While locating an instrument by fitting to a set of nominal points (see “Best Fit Points to Points”) is not a difficult task, you may find that using “Measure Nominal Points” is preferable for typical in-process alignments.
Measuring nominal points offers some notable benefits over Best-Fit:
Point naming is handled automatically, eliminating the potential for typographical errors in measured point names.
If the instrument is roughly aligned to the points already (for instance, it is being re-fit into the reference system after being bumped), it can easily be automatically pointed at the targets (if the instrument supports pointing). Single points or the en- tire set of reference points can be automatically measured with a single button click.
The quality of the fit is displayed at the time of measurement— therefore any blunders in measurement (resulting in errors in the fit) can be discovered much sooner than if the fit were be- ing performed later in the process.
There are a few things to keep in mind when using this command:
It must be used at the time of measurement. If the measured points already exist, Best-Fit Points to Points should be used.
You have much less control over the fit. You cannot control degrees of freedom (with the exception of scale), weighting, or advanced fit operations—and you cannot remove points from
Alignment & Transformations the fit unless you delete them.
You’re presented with much less detailed information about the fit results.
The best fit by measure nominals dialog is shown below:
Initial Settings .The upper portion of the dialog offers basic configuration controls that should be considered before beginning the measurement process.
Group to Contain Measured Points. This is the name of the group in which you’d like to measure the corresponding points. It is best to type this in before measuring any of the points, be- cause if you change it after measuring any points, the already- measured points will not move into the new group. The Apply button will update the table based on the entered group, and the ellipsis button allows you to select an existing group to use.
Vary Scale. When checked, scale is allowed to float during the fit. The initial, delta, and final scale values are listed next to this checkbox.
Hold Level (Rx Ry off). This option can be if you do not want the locate operation to tilt the instrument. It restricts the degrees of freedom at are used in the alignment and was added for use with gravity compensated instruments in mind.
Closest Point. With this option enabled you still have to measure the first 3 points in order, but after that, you can measure the point in any order and SA will detect which point you measured automatically.
Measurement Controls. On the right-hand side of the dialog are a set of measurement controls used primarily for direct instrument control. Measurements do not need to be triggered from this section if you are measuring directly but they offer greater automation if needed.
Measure Manually. When clicked, the selected target in the list is measured. If the target already exists, an additional observation is added.
Point At. Points the instrument at the selected target and at- tempts to lock on. (Applies only if the instrument supports pointing/target acquisition).
Delete. Deletes the selected target.
Automatic Measurement - Single Point. Attempts to point at, acquire, and measure the selected target.
Automatic Measurement - Multiple Points. Attempts to consecutively point at, acquire, and measure all points in the list.
Tolerance. You can specify a tolerance in this field. Any deviation magnitudes that exceed the tolerance will be highlighted red. Click the Apply button to apply the specified tolerance to the table.
Point List. The point list field is used both to display which points have been measured and to control the measurement sequence. Points do not need to be measured in order. After the first 3 points have been measured, and assuming you have the Closest Point check box enabled, the point associations are detected automatically. However, you can also change this order by selecting a particular point from the list and measuring it directly.
Standard workflow To Locate an Instrument by Measuring Nominals
Ensure the instrument interface is running.
From the menu, select Best-Fit > Measure Nominal Points.
If more than one instrument exists in the file, select the instrument to transform, as prompted.
Select the nominal (reference) points. The Locate Instrument by Measuring Nominals dialog is displayed.
In the Group to Contain Measured Points section, type in a name for the point group in which to place the measured points.
Measure points manually or automatically using the dialog as described above. Once three or more points have been measured, click the Finished -- Locate Instrument button at any time to accept the fit and complete the command.