First available in TPC Desktop 2019
Calibrating an XRef (4:48)
- 0:00 We have a DWG file that is not on the same coordinate system as our survey
- 1:07 Create a calibration with point pairs
- 2:50 Enter information about the calibration for future reference
- 3:30 Save the calibration to a file
- 4:22 Reuse the calibration any time
A calibration is a classic 2D or 3D coordinate transformation which rotates, translates and scales positions in one coordinate system to match coordinates in another. It allows you to transform coordinates from one coordinate system to another.
- Calibrate data from a CAD drawing in inches to your survey in feet, or metric to imperial or grid to ground
- Automatically calibrate geodetic points onto your local site (ground distances at 5000/5000)
- Scale, rotate and translate assumed field data coordinates to match your survey’s control points
- Reference an imported raster image to your survey as a background image
- Scale a detail in a drawing
Calibration TypesCalibration Types
TPC supports 2D/3D Conformal, 2D/3D Affine and other classic coordinate transformations.
You’ll also find translate only, scale only and scale and rotate only.
Depending on what you are calibrating, TPC will either select the appropriate calibration type for you, or allow you to select it. You change the calibration type in the Calibration Results dialog.
Reusing Existing Calibrations
Once a calibration has been created, it can be reused any number of times simply by recalling its .cal file.
Operations that use calibrations provide a calibration browse field. Just select the file folder icon in the control and TPC will list the calibrations that are available. You can also browse to other locations, like a shared calibration folder, and use any of those existing calibrations also.
Calibration ResultsCalibration Results
A calibration is a set of parameters that can transform a coordinate from one coordinate system to another. TPC calibration results list these parameters, along with the computed rotation, scale factor, reference variance,
Callibration PointsCalibration Points
Calibrations are computed from point pairs that relate uncalibrated points to their calibrated or target positions.
These are the control points, in the calibrated CRS, that you used to compute the calibration parameters.
Each control point has an uncalibrated point associated with it. In the CAL file, these are the From points. If you have 5 Control points, you will have at least 5 From points. The first Control point pairs with the first From point and so on.
Additional Un-calibrated Points
A calibration can also include additional uncalibrated points. These additional points do not have a corresponding Control point. However, when the calibration is computed, TPC also computes the calibrated positions and standard errors of these points. You can review these computed standard errors to determine if the error associated with extrapolating the calibration too far outside of the control point area is acceptable or not.
Calibration PropertiesCalibration Properties
Each calibration holds information about itself, in addition to its coordinate transformation parameters used in calibration computations.
This information is not dependent on the calibration type, but instead, helps to identify the calibration for future reference.
Calibration AssociationsCalibration Associations
Calibrations can be associated with one or more specific files. So if you import data from a CAD file and calibrate it, importing from that CAD file again can automatically apply that calibration.
Calibration FilesCalibration Files
TPC stores information about a calibration in a .cal file.
When you calibrate a photo to your survey, pick as many point pairs as you can find on the photo, then let TPC’s Least Squares calibration routine create the best fit for you.
TPC even generates a world file for the photo, so that when you insert it into the next drawing, it comes in at its calibrated position – automatically.
How many times have to gotten a CAD file that didn’t match your survey?
- Like the one state plane survey you just sent off to the lighting engineers, only to get the design back at local coordinates 0,0?
- Like the architect who uses 4 feet per inch
- Or like the drawing shown below, where the XRef is close, but not quite.
- It happens all the time.
We inserted this XRef into a drawing only to find that it doesn’t quite fit our survey. So we calibrate it. We show the first two point pairs here, so you can see how we snap to the same end of line in the XRef, then the drawing.
Calibrate Xref First Two Point Pairs
A few more clicks for a total of four point pairs, or as many as you prefer, and the XRef is calibrated to our survey. And because TPC uses a Least Squares best-fit routine to compute the calibration, you can look at the residuals to see how good the fit really is.
And one you’ve established the relationship between a CAD file and your survey, you can share data between between them using that calibration. See Calibration Associations.
Apply a reverse calibration when writing a new CAD file and it ends up in CAD coordinate system – automatically. See Reverse Calibrate.
- calibrate your base station’s Here position and RTK points with published state or county control
- calibrate GNSS session positions with local site coordinates
- typical of CORS / OPUS data that is post processed
Of course you can have TPC generate State Plane or UTM coordinates based on a Coordinate Reference System, but what about working with a local site survey. You know, where everything is in ground distance at 5000/5000?
With calibrations, it’s easy.
- Calibrate GNSS to local site
- Plot/use GNSS data in your survey’s local site coordinate system
- Reverse calibrate to send local site data out to GNSS for stakeout
- Happens automatically with new Site Calibration
In the example shown here, we occupied each monument with GNSS (400, 401, etc) then inserted the local site point (400:1, 401:1, etc) as a side shot. That all TPC needed to create the calibration. Of course there are other ways to use traverses to generate a calibration. Just choose the one that works for you.
Once we apply the calibration, either directly to the GNSS points or via a Site Calibration, the GNSS points get calibrated to their Local Site coordinates.
It’s all pretty seamless and easy to use.
Now you can calibrate total station data based on assumed coordinates and rock it right onto your survey.
Starting with assumed coordinates
How many times have you started a survey using assumed coordinate system in the field?
In the example shown here, we just assumed coordinates for our occupied point and backsight. So as you can see, it’s close, but not quite.
So now we want to rock it onto the two lots shown in white.
It’s easy to do because in the field, we tied a number of the existing lot corners that we found. This gives TPC everything it needs to easily create a calibration using Least Squares best fit for all the tied lot corners.
Sending data back to your data collector
The choice is yours.
- Send a new job back to the data collector using the survey’s coordinates
- Or reverse calibrate to send any survey points back to the data collector in its assumed coordinate system for stakeout
It’s really the best of both worlds.
As you will see in this video of a project from one of our users in Vermont, when you have a mix of survey objects and drawn objects that need to be calibrated, it adds one simple step to the process. This is because of the underlying difference in the object types. The beauty of it is that you simply apply the calibration you created and used for the survey data to the drawn objects. As you’ll see, it is really easy!
TPC’s Site Calibration is pretty amazing.
Use any existing calibration as the Site Calibration or create a new one, and TPC will automatically calibrate GNSS data to your survey.
GNSS points always retain their original geodetic positions, but take on calibrated coordinates, so they plot right over the top of your survey and work for all COGO computations like offsets and inverses.
In the example here, we identified some lot corners in Google Earth, imported them into TPC, created a calibration to tie Google Earth to our survey then selected it as our Site Calibration.
Now we can work seamlessly between TPC and Google earth to
- Plot your local site data directly in Google Earth
- Create points and figures in Google Earth and plot/use them in your local site survey
Survey CalibrationsSurvey Calibrations
A survey can use any number of calibrations. Once you use a calibration, it is automatically added to the survey and indexed.
As you create calibrations in a survey, TPC adds them to the survey, giving them a calibration index for that survey. TPC may also prompt you to save the calibration to the survey’s file folder (default).
Survey (TRV) Files
When you save a survey (create or update it’s .trv file), TPC writes abbreviated information about each calibration used in the survey to that file.
This provides information about each calibration for future reference and to help manage the calibration index stored with the survey’s data and drawing objects. If you move a survey file to another folder, this calibration information is still available to the survey, so the integrity of the calibrations is maintained.
Managing a Survey’s Calibrations
In this dialog, you can
- review the calibrations that have been used by a survey,
- identify any calibrations that are no longer used (no survey objects are currently calibrated by it)
- remove any calibrations that are not currently in use
- edit the properties of any calibration
Removing a Calibration
Calibrations can only be removed from a survey if they are no longer being used.
Removing a calibration from the survey does not delete its calibration file. As a result, even though you have removed a calibration from the survey, as long as you haven’t also deleted or moved its calibration file, you can still reuse the calibration anytime in the future.
When you calibrate data, TPC looks to see if a particular file is involved. If so, TPC stores a reference to that file with the calibration.
This is very handy when calibrating CAD objects. Once TPC knows what calibration to use, it can continue to use that calibration as you interact with that CAD file.
When you send or transfer data between the CAD View and the Drawing View, TPC uses the calibration referenced by the CAD file to automatically calibrate data between the two.
Calibrations can also be applied in reverse, called reverse calibrating.
You can edit any existing calibration, any time. See What is a Calibration above.
If you change any of the calibration’s properties, TPC prompts you to save the calibration. You can use the existing calibration file and overwrite it or save it using a new calibration file name.
If you modify the calibration type so that it recomputes new calibration parameters and you have already use that calibration to calibrate survey data, TPC will ask if you want to re-calibrate that calibrated survey data. See Re-Calibrating.
Recalibrating happens to calibrated data when the calibration parameters are change.
For example, if you created a calibration with its computed scale factor and calibrated some survey data, then modified the calibration parameters to hold the scale factor to 1.0, TPC would prompt you to recalibrate those calibrated objects.
When objects are recalibrated, they are first uncalibrated using their original calibration parameters . Then they are recalibrated using the new calibration parameters.
When you create a calibration, you can also create and save a .cal file for it in the survey’s folder. This file contains all the information about the calibration. Its points, results, properties, origins, etc.
If desired, this file can be shared with other surveys. Either share the file from a common folder or copy to file to the survey’s folder. Either way, it can be selected like any other existing calibration from the survey.