where the tilt would have saved them time.

A Spatial Technologies customer that has found tilt useful for numerous applications is Lucas Geomatics, a surveying firm based in Surrey B.C. “When I set up GNSS units for construction companies, tilt is great as they don’t have to be super accurate,” said Peter Smith. “I mean, here’s a machine with a bucket that’s 3 f wide, and the bucket is about an inch thick. The grade checked does not have to be super accurate for the machine to hit it, because end users in construction companies do not have to do precise surveying. They’re great guys who dig trenches and tilt gives them more than enough precision for their needs.”

Among the many uses Smith has found for tilt, he has also adapted it in a very creative way to deal with areas of deep foliage. A common approach to working in thick foliage is to raise the GNSS rover up on a tall or extended pole; this can increase the number of satellites viewed and reduce multipath. However, working with a bubble low enough on the pole to see makes it quite difficult to keep the rover at the top sufficiently still and plumbed over the tip. I remember some clever (albeit questionable) solutions folks cobbled together to help plumb very tall poles, such as a small live video camera pointing down over a bubble near the top of the pole to view on a phone. Smith said that tilt solved this problem, and he uses various tall rods, including one that extends to as much as 12 m. He chose a non-conducting pole, such as those used by utility companies for high foliage.

He does prefer to use the rover on a bipod, though, and bubble for control and points requiring very high precision. He sets the data collector software to log positions at 5 Hz or 10 Hz (standard in most systems) and has the software average multiple positions over the course of a minute or more.

Smith said that tilt has made a lot of difference in their surveys, especially where they want a lot of productivity, but do not need very high precision. Part of why he is impressed with the GS18 T is that they had upgraded from an older system, that only used two constellations, to full constellation support on their new rover.

Stability through motion

It sounds counter to one of the key principles for surveying measurement: the instrument and pole must be kept very still. However, in other data collection technologies, including aerial mapping and mobile mapping, leveraging predictable motion, acceleration, and trajectory caught on decades ago. There are numerous integrated GNSS + IMU solutions from, for instance, Applanix and NovAtel, that are the key positioning components for kinematic mapping systems. Such integrated sensor solutions are also in broad use now for UAV real-time and post-processing workflows.

One challenge for integration into survey rovers, was miniaturization. Additionally, such solutions needed a wealth of satellites and signals to be usable at tilted angles. The Galileo and BeiDou constellations reached full complement at about the same time as no-calibration tilt was introduced. Some manufacturers even integrated new antenna designs to better utilize satellites at tilted angles, for example in the Leica GS18 T.

The electronic bubble aspect of such solutions was in some ways the easiest to achieve. Depending on the quality of components, multiple tilt sensors can measure the angle of tilt at precisions matching, or even bettering that of typical pole bubbles. Plus, they are built into the rover boards with a direct relationship to the axis and phase center, whereas the bubble is external, down on the pole.

Integrated IMUs, with as many as nine axes, are highly sensitive, as are accelerometers (if an integration utilizes those). Skeptics always point out that IMUs are subject to drifting over time. However, the observed high-rate GNSS positions and motion sensors are continuously updating the calibration of the IMU. It is true of no-calibration tilt systems that if you hold the pole still too long, it will lose its calibration. Or if you move it too fast. Though on every tilt rover I’ve tried, I moved it around vigorously and spun it (more than would happen in normal operations) and it still kept its calibration. There can also be instances of environments with excessive multipath hazards — such as heavily wooded areas, urban canyons, or congested construction sites — where users often find it best to turn off the tilt for certain shots.

Industry penetration

While manufacturers have approached the GNSS/IMU solution in varied ways, the fundamentals are the same. Once some of the major vendors developed and integrated tilt, they began offering this feature for OEM customers, and in recent years we see tilt on many other brands worldwide. There are relatively new players in the market that took a different approach, developing not only their own GNSS boards and positioning engines, but IMU solutions as well.

Tersus GNSS has rapidly gained a presence in various global markets, though relatively new in North America. While starting as an OEM, the company pivoted to developing its own boards in 2015, and GNSS + IMU integrations more recently. It recently published a paper about what they call its “Extreme RTK Solution” that has a section with data from their own tests for both plumbed and tilted observations.

I did some quick tests with a Tersus Oscar Ultimate, at various angles of tilt, and in mixed environments. The results aligned closely with those in the paper, as did tests with other tilt rovers. I have had the opportunity to try rovers of several different brands, to check precision at various tilt angles against points established with static observations (to see how much the tilt added to the total error). While these were not comprehensive tests, I did compare notes with surveyors who did their own tests, and we’ve all been finding out the practical limits of tilt. Perhaps part of why tilt took a while to catch on was that surveyors needed some time with these units in real-world environments, to get a feel for sweet spots for tilt for different tasks that have specified error budgets.

To get an idea of potential productivity gains, I did a small topographic survey of an area I had previously surveyed with a conventional rover, total station, and scanner. The total station and tilt-less GNSS took about the same amount of time — but with tilt it took about half the time. Many variables can and do come into play, but the figure I keep hearing of up to 30% efficiency gain for many applications seems realistic. Certainly, for asset and resource mapping, tilt could easily fit the looser precision requirements.

As for degradation at various angles of tilt, checks against static points (beyond standard GNSS error) showed negligible differences under 5°, 1 cm up to 15°, 2 cm or more around 45°, and 3 cm or more at 60°. This was just a cursory look, and indeed any surveyors that use tilt should do their own testing. I did notice in the data that when doing simple topo shots, just moving around the site, the pole did not often exceed 5°. Therefore, moving around quickly and efficiently for topo, not having to look at the bubble, improves productivity without significantly compromising quality.
Layout, as surveyors and construction folks who use tilt say, can be quite a snap compared to the old “plumb-shoot, move-plumb-shoot, move-again-plumb-shoot, etc.,” process. You simply move the tip of the pole around until you are on the point.

Enabling further sensor integration

No-calibration tilt, and multi-constellation GNSS, have enabled further developments that may not have been practical otherwise. Leica has since added an image point extraction feature to its GS18I. This marries the tilt and a camera with a clear path to processing the images in the data collector software. With tilt running, and at ranges under 6 meters, you can roughly aim the camera side of the rover toward say, features under an overhang, that you would not otherwise be able to shoot with a GNSS alone. You walk past the features as the camera takes a series of images. Then, in the software, you identify the points in multiple images and photogrammetrically it gives the offset. You can also process the image series into point clouds. There were several attempts at this sort of solution in the past by various brands. However, without tilt it was too cumbersome as you would need to stop and plum for each image in the series. As one user told me, the new image point features are “like having a UAV on a pole.”

Tersus GNSS has taken a slightly different approach to their image point solution. You pick the point you desire in the camera view on the data collector, and then move along as the software automatically identifies the same point in subsequent frames, until it has enough matches from multiple angles to calculate the offset. CHC Navigation has just announced its own image point feature, with a two-camera integration in the i93 Visual GNSS RTK rover.

Pole tilt also has been integrated into non-GNSS solutions. For instance, the recent release of a prism-pole tilt solution by Leica, the AP20. A constant stream of positions of the prism, from the total station, takes the place of GNSS in this application. They’ve also included a rather clever automated pole-height feature.

What could be next? Perhaps small solid state lidars on rovers, or combined lidar/camera solutions such as on an iPhone/iPad, or a tiny SLAM scanner (that could also aid in position stabilization)? Not to mention what might be coming in the not-too-distant future in the realm of quantum sensing.

In considering all feedback about no-calibration tilt, it seems it is very much here and here to stay. There are many who love it and try to use it for everything (perhaps, in some cases, too many tasks). For others, it is conditional love: use where appropriate. While others still hate it immediately, perhaps on principle, though I find those folks typically have never tried it. Legacy tools and methods provide comfort and known levels of risk. New features such as tilt, provided some time is spent gauging its performance and appropriateness for various tasks, can deliver productivity gains that should prompt reevaluation of some long-held assumptions.