A level compensation is different from level measurement. Level compensations 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. The Z axes of the tracker then BECOMES the Measured Gravity line when an OTG was performed. All measurements sent to SA after this level compensation is performed have angle correctiosn for gravity.
Note: This is not true for a level frame measurement. Level frame measurements will still return the current gravity direction relative to the uncompensated tracker base.
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 Any time an instrument is moved that level compensation is no longer valid and a new one performed.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:
A level compensation should be performed after each instrument station move - prior to any point measurements.
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.
Leica’s LMF trackers (AT9x0, AT600 and AT500) offer the ability to perform an OTG operation. This can either be performed within tracker pilot or SA.
If you open Tracker Pilot there are two settings of note:
Inclination Monitor. Which allows you to monitor level and works much like a drift check
Inclination to Gravity. This is where an Orient To Gravity option is offered.
To perform an Orient to Gravity (OTG) operation do the following:
Select the Inclination to Gravity option from the Settings.
Ensure the bubble level is within range and then perform a level Measure.
Enable the option to Use Inclination values for the measurements.
The plum bob icon means OTG is enabled. Starting in 2024.1 SA also uses the same icon to help clarify the current status of the instruments Level Compensation.
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.
But only 1 OTG can ever be performed for the same instrument plant or SA Instrument in the Tree bar. For each instrument change position, bubble change (as for each temperature change), perform a jump instrument operation or manually add one .
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.
The current status of level compensation can be verified in the Instrument History as well as the SA Log file. Additionally the status of the level compensation is saved in the measurement info for a point:
The one time that gravity compensation is necessary within SA and the direct level measurement and straighten process is not going to work s is when a large area network is needed and you wish to hold level relative to the curvature of the earth.
If you have a large enough network this becomes quite important because your in a scenario where holding a planar reference is introducing error:
However a spherical comparison is not completely accurate either. The level measurement process is a measurement of the gravitational pull direction. This pull can be quite complex.
For more information see https://en.wikipedia.org/wiki/Gravity_of_Earth.
USMN does have the ability to hold an instruments current standing axis while performing an alignment. To do this two things are necessary:
You need to check the Instrument Frame option in the Instrument Solution Reference Frame section. This tells USMN to use the instruments own frame as the reference frame during the alignment rather than a single central reference.
The Rx and Ry degrees of freedom for each instrument need to be disabled. This ensures that the instruments are not allowed to tilt off their own axes.
But 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 used to 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.
For each instrument station:
Perform a level compensation (OTG or VL). This must be performed first in order to align gravity with the instruments base frame.
Measure all reference points and features of interest from each station.
Perform an initial full 6DoF network alignment to get the instruments correctly located with respect to each other in the job file.
Straighten each instrument with respect to the center of the earth or the local geoid reference direction in its current location. This will tilt each instrument slightly with respect to the current network position.
Edit the USMN network settings:
Ensure that the Instrument Frame selected in the Instrument Solution Reference Frame
Disable Rx and Ry within the Instrument Settings and Apply to All.
Re-Run the Network Solution.
The straighten operation in step 2 above will correctly straighten the orientation of the instrument with respect to its specific location in space. However, when the USMN solution is run the instruments may move to some extent. For this reason the straighten operation may need to be run again and the network solved a couple of times to find the best possible solution.
For this reason an MP driven alignment may be the best solution. An example of this process aligning to an earth sized sphere can be found on our FTP site here: FlatEarthRelatedMP.zip . This MP allows you to build an exaggerated curved earth solution and demonstrate a flat network solution and then see it alignto a curved network solution as shown below.
This process will pull the network into the correct orientation relative to the geoid or gravity reference: