Instrument Shots
Show Instrument Shots check box works along with the Shots display options in the Users Options. to display lines in the graphics connecting an instrument to the points measured by that instrument (polar instruments only).
A measured point (also referred to as a target) is a point which has been determined from one or more direct instrument observations. For traceability purposes, targets they maintain a record about who and what measured them, when and how they were measured, weather data, sampling statistics, raw measurement data from the instrument, and any other pertinent metadata. Because they are tied to instrument measurements, measured points cannot be created through typical point construction or analysis methods--they must be measured. Measured points are represented in the graphical view by a 3-D plus symbol with a diamond around it (the size of which can be controlled in the User Options Display tab:
When measurements are recorded, SA always records the raw measurement data that came from the instrument. For theodolites, this means a horizontal angle (azimuth) and vertical angle (elevation). For other line-of-sight devices, it means azimuth, elevation, and distance, and for non-spherical measurement devices (such as portable arms) the coordinates are provided from the instrument in Cartesian space.
The instrument always provides the location of the measurement relative to its own internal reference system—typically at the optical center or at the base of an instrument. Since each instrument has a three-dimensional position and orientation in the job file, SA takes these raw measurements and calculates their coordinates in job space based on the position of the instrument. When an instrument moves, the position of all of its measurements are recalculated.
A constructed point is a point that is not associated with any instrument observations or measurements. There are many ways to create constructed points. They may have been created manually by entering coordinates, by intersecting a line with a plane, or may be the result of importing data from an ASCII file. A constructed point may also result from deleting a target’s individual measurement observations. A constructed point is represented in the graphical view and the SA tree by a 3-D “plus” symbol
About Target Observations
An observation is a single reported measurement from an instrument. A target, by definition, must have one or more instrument observations. While the coordinates for a constructed point are easy to understand and unambiguous, the coordinates for a target take more explanation. It is fairly common, particularly with theodolite measurements, for a given target to have multiple observations that together define the coordinate of the point.
It would be easy to simply take an average of each observation to determine the computed target location, but that is not an optimal technique, because it assumes that each observation has equal error. Instead, SA uses a more accurate technique of computing the coordinate for the target based on an optimization.
The closest instrument is most accurate, and gets the most influence in calculating the target location. The net result of this optimization technique is that the computed target location’s coordinates are more accurate than they would be if the average of the observations were used instead.
Different instrument types require different default approaches to shot computation. Within the User Options on the Analysis Tab is a selection for how measured points are computed based on the contributing observations.
Measurement Details
To view the detailed measurement information that comprises a target, open the point's properties and click on the Measurement Details button. (Note that the button will display the number of active observations that are being used to calculate the target position).
In this example , there are two active instrument observations that are being used to calculate the target coordinates. The checkboxes at the left side of the observation controls its status. If an observation is unchecked, it will not be used in the computation of the point location. As shown below, if all of the observation are turned of for a particular point, the shot line will turn red and the point will remain behind should the instrument be moved.
At any time all of the shots can be re-enabled using the Activate All Measurements command.
A slightly different scenario may occur where a point group is moved. In this case the observations on the points are still valid and active. However, because the point coordinates have changed the points will no longer appear to be measured. As long as the observations are still saved within the point they can be re-activated by using the command Re-Compute Targets from Shots.
Several job cleanup tools are also available:
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Remove Measurements by Instrument offers an option to remove the measurements from the selected instrument.
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Remove All Measurements. Provides the ability to remove measurements from all measured points in the job file.
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Remove All Measurements and Associated points. Will effectively delete all measured points, which can be a convenient means to setup a template file.
Additional Instrument Shot Commands
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Hidden Point Bar. Provides a means to build a hidden points from two selected points using the hidden point bar definitions in the User Options.
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Re-Compute Hidden points. Can be used to force a re-computation of the position of points created using the naming hidden point bar naming convention.
Imported points can be linked to an instrument station as measurements by using the Fabricate Measurements command. This will create a simulated observations of the points from the selected instrument, and can be done without starting an instrument in Simulation mode.
Measurement error can be injected to simulate real world conditions. The error is based on the uncertainty model of the selected instrument. The simulation process can also be limited by distance to more closely replicate true instrument operations.