Conventional, Postprocessed, or Real-Time?
This is blog 4 in the series “Real-Time GNSS Corrections: Productivity, Profitability, and Practical Considerations” written by Jason Evans, Portfolio Manager, Trimble Positioning Services and NYS Registered & Licensed Land Surveyor. The blog series discusses important considerations for your GNSS surveying workflows: When should you use a base station? Are you missing the opportunity to gain productivity by including correction services in your workflows? Can differential base-rover RTK benefit from correction services? Reading the full series will not only help you to answer these questions, it will enable you to calculate the cost savings of a real-time GNSS correction subscription.
Selecting the best workflow for site and base coordination based on error budget.
The Known Factor
Tools for surveyors have evolved significantly in the last decade, and while working with well-known familiar tools is often preferred, new techniques can provide significant time and cost savings while also meeting precision and accuracy requirements for many applications.
One of the main considerations in determining whether a conventional optical, postprocessing or real-time workflow is best for a project is the level of confidence required for the accuracy of measurements. This alone could sway a surveyor from adopting a different technology—due to lack of confidence in the measurements or how to use the technologies to improve efficiency and increase accuracy.
The Error Budget
Each project has an error budget that provides insight into the right tool for the job. The level of accuracy required will vary, determining the thresholds and specifications for how rigorously to measure and adjust. For example, the error budget for surveying a property in a dense, urban setting may be much smaller than the error budget for a large farm property due to the required specifications for the survey or, when working on a project to establish first order control, there are specific accuracy requirements that must be met by the surveyor. Some projects may require producing control reports, standards on errors of closure, and confidence of position. But applying these standards to every survey results in more effort than necessary, extra costs, and wasted time.
The following are some examples of desired accuracy, and methods used to achieve these:
A control project that requires vertical accuracy for control of <1 cm 2σ (95% confidence). You may have no other choice than to perform a closed loop with a digital level. For <2 cm 2σ you could have more options, and adjusted traverse with your total station, trig leveling, or rigorously constrained long static GNSS, postprocessed.
For horizontal accuracy of <1 cm horizontal, a closed traverse is standard, but you may be able to meet this requirement with rigorously constrained and execute long static GNSS, postprocessed.
Pre-design topographic surveys and construction projects are examples of where a slightly lower standard of accuracy may be acceptable. For example, when installing a parking lot and lighting, being half an inch off in the wiring is not going to result in a major difference. Whereas half an inch off vertically for a mile of pavement on a roadway project would be extraordinarily expensive because of additional material and labor costs that would be involved to lay more pavement.
If a specific construction element requires <1 cm 2σ, you may be limited to conventional methods with total stations and digital levels.
At <2 cm 2σ, the options grow. You could easily achieve this with static postprocessed, but that is impractical for the collection of the many points needed for a topographic survey or construction stakeout. For topographic and most construction tasks, conventional and RTK have become the go-to, provided you set up your instruments or RTK on rigorously established control. Local real-time networks (RTN) or correction services like Trimble VRS Now, or Precise point positioning (PPP) correction services like Trimble CenterPoint RTX can easily meet these requirements in many situations.
There may be accuracy requirements expressed as 3D values (e.g., <3 cm 3D 2σ) or separate values for horizontal and vertical (e.g., 2 cm H, 4 cm V). While dependent on specifics of the site, environment, and other conditions, if properly executed, conventional, postprocessed, and real-time can all deliver 3 cm 3D with high confidence. Many surveyors triage their options by looking at the most time and cost efficient method, determining if it will meet the requirements (which often involves testing at the site), and if it does not, they move on to the next option.
To recap, based on the error budget, the level of accuracy needed, required setups and reporting will determine the right workflow for the job. With the right workflow, efficiency and profitability are maximized.
Surveyors have more options today than ever before when selecting hardware, software, and correction services for their jobs. One decision that needs to be made is to take either a postprocessed or real-time approach, and the selection that is right for the job should be determined by the error budget and requirements of the job.
Postprocessing data is a well-established method with equipment and services that are tried and true. Measurement accuracy can be proven, meeting the confidence required for static control, design, and topographic and boundary surveys.
Benefits of postprocessing include:
- It is not susceptible to the communication link drops of real-time options
- Validated positions and adjusted field data, constrained to a geodetic datum
- Correlation of biases
- Inclusion of survey control monuments, when required
- The ability to analyze and remove noisy data from observation sets
- Repairing cycle slips and fixing integer ambiguities
- Least-squares (single baseline) and constrained least-squares (network) adjustments
Postprocessing typically comes with increased costs on hardware, software, skill, and labor. By properly setting up and maintaining control points and spending extensive time in the field and the office, this approach provides both proof of confidence in the results and that you have met the error budget.
As we examined in the previous blog Bases, Correction Services, and Control, online postprocessing services provide methods to establish control for setting up a base, new project control, or evaluating the quality of previously established control.
Services provided by federal agencies, like the U.S. National Geodetic Survey (NGS) Online User Positioning Service (OPUS), provide standard reports with quality and accuracy details. There is standard OPUS for observations >2 and <48 hours, and an OPUS-RS for observations >20 and <120 minutes; you must determine which suits their error budget. Many surveyors view services like OPUS as the authoritative answer but fail to look at details like the peak-to-peak values in the extended reports for the few instances where it might not meet your error budget. Still, you must do the same long observations as you would if you did your own postprocessing, but there are alternatives.
Commercial services like Trimble CenterPoint RTX Online Postprocessing can often deliver the same precision as federal services, but with dramatically shorter observation session times. This is possibly due to several factors: improved PPP clock and orbit data and multi-constellation support. Taking this a step further, many regional real-time networks (RTNs), like those using Trimble Pivot software, offer online postprocessing. These leverage improved clock and orbit data to the same kind of baseline processing you would do with your own office software, but the task of processing is greatly automated. The PPP and RTN services provide standard reports with quality and precision data.
Real-time data was developed as a workflow over the past few decades, as this primer on GNSS correction types examines. For many surveyors, the standard real-time approach has been differential kinematic using a base station and rover(s) with radios. More recently, correction services from Real-time Networks (RTNs) and Precise Point Positioning (PPP), using real-time corrections via satellite or internet data link, have grown in popularity.
There remains a high degree of confidence in measurements using real-time corrections, and results are achieved more efficiently than traditional methods, and, when compared to base-rover RTK, eliminate several factors including:
- Need for a base station, receiver, antenna, and radio
- Time spent setting up and breaking down a base station
- Errors in base station setup
- Time spent checking control points to verify base station setup
- Scouting a site and guarding a base station from theft or disturbance
- Radio broadcast license
- Risk of broadcasting UHF signals across international borders
- RTN corrections are not as susceptible to the degradation of solution over distance that long-baseline RTK experiences. Base-rover RTK can yield varied results if you mix baselines of different lengths
- RTN can be better at modeling changing space weather (ionospheric) and tropospheric conditions over wide areas
- Satellite-delivered PPP correctors, can go one-better than RTN, eliminating the required cellular connections
There are both survey field and survey office software platforms, like Trimble Access and Trimble Business Center that seamlessly work together to offer the flexibility of choosing postprocessing or real-time data depending on the job. For example, office software can automatically download data from a continuously operating reference station (CORS) and perform the calculations required to confirm measurements— analyze and edit observations, process baselines, and perform adjustments.
A real-time workflow either using a CORS network or a satellite delivered service is ideal for projects where error budgets need to be met—which has increasingly become the case—affording the ability to save significant time and costs.
The powerful field and office software platforms also enable the import of vector data from real-time GNSS receivers and vectors from total station surveys, combining these in postprocessed network adjustments.
A recent example of the rise in confidence in real-time GNSS correction methods is the development of a standard GNSS vector format (GVX) by the NGS. GVX can be used for submission to the NGS OPUS Projects Beta application for incorporation in Bluebook and GPS-on-Benchmarks processes. The NGS will accept vectors from RTK and RTN surveys from sessions as short as 3 minutes (two 3-minute sessions minimum, separated by at least three hours). This is a huge leap forward from the legacy standard method of observation sessions of 4 hours or more.
Several factors have contributed to this change. One is the proven high quality of RTK/RTN results, and that RTN bases are typically rigorously constrained to fiduciary active control networks (e.g., the nationwide NOAA CORS Network in the U.S. and similar networks in other countries). The reduced observation times are also made possible through the multi-constellation capabilities of RTN and modern base-rover systems.
GVX has been incorporated as a standard output in Trimble Business Center and some other office software; the adoption of GVX and formats for other similar modernization initiatives.
Whether manually processing or using an office software to automatically process, time and expense required should be weighed against the error budget.
The Best Option For The Survey
Unless there are specific legal or contractual requirements precluding the use of either postprocessed or real-time GNSS correction approaches, both can provide the data required by the survey, assuming they meet the error budget.
We should also not overlook the safety advantages for the surveyor, crews, others on the site, including members of the public. The less time you need to spend doing observations in hazardous environments the better.
After reviewing the factors present in a survey project—communication options, site accessibility, accuracy requirements, crew skills, and tools available—it should become clear whether a postprocessing or real-time workflow is the right option. With advances in both real-time GNSS correction services and hardware, it’s important for surveyors to know what’s-what in order to make an informed choice. It is worth the effort to consider all options knowing the benefits afforded by each, for the surveyor and their customer.
In the next and final blog of this series, Site Calibrations, Bases, and Correction Services, we'll look at how site calibrations are an essential element of many surveying projects and how to approach them.