In the field of surveying, accuracy and precision are paramount. One of the key considerations in achieving precise survey measurements is performing GNSS (Global Navigation Satellite System) site calibrations. GNSS site calibrations involve the adjustment of GNSS receiver measurements to account for errors and discrepancies caused by various environmental factors.
This comprehensive guide aims to answer key questions surrounding GNSS site calibrations, providing valuable insights into the importance, methods, and benefits of these calibrations for surveyors and construction professionals. By understanding the principles and best practices of GNSS site calibrations, surveying accuracy can be optimized, leading to more efficient and
reliable results.
1. What is a GNSS Site Calibration and How Does it Work?
A GNSS site calibration involves the process of establishing a relationship between GNSS receivers’ measured coordinates and the desired coordinate system. It aims to associate the Geographic Latitude Longitude and Ellipsoidal Measurements to the Local Coordinate System.
The calibration process involves collecting observations at control points with known coordinates. These observations are then used to compute the differences between the measured and computed coordinates, allowing adjustments to be made to the GNSS receiver’s measurements. This adjustment helps to account for systematic errors and improves the accuracy
of subsequent survey measurements.
2. Do I Need to Calibrate?
The need for GNSS site calibrations depends on various factors, such as the type of coordinate system the project is using. I.e.: UTM Projection or a Local Site.
In some cases, performing a site calibration may not be necessary. Some projects are prepared in a Mapping projection that will require no Calibration when using a Network RTK service such as WizNET which is already integrated into the spatial fabric and broadcasting corrections from a known and integrated position. However, for projects where high accuracy is crucial, such as engineering or construction projects, or projects using a ground-level coordinate system performing a GNSS site calibration is highly recommended to ensure precise results.
Factors to consider when determining the need for calibration include project specifications, coordinate system, required accuracy levels, and whether the user is working with an RTK Network or a local base station set up on an unknown point. Consulting with experienced surveyors or GNSS specialists can help make informed decisions on whether a calibration is necessary.
3. What Results are Considered Good?
The accuracy of GNSS site calibrations is typically assessed based on various metrics, including calibration residuals and scale factor values. Calibration residuals represent the differences between the measured GNSS coordinates and the ground truth coordinates. Lower residuals indicate better calibration results, as they reflect a smaller discrepancy between measured and actual positions.
Similarly, scale factor values are used to assess the accuracy of GNSS calibrations. Scale factors indicate the systematic scaling errors in the measurements. Ideally, the scale factor should be close to 1, when working in a ground-level system or very close to the provided Combined Scale Factor as provided by the designer, Legal Surveyor or determined by Online Services such as NRCAN’s TRX Service indicating minimal scaling errors and high level of accuracy.
It’s important to note that acceptable calibration results may vary depending on the project requirements. For high-precision applications, such as land surveying or construction layout, tight calibration residuals and scale factor values are desired, usually within a few millimetres. For earthworks and other types of work that require less precision 2-3cm residuals are acceptable and will still yield good results.
4. How to Calibrate GNSS Tools Effectively?
Effective calibration of GNSS tools involves following best practices. Here are general steps to perform a GNSS site calibration effectively:
- Identify suitable control points: Select control points with accurately known coordinates that are representative of the desired survey area.
- Each measurement will allow for holding Horizontal and/or Vertical coordinates: Best practice is to use a minimum of 3 points for Horizontal and 1 Vertical control point typically in the middle of your projects vertical range.
- Collect data: Measure and record GNSS observations at the control points using the GNSS receiver. Choose whether to hold H, V or Both.
- Evaluate the results: Assess the residuals and scale factor values to ensure the calibration achieved the desired accuracy levels.
- Adjust which points are used for H, or V to yield the best results.
- Preform QC checks by staking out a control point that is NOT PART of your calibration to prove the results.
Site Wiz Surveys offers support and training to assist in the calibration process, allowing surveyors to optimize accuracy and efficiency in their GNSS measurements. For users who are uncomfortable with this process we offer onsite assistance, or we can preform the calibration for you and provide a field ready GNSS calibration allowing you to plug and play.
In our local market: We also offer remote support for users outside of our local service area via TeamViewer ™ or Anydesk ™ RDP services.
Conclusion
GNSS site calibrations play a crucial role in achieving accurate and reliable survey measurements, as explained in this guide. Leveraging advances in GNSS technology is also an important factor in optimizing the accuracy and efficiency of survey work. Site Wiz has implemented WizNET, an advanced GNSS Network solution that offers wide coverage and fast signal acquisition, ensuring maximum productivity for users. The platform leverages the latest GNSS technology and provides a range of features designed to make surveying easier, faster, and more accurate.
Whether you need to perform a calibration or survey work in challenging environments, WizNET’s advanced features, and comprehensive GNSS signal coverage ensure reliable and accurate results every time.
To learn more about Site Wiz’s WizNET solutions and how they can enhance surveying success, visit WizNET’s webpage and discover the future of surveying and mapping.
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