How do you deal with GPS errors and uncertainties when surveying monument coordinates?

1 Sources of GPS errors

GPS errors can be caused by a range of factors that impact the precision and accuracy of the satellite signals and receiver measurements. Common sources of error include atmospheric conditions, like ionospheric and tropospheric delays, that can cause refraction and distortion of the signals; satellite geometry, such as the number, position, and orbit of the satellites, that can influence signal quality and reliability; multipath effects, like reflection and interference of signals from nearby objects like buildings, trees, and vehicles, that can create false or noisy measurements; and receiver errors, such as clock drift, antenna misalignment, and software glitches, that can introduce bias and noise in the measurements.

2 Types of GPS uncertainties

GPS uncertainties measure the quality and confidence of GPS measurements. These uncertainties can be divided into two main categories: random and systematic. Random uncertainties are caused by random noise and errors, which can be reduced by increasing the sample size, duration, and frequency of measurements, as well as using statistical methods to estimate the mean and standard deviation. Systematic uncertainties are due to consistent or predictable errors, which can be corrected by applying calibration, adjustment, or transformation methods to the measurements and using reference data or models to account for known sources of errors.

3 Methods and standards for GPS uncertainties

Different methods and standards are used for measuring and reporting GPS uncertainties, depending on the purpose and context of the survey. Horizontal and vertical accuracy are measures of how close the GPS measurements are to the true or accepted values, usually expressed as a radius or height interval. Precision and repeatability measure how consistent the GPS measurements are with each other, typically expressed as root mean square error (RMSE) or standard deviation, or as the difference between repeated measurements. An uncertainty ellipse graphically represents the horizontal accuracy and precision of GPS measurements, with a circle or ellipse containing a certain percentage of measurements, showing the direction and magnitude of major and minor axes. The National Spatial Reference System (NSRS) is the official coordinate system and datum for the United States, defining reference frames, models, and standards for geodetic surveys, including GPS surveys. It also provides guidelines for determining and reporting GPS uncertainties in relation to NSRS.

4 Best practices for minimizing GPS errors

To minimize GPS errors and improve the quality of your survey work, it is important to plan your survey carefully, considering the site conditions, objectives, and GPS equipment and settings. Select the optimal time, location, and duration to avoid periods of low satellite availability, high multipath effects, or poor atmospheric conditions. Additionally, use appropriate GPS equipment and settings such as high-quality receivers, antennas, and software. Differential or network GPS techniques such as real-time kinematic (RTK) or post-processing (PP) should be used to enhance accuracy and precision. Furthermore, it is essential to follow proper field procedures and protocols like checking and calibrating equipment, recording and documenting data, verifying and validating results. Control points, benchmarks, or monuments should be used to establish or check coordinates. Quality checks such as redundancy, blunder detection, or error analysis should be used to identify and correct any errors or anomalies in your data.

5 Tips for mitigating GPS errors

Sometimes, GPS errors may be unavoidable or unpredictable in urban areas, dense forests, or near power lines. To mitigate GPS errors and reduce their impact on survey work, it is recommended to use alternative or complementary methods such as conventional surveying techniques, optical or laser instruments, or mapping software. Additionally, interpolation or extrapolation methods like splines, curves, or surfaces can be used to estimate or fill in missing or unreliable GPS measurements. Furthermore, professional judgment and experience can be used to evaluate or modify GPS measurements in order to detect discrepancies and apply necessary corrections.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?