Newsletter Article

Holding Level in SA

Many metrology applications require equipment, machines, structures, and foundations to be both flat and level. One example is aligning carts and rails that are used to transport heavy materials.  The ability to accurately measure gravity and keep track of level throughout repeated instrument moves and part alignment operations can be a critical component in a measurement job’s success. Understanding the underlying principles and tools in SpatialAnalyzer can help users utilize SA for many level measurement applications.

Holding Level in SA

 

Many metrology applications require equipment, machines, structures, and foundations to be both flat and level. One example is aligning carts and rails that are used to transport heavy materials.  The ability to accurately measure gravity and keep track of level throughout repeated instrument moves and part alignment operations can be a critical component in a measurement job’s success.  Understanding the underlying principles and tools in SpatialAnalyzer can help users utilize SA for many level measurement applications.

 

Flatness vs Level to Gravity
It is important to understand the fundamental difference between the terms “flat” (or planar) and “level” with respect to gravity.  

 

Flat refers to a planar property and measurements of flatness, parallelism, and perpendicularity can be accurately determined and perfectly described in a Cartesian coordinate system.  

 

Level, on the other hand, is a measure of orientation relative to the earth’s gravitational pull; it is a directional measurement that depends upon the location on earth where the measurement was taken.  If a measurement job encompasses a small area such as a shop floor, multiple measurements of gravity will appear nearly planar.  For this reason, it is often helpful to use your instruments to determine a single level reference at a particular location, or an average of several level measurements, and then consistently reference this level plane throughout the job.

 

Compensated vs Uncompensated Measurements
Another point to consider is that some measurement devices have built-in level measurement capability, while only a subset of those actively compensate measurement back to level.  For example, theodolites and total stations typically level compensate measurements, while laser trackers can have levels but they don’t compensate the measurement back to a level plane.  There are several instruments that offer options to do both; so it’s important to know when to choose the option that is most appropriate for your application.

 

Level compensators automatically adjust measurements relative to the gravity vector.  They always have the gravity vector as the zenith, which makes the horizontal plane level to gravity.  This happens in the instrument before the data is sent to SA.  Most laser trackers don’t use or have a level compensator, so they always report data relative to the standing axis of the instrument.  In most cases you can measure the difference between the standing axis and the gravity vector.  SA represents that as a Frame where the Z-axis is aligned with the gravity vector.  When you make the frame the working frame in SA, measurements are reported relative to a level plane.  

 

The difference between level compensated and standing axis instrument models can be seen in SA as shown in the figure below.  

 

The instrument model above on the left is using level compensation, so the data is automatically aligned to gravity.  The instrument model above on the right isn’t using level compensation, so its data is shown relative to the standing axis.  Note that the points are the same, but the instrument models are in different orientations.  This difference is often very slight, but can be clearly shown in the graphic above since the angular difference is exaggerated by 20⁰.

 

Gravity Measurements in SA
A level measurement is inherently a planar measurement where the plane’s normal vector is aligned to the gravity vector.  When measuring level with an instrument (without a level compensator), SA creates a frame at the instrument’s origin where the Z-axis is aligned with gravity.  This measurement is made relative to the instrument standing axis.  When the measured level frame is made the Working Frame, the station’s measurements are reported relative to the level plane centered at the station.  

SA automatically associates the level frame to the instrument model.  That association means when you locate the instrument relative to the object being measured, the measured level frame automatically moves with the instrument.

 

Performing Alignments while holding Level in SA

 

 

Examples of How to Hold Level in SA:

 

 


 

 

The instrument’s level can be held in this way for most alignment operations in SA, such as relationship fitting and even USMN.

 

Questions?  Do you have any hints or tips of your own that you’d like to share?  Contact NRK at support@kinematics.com.

 

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