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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 accuratelymeasure gravity and keep track of level throughout repeated instrument moves and part alignment operationscan be a critical component in a measurement jobs success. Understanding the underlying principles and tools inSpatialAnalyzer 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 withrespect to gravity.

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

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

Compensated vs Uncompensated Measurements Another point to consider is that some measurement devices have builtin level measurement capability, whileonly a subset of those actively compensate measurement back to level. For example, theodolites and totalstations typically level compensate measurements, while laser trackers can have levels but they dont compensatethe measurement back to a level plane. There are several instruments that offer options to do both; so itsimportant 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 thegravity vector as the zenith, which makes the horizontal plane level to gravity. This happens in the instrumentbefore the data is sent to SA. Most laser trackers dont use or have a level compensator, so they always reportdata relative to the standing axis of the instrument. In most cases you can measure the difference between thestanding axis and the gravity vector. SA represents that as a Frame where the Zaxis is aligned with the gravityvector. When you make the frame the working frame in SA, measurements are reported relative to a levelplane.

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

The instrument model above on the left is using level compensation, so the data is automatically aligned togravity. The instrument model above on the right isnt using level compensation, so its data is shown relative tothe 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 isexaggerated by 20.

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Gravity Measurements in SA A level measurement is inherently a planar measurement where the planes normal vector is aligned to thegravity vector. When measuring level with an instrument (without a level compensator), SA creates a frame atthe instruments origin where the Zaxis is aligned with gravity. This measurement is made relative to theinstrument standing axis. When the measured level frame is made the Working Frame, the stationsmeasurements 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 locatethe instrument relative to the object being measured, the measured level frame automatically moves with theinstrument.

Performing Alignments while holding Level in SA

Construct a Level Frame on the Object using Frame Wizard Constructing a Level Frame on the objectbeing measured is a common technique in leveling applications. That is easy to do using the measured levelframe. Use it to set/control the constructed frames primary axis with SAs Frame Wizard. When theconstructed frame is made the working frame, data is reported level and relative to the object (e.g., machine,foundation, and structures). The process steps are detailed below:

Measure a set of points on key features of the object.

Use the Instrument Interfaces function: Devices >>Level >>Measure. A level frame is constructedat the origin of the station.

Start SAs Frame Wizard: Construct>>Frame>>Frame Wizard.Select the ZAxis of the Measured Level Frame as the primary axis control.

Select key feature points on the object to define either the X or Y axis direction.

Select a key feature on the object as the Frames Origin.

Name the Frame.

Make the new Leveled frame the Working Frame.

Measure at least three (preferably six or more) easy to see target locations to use to align/locate newstations to the gravity reference.

Aligning New Stations to the Gravity Reference Level FrameWhen a new station is added to a job and itdoesnt have a level compensator, it needs to be aligned to gravity reference or Level plane. There are anumber of techniques to get it gravity aligned.

Locate the station using at least three or more points that are known relative to the gravityreference.

Measure a Level Frame from the new station and do a Locate>> Frame to Frame using a knownLevel Frame as the Reference and the newly measured level frame.

Use a Relationship Fit to locate the station relative to level points and features on the object.

Transformation Degrees of Freedom After a station is set up and leveled in SA, it has to stay leveled. Thedegrees of freedom control section in SAs transform dialogs provide the tools to hold level during alignments.They work by restricting motion relative to the working frame, not the individual instruments frame.Therefore, to prevent rotation of the instrument relative to its own gravity measurement (Rx and Ry), youmust turn off Rx and Ry when solving for the transformation to prevent the instrument from rotating off itsgravity reference. Otherwise, the rotational axis of the instrument will change and the level constraints willbe lost.

Straightening Instrumentsit is important to align an instrument with the current level working frame toensure that correct degrees of freedom are constrained during the alignment. In SA, this can be accomplishedusing either one of the Instrument>Straighten commands. There are two separate commands depending onwhether or not your instrument has an active compensator.

Instrument>Straighten (make Zaxis align with working frame)this provides a quick way to orienta compensated instrument or the instrument model with the working frame.

Instrument>Straighten (align level frame Zaxis with working frame)This command was recentlyadded to provide an easy to way to use the level measurement taken by an uncompensated instrumentand rotate the instrument so that its level measurement is aligned with the working frame (not themodel frame). This command can operate on any frame associated with an instrument (by usingInstrument> Object Associations> Associate Objects with Instrument). This can be used to align astation(s) to an averaged level frame, if needed.

Examples of How to Hold Level in SA:

Checking the position of your measurements directly: The biggest advantage to having the levelmeasurement frame tied to the instrument is that it moves with the instrument station as the instrument ismoved in the job file. Multiple instruments can be moved, adjusted, and aligned as necessary without thethreat of losing the level measurement relative to the measured points. At any time, a level frame from anyinstrument can be set as the working frame and in doing so, the X, Y, Z position of all points or objectsmeasured by that station in the job will be updated to report relative to this new frame. With the level frameas the working frame, the Z heights of points or objects can be directly compared to determine if they arelevel relative to each other.

Comparing measurements to a level plane: It is often required to compare a set of measurements directly toa level plane to determine if the part is level and determine the individual measurement deviations from thislevel plane. To do this, follow the simple procedure below.

1. Measure your part and take a level measurement (Devices>Level>Measure in the instrument interface ifyou dont have a compensated instrument) or simply build the instrument model frame usingConstruct>Frame>On Instrument>Base if the instrument is compensated.

2. Make this level frame the working frame in the job.3. Fit a plane to the points and built its centroid cardinal point.4. Use Construct>Plane>Normal to Object through Point (selecting the Level Frame measurement and the

center point of the fit plane for its location). This will build a level plane at the centroid ofmeasurements for your reference.

5. Use Query>Points to Objects toselect the measurements and the level plane to build a vector groupdisplaying the point deviations from level.

BestFitting and comparing measurements to a nominal data set while holding level: Another commonscenario is to compare measured points directly to nominal points while holding the instruments level, orleveled BestFit. A simple method to accomplish this is below.

1. Import the nominal points into the job file. Data is imported relative to the working frame, so be sure toset the frame representing Level in your job (such as world) as the working frame before bringing in thepoints.

2. Measure the nominal points and take a level measurement (if you dont have a compensated instrument)or just build the instrument model frame using Construct>Frame>On Instrument>Base.

3. Use the appropriate Instrument> Straighten command to align the instruments gravity measurement withthe working frame.

4. Rightclick on the instrument and select Locate>BestFit, then turn off Rx and Ry in the Degrees ofFreedom section when you perform the fit.

The instruments level can be held in this way for most alignment operations in SA, such as relationship fittingand even USMN.

Questions? Do you have any hints or tips of your own that youd like to share? Contact NRK atsupport@kinematics.com.

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