A recent thread spurred a side conversation regarding Tilt Compensation for pole tilt for a GNSS receiver. I'm most familiar with the Javad GNSS tilt compensation capabilities, limitations, and processes, but I'd invite any others with knowledge of other systems to feel free to add their views as well.
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The tilt compensation for the Triumph-LS and other Javad systems incorporates a combination of two types of sensors: accelerometers and a magnetometer. The accelerometers determine tilt using the force due to gravity (acceleration) as measured in three different axes. The tilt then is known for the receiver, but to find the tilt direction relative to the user's coordinate system, the orientation of the receiver relative to the Earth must be known. This is done using a magnetometer to determine magnetic North. The compass direction is further refined using the Worldwide Magnetic Model to compensate for magnetic declination. With this known, the tilt of the receiver can be determined relative to North. One thing we observed early on in testing was that the tilt sensors may not necessarily be precisely placed perpendicular to the receiver housing, or perhaps the threads may not be precisely perpendicular to the receiver, or perhaps the pole threads may not be precisely in line with the pole. These potential misalignments are very small in my experience and are typically less than 1?ø but must be accounted for by comparing the computed tilt from the accelerometers to a well calibrated bubble vial on a pole. Once this calibration is made, the tilt compensation is usually good for less than 0.2?ø, which at normal pole heights is less than 0.02' or sub-centimeter. The accuracy of the tilt may degrade as the magnitude of the tilt increases. In the field, the main concern for the operator when using a properly calibrated tilt sensor is potential magnetic interference. If the direction of the tilt is not known accurately, then the correction will be wrong and likely produce even more error. So taking a leaning shot next to a vehicle, pipe post, metal building, etc. can be problematic.
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As I understand, the Leica GS18T uses a different method to determine the direction of the tilt: gyroscopes. The gyroscopes are zeroed by comparing its measured acceleration to the movement of the receiver (when fixed) as determined by RTK. The direction is then carried forward by the gyros based on previous headings from the RTK which is impervious to magnetic interference, since a magnetometer is not used for determining orientation. The downside to gyros is that they drift over time. As more time passes from the last calibration to the RTK heading, the less accurate the gyro determined orientation becomes. From the product videos I've seen from Leica, they've limited the application of the gyro heading to 60 seconds, after which a tilt measurement cannot be made. In open environments that require a tilt measurement due to obstruction, this limitation may not be a problem, however, in difficult canopy, this approach could be an issue. The receiver requires an RTK fix and motion to calibrate the gyros and, once on point may must be measured in less than 60 seconds. It's difficult to maintain a fix under canopy already, but to do so while in motion for the gyros to zero may be impossible. Furthermore, if the calibration were somehow valid, it may be that the receiver cannot fix before the calibration timer expires.?ÿ
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Perhaps the best approach would be some hybrid that relies on a combination of both methods, and perhaps Leica does this already. For the surveyor it is important to know the capabilities and limitations of the technology we use. I like using tilt compensation, but I only turn it on when I need it and I usually have some sort of method for checking the results after the fact. If I can use a conventional level bubble I will leave the tilt compensation turned off. One last addendum: for tilt compensation to properly work, the pole height must be known as the pole tip is calculated based on the tilt values, the orientation of the receiver and the length of the pole from the receiver.
Shawn,
The R10 does the same thing, accelerometers to determine tilt and magnetometers to determine direction. There are three calibration procedures that must be carried out in order to perform tilt compensated measurements. From my experience, just about any significant magnetic anomaly or electromagnetic interference will create too much interference to get a compensated shot with reliable results.
However, I do like the e-bubble. If it's calibrated on a good tripod with a well calibrated tribrach it's more accurate than the level vial on the pole, plus I don't have to take my eyes off the collector. Tilt auto measure is a nice feature for topo, especially if you're doing an elevation grid and the feature code isn't changing. With the R10, you don't need to calibrate the magnetometers if all you care about is seeing the e-bubble. I'll set it up on a tripod and tribrach, calibrate the bubble, and then rotate it 180 degrees and check it. usually it comes back into the center but if not you just take out half the error with the leveling screws and recalibrate, one or two iterations of this is always enough in the rare instances that the tribrach level bubble is out.
It sounds as though Leica has the best solution for compensated points, from what I've heard. I've never actually seen it work though.
I also discovered that when you use Integrated Survey (robot and RTK on the same pole) you get the benefit of the R10 e-bubble even on the robotic shots, which I thought was pretty awesome. Put the robot EDM in Tracking and hit Store as soon as the bubble goes green.
Do the Javad and Trimble ones not use Inclinometers? Like in a total station?
Accelerometers sounds the same as IMU to me.
@gschrock
Interesting that the receiver doesn't need be fixed. Perhaps they are using displacement from the GNSS (which doesn't require a fixed position) but provides fairly precise XYZ displacements over time.?ÿ But it apparently does need to be moved during the measurement (should the measurement require very much time).
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The tilt sensors in a total station are much different from the tilt sensors used in the Javad and Trimble systems which are more akin to what is found in a cell phone. The total station sensors vary but I think typically use some sort of liquid that settles in a pool that is normal to gravity with some sort of laser that bounces off the surface onto some sort of sensor array. But I don't know much more about it (and may not even know what I just said LOL). They're pretty limited in range, I think, so it probably would not be a good application for a pole sensor.
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Tilt auto measure is a nice feature for topo, especially if you're doing an elevation grid and the feature code isn't changing.?ÿ
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This is my favorite use of tilt compensation as well. Shots taken about as fast as you can walk with no need to touch the keyboard.?ÿ
Plumb bob.
Box tape
Suunto compass, (calibrated, btw) and, offset away from magnetic stuff. It still works!
yes, tilt compensation doesn't need to be fixed because it's relying on the rover's velocity which comes from gnss signal tracking algorithm like doppler effect. so it works under single or float when you do tilt compensation initialization.