Working With Level Within SA

The following steps apply to a Laser Tracker or similar instrument that does not have a Level Compensator or that level compensator is not enabled. For details on Level Compensation go Here.

 

The steps to working with level within SA are as follows: 

  1. Connect to the Instrument

  2. Adjust the instrument such that the level measurement is within range:

  3. Measure level

  4. Set the Level frame as Working... or straighten the instrument in the job file.

 

Basic Level Comparisons

An additional advantage of measuring level as a frame is that performing level comparisons from a single instrument station can be quite easy to perform.

To compare the relative elevation of measurements with respect to gravity you can measure level and then set the level measurement frame as the working frame in the job file. When you do so, the coordinates of all measurements are reported with respect to this gravity frame. The Z component values will then correspond to the level plane displacement and can be directly compared.

 

Instrument Level Frame vs Base Frame

Measurements are recorded from an instrument with respect to the “Base Coordinate” of the instrument. This “Base” does not refer to the attachment to the tripod, but instead refers to the centroid or origin of the measurement process. This location is typically at the center of rotation of the encoders as shown here:

A level measurement is recorded relative to the instruments base frame just like all point measurements. A measurement is taken and then a frame is created at the same origin point but tilted such that the Z-axis aligns with gravity. The image below depicts the resulting level measurement frame had a measurement been taken in the same orientation (level measurements are typically only possible when an instrument is nearly level, so this is for illustration only).

 

Alignments with Level

Its often necessary to align multiple instruments as part of a job that includes level measurements. There are two ways to handle this sort of scenario.

  1. You can perform a network alignment as usual. Level measurements are tied to the instrument that measured them and will move with the instrument during an alignment. This means that they will correctly indicate the gravity direction after an alignment is complete. A comparison of level measurement between stations can then be an indicator if there is any issue with the alignment and can be averaged to give you a best indication of gravity for the job.

  2. You can first straighten your instruments with respect to the working frame and then hold level during an alignment. This option works great in situations where you don’t have enough measurements to constrain the alignment such as measuring in a tight hallway.


In order to hold level during an alignment an instrument must first be straightened so that its level measurement frame is aligned with the working gravity frame. This is accomplished by using the Instrument Station Controls>Straighten commands.

 

 

Checking Level Status

It can be helpful to double check that your instrument is still correctly oriented. To do so, select the level measurement frame measured by the instrument in the tree and look at its transform in the report bar. Its important to look at the level measurement not the instrument’s transform because your measurement frame is the gravity reference.

When you look at the transform check the Rotation (deg) row and verify that the Rx and Ry values are exactly zero.

The yellow Rx and Ry values indicate that the tracker has been tilted off axis relative to gravity. You will need to use the command Align Level Frame Z-axis with Working Frame. To correct for this.

Note: If your instrument has an active compensator or if you have performed a level compensation OTG or Virtual Level for example, the base frame of the instrument can be used as a gravity reference. In that case go Here.

 

Flat Earth Alignments - Holding a level plane

Holding Level - Degrees of Freedom

In order to hold level during an alignment process, you need to restrict the degrees of freedom. Some alignments such as Measure Nominal Points includes a “Hole Level (Rx Ry off )” check box. What this does is turn off the Rx and Ry portions of the full degree of freedom selection offered in alignments such as Best-Fit which offers a more comprehensive set of controls.

 

A similar set of controls can be found for each instrument within USMN as well for larger networks. The alignment process therefore would go like this:

For each instrument station:

  1. Perform a level measurement. This must be performed first in order to align gravity with the instruments base frame.

  2. Straighten the instrument to the World frame or job frame using ( Instrument Station Controls>Straighten> Align Instrument Z Axis with Working Frame).

  3. 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. Select Working Frame for the Instrument Solution Reference

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


  1. Solve the network.

 

For a Curved Earth solutions you will need to use a level compensation process.