Drift Check is an essential tool to verify that an instrument has not been bumped or physically moved in any way, ensuring measurement accuracy. It can easily be accessed by directly right-clicking on the instrument you want to check. It is a good practice to perform a drift check at the end of each measurement task, multiple times during a long measurement job, and any time that you suspect the instrument may have moved in any way.
The Drift Check dialog may appear familiar. The group name control and measurement controls are the same as those used in the Measure Nominal Points alignment, and in many ways it’s the same operation. It allows you to select a set of points and measure them in a fast and efficient manner. In fact if you choose to Add a new instrument and align, the Measure Nominal Points dialog will open for you automatically for greater control over the alignment.
However, the advantage of Drift Check is that it provides both a report of the current deviations and a preview of the RMS and Max deviation you would see if you were to re-fit, aligning using a Best-Fit to the nominal points. This is quite helpful because the deviations can be compared, and an informed decision can be made as far as how to proceed.
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.
Two additional controls can be found just below the point list.
Use Closest Point to Manual Measurements 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.
Minimum Point Count. This is an optional field that forces a user to measure a minimum number of points before a transform operation can be computed.
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.
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.
Configure Instrument. The Configure Instrument button can be used with select instruments to directly access the instruments measurement settings. For example, when working with a Leica tracker it will open a dialog that presents selection between Fast, Standard and Precise measurement profiles.
The Point List is interactive. You can select a reference point and use the point at target button to point the instrument at the reference point. Measurements will also be directly associated with the selected point in the list, so measurements can be taken in any desired order.
Best-Fit (Add New Instrument: Transform). Pressing this button will automatically add a new instrument model to the job file, disconnect the interface from the original model, reconnect it to the new model and perform a best-fit using the points measured in the drift check to align the new instrument station.
Best-Fit Scaled (Add New Instrument: Transform and Scale). Pressing this button will also automatically add a new instrument model to the job file, disconnect the interface from the original model, reconnect it to the new model and perform a best-fit using the points measured in the drift check to align the new instrument station. However, it will allow scale to float as part of the best-fit and the new station will be scaled accordingly.
Finish -- Drift Acceptable. Pressing this button will record an event in the tree as a record but will not have any effect on the existing instrument station.
Drift Checks should be performed frequently, and on regular intervals. The frequency will need to increase the tighter the tolerances. Minor shifts in the instrument placement can add up and drift checks are the best way to make sure either: that things are still in a good condition, or to identify that you’re instrument needs to be re-aligned. The sooner you realize an instrument was bumped the sooner this can be fixed.
Get to know your instrument and your working environment to establish an expected threshold for a drift check. Drift checks should be quick an painless but in real life there will always be error. If you have an idea of how accurate your instrument is under your expected working conditions, it can become second nature to accept and continue or add a new instrument and re-align.
You may not want to align to your reference group. It is very important to not create an error stackup situation. If you align one station to the next in sequence, errors can accumulate. You always want to align to your original nominal point set, but when these are imported from a CAD model the errors may be pretty big to start.
Right-click on your live instrument model in the job file and select Drift Check.
Select a Point Group to measure as a reference. This should either be an imported reference group or set of prior measured reference points. ***See Key Points.
Measure each point in the list by selecting and manually measuring it (you can use Point At to find which point is which). If you have a set of targets already in place you can also use the Automatic Measurement option to measure all the points automatically.
Compare the drift results to the expected results if you were to relocate. Choose to Add a New Instrument and Transform if this difference is significantly greater than you would expect with your instrument under your conditions. ***See Key Points.
When you start the job you can measure the CAD points and align, noting the significant errors in alignment.
When you perform drift checks you will want to select the first measured point group so that the errors displayed in the Drift Check are 100% linked to the station placement.
Then when the instrument needs to be re-aligned you will want to again align to the original CAD points to prevent any error stackup. The Add New Instrument and Align option within the Drift Check dialog should only be used when the original measured group is the defining point group for the nominals within the job file.