On Sept. 16, the National Geodetic Survey (NGS) released the latest beta version of OPUS, called Beta OPUS Projects 5.0. This version of OPUS now accepts real-time kinematic data and post-processed GNSS vectors from vendor software. See the box titled “Beta OPUS Projects 5.0 Webpage” on the website.
As stated in the announcement, NGS has developed a file format for submitted real-time kinematic (RTK) data and post-processed GNSS vectors from vendor software to NGS. It is denoted as GNSS Vector Exchange Format (GVX). This format enables NGS to incorporate the data into its GNSS processing routines.
This is similar to the original Receiver Independent Exchange Format (RINEX) developed for making post-processing more efficient when combining GNSS data from manufacturers outputting raw GPS data in varying file formats. In my opinion, this is a significant improvement to NGS’s OPUS web utility.
Beta OPUS Projects 5.0 Webpage
Image: NGS website
Users can obtain background information about the GVX file format by clicking the link GVX file format. More detailed information about the GVX format can be obtained by clicking on the Documentation link.
GNSS Vector Exchange File Format Webpage
Image: NGS website
Basically, GVX is a standardized format for exchanging GNSS vectors derived from GNSS survey data using any manufacturer hardware and software results (see the box titled “Excerpt from Documentation of GVX”). NGS designed the format so that it included all of the necessary data (including metadata) of a GNSS vector for incorporation into a survey network for performing a least-squares adjustment.
Excerpt from Documentation of GVX(Link to PDF of GVX Documentation) To this end, this document proposes a new standardized file format known as the GNSS Vector Exchange Format (GVX). GVX aims to provide a standard format for exchanging GNSS vectors derived from varying GNSS survey methods and manufacturer hardware. The file format includes all of the necessary data of a GNSS vector for inclusion in a survey network for least squares adjustment, as well as metadata which describes the vector. The format is meant for any type of GNSS vector, whether it was derived in real-time or from baseline post-processing. GVX has been written in extensible markup language (XML). XML was chosen because it was designed to carry and store data in plain text format, it is easy to expand and/or upgrade to new operating systems, and it can be read by both humans and machines. |
A sample GVX file can be obtained by clicking on the link titled “Example of GVX file, project day 066, day 052, day 053, day 054.” As NGS states in the documentation, the output can be read both by humans and machines. What’s important is that it can be read by machines so the information can be incorporated into software programs. GNSS vendors have all the information they need to generate the output file to enable users to import the data into OPUS Project 5.0. Users will have to contact their software providers to determine whether their software routines generate the GVX output files.
Example of GVX file, project day 066
Image: NGS website
As I previously mentioned, this new option in OPUS Projects 5.0 is a significant improvement because many surveyors use RTK networks to obtain coordinates of marks. It will also facilitate the occupation of benchmarks with GNSS equipment to support the NGS 2022 Transformation tool. North Carolina, my home state, has a real-time network (RTN) that includes 96 GNSS CORS. (See the box titled “NC GNSS CORS and Real-Time Network.”) Currently, the North Carolina GNSS CORS and RTN has 4584 RTN service subscriptions.
NC GNSS CORS and Real-Time Network
Image: North Carolina Geodetic Survey Website
I could not find a current list of public RTK networks in the United States, but I did locate a Jan. 7, 2014, GPS World article by Eric Gakstatter that provided a list of public RTK base stations in the country. It’s not up-to-date, but it highlights that, more than seven years ago, more than half of the U.S. states had some kind of public RTK network. I would like to update the table, so I’d appreciate receiving information on the status of any public RTK network. Please feel free to send me an email at geospatialsolutionsbydbz@gmail.com.
List of Public RTK Base Stations in 2014Based on GPS World 2014 Article by Eric Gakstatter Note: States not listed did not have a public RTK network.
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Why do I believe that this new option in OPUS Projects 5.0 is so important? Because it facilitates the incorporation of accurate GNSS-derived ellipsoid and orthometric heights into the National Spatial Reference System (NSRS). With the development of improved algorithms, the results of coordinates computed using GNSS CORS/RTNs are more accurate today than ever before. During the last decade, there have been many studies analyzing GNSS data to estimate the accuracy values of coordinates from RTN data.
A study titled “Accuracy of GNSS Observations from Three Real-Time Networks in Maryland, USA” by Daniel Gillins, Jacob Heck, Galen Scott, Kevin Jordan and Ryan Hippenstiel presented at FIG Working Week 2019 in Hanoi, Vietnam, April 22–26, 2019, provided a comparative evaluation on the accuracy of three independent RTNs constructed with differing hardware and software. Their study was based on 486, 5-minute duration GPS + GLONASS network RTK (NRTK) observations. The results indicated that repeat NRTK vectors could be combined to meet 1 cm horizontally and 2 cm vertically (ellipsoid height) accuracies at 95%. confidence. See the box below. It should be noted that the repeat observations should be observed at different times of the day (for instance, separated by > 2–3 hours), as well as, in my opinion, if possible at least more than two different days.
Conclusions from Daniel Gillins, et al. 2019 FIG PaperA total of 486, 5-min duration, GPS+GLONASS NRTK observations were collected on nine bench marks distributed over a 4,000 square km area with rovers connected to three different RTNs in Maryland. Each RTN was developed with equipment and software from a different manufacturer, yet all three RTNs performed similarly in terms of accuracy. When differenced with coordinates from a static GNSS survey campaign, the horizontal and vertical RMSE of the NRTK-derived coordinates was 2.3 cm horizontally and 4.5 cm vertically at 95% confidence. Repetitive NRTK vectors on each baseline differed between ± 2.4 cm horizontally and ± 3.4 cm vertically at 95% confidence. As a final accuracy evaluation, hybrid survey networks consisting of repeat NRTK vectors and baseline solutions from post-processing static GPS data collected at RTN base stations and CORSs were adjusted by least squares. Prior to adjustment, the VCV matrices of the vectors were scaled by variance-component estimation. Adjustment of hybrid survey networks with four repeat NRTK vectors per bench mark produced network accuracies at 95% confidence for the adjusted coordinates at all bench marks less than 1 cm horizontally and 2 cm vertically (ellipsoid height). In addition to the benefits of using efficient and accurate NRTK vectors, the hybrid survey network approach makes use of redundant vectors for checking data and avoiding blunders. The approach also provides traceability because the NRTK vectors are tied to an RTN base station which is tied to CORS. Finally, these networks ensure the survey is referenced to the published coordinates of the CORSs which are held as constraints in the adjustment. |
Lastly, I would like to remind users that only three months remain until the December 31, 2021, cutoff to submit GPS on Benchmarks data that NGS can guarantee will be analyzed to compute the initial set of 2020.0 Reference Epoch Coordinates (RECs) that will be released with the Modernized NSRS. This initial set of RECs is currently the only set that NGS can guarantee will be used to build the 2022 Transformation Tool. Once the transformation model is finalized, the NAVD 88 – NAPGD 2022 transformation values will be locked in and will not be updated as additional sets of RECs are computed. If you have questions or concerns about this cut-off date, please contact your NGS Regional Geodetic Advisor, or drop NGS a line at ngs.gpsonbm@noaa.gov.
Beta OPUS Project 5.0 is a web-based tool that makes it easier to submit data to NGS. I would encourage NSRS users to occupy as many benchmarks with GNSS equipment and submit the data to NGS before the Dec. 31 deadline. Not only will these data help in improving the transformation model, but the marks will be included in the first computation of Reference Epoch Coordinates (RECs). You can obtain information about Reference Epoch Coordinates in NGS’s NOAA Technical Report NOS NGS 67 publication titled “Blueprint for the Modernized NSRS, Part 3: Working in the Modernized NSRS.” A future column will address the different types of coordinates that will be distributed by NGS with the modernized NSRS.