Level Compensation

Level compensation is different from level measurement. Level compensation changes the base reference for points recorded from an instrument. Much like an ADM compensation or axis compensation, this operation changes the base reference from which measurements are recorded.

Operations such as Leica’s Orient to Gravity (OTG) or API’s Virtual Level operations perform level compensations.

 

There is an important distinction between level compensation and other instrument compensations however, in that a level compensation is only valid for a single instrument plant. Any time an instrument is moved that level compensation is no longer valid and a new one performed.

Its important to note - Comparing measurements before and after a level compensation would be similar to comparing different instruments to each other. This means two things:

  1. A level compensation should be performed after each instrument station move - prior to any point measurements.

  2. If measurements have already been taken at an instrument plant, a jump instrument operation must be performed prior to performing a level compensation and taking additional measurements.

 

Additional Notes on Leica’s Orient to Gravity (OTG) Operation

Leica’s AT9x0, AT600 and AT500 trackers offer the ability to perform an OTG operation. This is performed within tracker pilot not SA. If you open tracker pilot there are two settings of note:

 

To perform an Orient to Gravity (OTG) operation do the following:

  1. Select the Inclination to Gravity option from the Settings.

  2. Ensure the bubble level is within range and then perform a level Measure.

  3. Enable the option to Use Inclination values for the measurements. option transforms the measurements sent to SA based on the level measurement taken in tracker pilot. This means OTG is enabled. When unchecked its not. So this icon is the key

 

The advantage to this approach is that the instruments base frame is now the level reference and you can directly look at the instruments transform to see its relative position in the job file.

 

Active Compensation

Some instruments, including most total stations and the Leica AT40x trackers, have active compensators. These devices are actively adjusting the measurements using the gravity sensor which ensures that the measurements are always gravity compensated as long as the compensator is enabled. Orient to gravity operations are not necessary for these instruments because compensation is part of the measurement process.

However, if the compensator is turned off for any reason the same rules apply and a jump instrument and re-alignment operation should be performed if any prior measurements were taken.

 

When to Perform a Level Compensation

Level compensation is not necessary for most applications. Even when you intend to hold level with respect to a local gravity plane, all that is really needed is a gravity measurement and to straighten the instrument model with respect to the measurement. This takes a record of the current gravity orientation at that location and, by straightening the station, aligns the instrument to the gravity plane. A network alignment can then be performed while holding level.

The one time that gravity compensation is necessary within SA is when a large area network is needed and you wish to hold level relative to the curvature of the earth. Depending on the scale, the direction of the gravitational pull can be quite complex (Figure 11-57). For more information see https://en.wikipedia.org/wiki/Gravity_of_Earth.

 

SA’s large network alignment solutions such as USMN do not offer an option to hold a non-planar reference during the network solution. For this to happen we would need to both keep track of the instruments to use and the corresponding gravity measurements to using constraining the solution. That’s a lot keep track of. In that scenario a level compensation such as OTG or VL is necessary. It aligns the instruments base frame with the gravity measurement. Once a level compensation has been made USMN does offer the ability to perform an alignment while holding the instruments base frames level. This sequence then allows you to hold a local level for each instrument during the alignment process.

 

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We disagree with the process that is presented in page 346. For the second point 2 , you request to straighten the instruments with respect to a common working frame and then NoRy NoRy . This is not correct ; you are constraining all instruments normal to a common plane and certainly not to follow earth curvature.

We have a similar approach where we constraint all instruments to be normal to a sphere ( our representation of geoid) and then NoRx NoRy in their respective instrument frame . This gives us very good results , that we have recently presented in the international conference for particle accelerator alignment ( IWAA24)

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Therefore the steps to perform a USMN with a large non-planar network would be as follows:

For each instrument station:

  1. Perform a level compensation (OTG or VL). This must be performed first in order to align gravity with the instruments base frame.

  2. Measure all reference points and features of interest from this station.

 

Once this is complete you can align the network using USMN and these settings:

  1. Perform an initial network alignment to get the instruments correctly located with respect to each other in the job file.

  2. Straighten the instrument with respect to a common work- ing frame. This ensure that the instrument base frame is aligned with the working frame ( Instrument Station Controls>Straighten> Align Instrument Z Axis with Working Frame).

  3. Select Instrument Frame for the Instrument Solution Reference

  4. Open the Instrument Settings, disable Rx and Ry in the Degrees of Freedom section and press Apply to all.



  1. Solve the network and consider any outliers as normal.