NGS to perform another Multi-Year CORS Solution

The National Geodetic Survey (NGS) is performing another Multi-Year CORS Solution (MYCS) of the National CORS. This will mean the CORS coordinates will be updated to be consistent with the latest International Terrestrial Reference Frame of 2014 (ITRF2014). NGS has provided preliminary information from the new MYCS at this website. It should be noted that these values are considered beta so they are not final. They may change before they are adopted by NGS for publication. This column will provide potential changes in ellipsoid heights based on the updated beta CORS coordinates (downloaded on January 11, 2019).

NGS has a website that describes the CORS coordinates and how they are established. (See box titled “Excerpt from web page https://www.ngs.noaa.gov/CORS/coords.shtml.)

What is a Multi-Year CORS Solution? NGS provides a short explanation on their web page (see box titled “Description of MYCS1.”)

As noted in the explanation, MYCS1 was the first of a series of reprocessing projects that will occur periodically in the coming years. The current CORS coordinates are referenced to IGS08 epoch 2005.00 (see box titled “Description of IGS08 epoch 2005.00 Coordinates”). The updated coordinates will be consistent with the International Terrestrial Reference Frame of 2014. ITRF2014 is the latest frame realization of the International Earth Rotation and Reference Systems Service (see box titled “Description of ITRF2014”). What will be important to users is how much have the coordinates changed due to the reprocessing.

How does this fit into what surveyors use, that is NAD 83 (2011, MA11, PA11)? NGS provides a description of CORS coordinates and NAD83 (2011, MA11, PA11) on their web page (see box titled “Description of NAD83 (2011, MA11, PA11)”). NGS provides CORS coordinates in both IGS08 epoch 2005.00 and NAD83 (2011, MA11, PA11) epoch 2010.00. See box titled “Current CORS Coordinate Listing for a CORS in Monroe, NC (NCMR)” for an example of a CORS in North Carolina. The beta website contains links to the updated CORS coordinates based on ITRF 2014 (see box titled “Updated CORS Coordinate Listing for a CORS in Monroe, NC (NCMR)” – ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/ncmr_14.betacoord.txt).

A March 2017 presentation by NGS employee Kevin Choi does an excellent job of explaining the CORS status and why NGS is reprocessing the CORS to be consistent with the latest ITRF. It is dated because it was presented in March 2017 but the reasons for reprocessing and recommendations are still valid today.

The text box titled “Slides from NGS Presentation, titled ‘CORS, OPUS, and Reprocessing status by Kevin Choi,'” contains four slides from Kevin Choi’s presentation that explains why NGS periodically performs a multi-year reprocessing of the National CORS. As stated in the slides: (1) some CORS coordinates are outside their allowable 2/4 cm (H/V) threshold, (2) some stations that had modeled velocities will now have computed velocities, (3) there are new CORS stations since the last reprocessing, and (4) there is an updated ITRF (ITRF2014 and a corresponding IGS14). What’s really important to the user of CORS is, how much have the coordinated changed and what does it mean to me. This column will focus on changes in ellipsoid heights. I downloaded the CORS coordinate information for both the updated ITRF2014 values (ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.comp.txt and ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.htdp.txt) and the current IGS08 values (ftp://www.ngs.noaa.gov/cors/coord/coord_08/nad83_2011_geo.comp.txt and ftp://www.ngs.noaa.gov/cors/coord/coord_08/nad83_2011_geo.htdp.txt) from NGS’ website. See box titled “Excerpt from ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.comp.txt” for a sample of the contents of the file of the stations with computed velocities and the box titled “Excerpt from ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14/nad83_2011_geo.htdp.txt” for a sample of the contents of the file of the stations with modeled velocities. As previously stated, the ITRF2014 coordinates are “Beta” and can change before being officially published. I generated several plots that depict the difference between the two sets of ellipsoid heights referenced to NAD83 (2011).

The column labeled “Site Status” indicates whether the site is (1) Decommissioned, (2) IGS station and not a National CORS, (3) Non-Operational, and (4) Operational. A summary of the status of the Stations is listed in the text box titled “A Summary of ITFR2014 CORS Stations.”

The file also provides the velocity of the station (modeled or computed) in the north (Vn – units mm/yr), east (Ve – units mm/yr), and up (Vu – units mm/yr) component. The box titled “A Summary of the Velocity Values of ITRF2014 Stations” provides a statistically summary of the velocity components of the stations.

The text box titled, “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS” depicts the differences in NAD83 (2011) ellipsoid heights between all common CORS between the two set of values in conterminous United States. There appears to be several CORS that have very large changes in ellipsoid heights.

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights
and Published CORS – Less Than +/- 1 cm” depicts the differences that are between -1 cm and 1 cm.

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS – Greater Than +/- 1 cm” depicts the differences that are less than – 1cm or greater than 1 cm. As the plots indicate, most of the differences are between +/- 1 cm.

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS – Greater Than +/- 2 cm” depict the differences that are greater than absolute 2 cm (that is, less than – 2 cm and greater than 2 cm). The plot clearly indicates that most of the station coordinates will change less than +/- 2cm.

The text box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in North Carolina” depicts the differences in NAD83 (2011) ellipsoid heights between all common CORS between the two set of values in North Carolina. Most of the differences are less than a centimeter.

The box text titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in North Carolina – Greater Than +/- 5 mm” depict the differences greater than absolute 5 mm. The plot clearly shows that the coordinates of most stations in North Carolina will change less than 5 mm.

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in Alaska” depict the differences in Alaska. Most of these differences are less than a couple of cm but there are a few large differences.

The box titled “Differences between 2014 Reprocessed CORS Ellipsoid Heights and Published CORS in Anchorage, Alaska, Region” depict the differences in the Anchorage, Alaska, region. There are a couple of stations in the Anchorage region that their ellipsoid heights will change over 10 cm.

This column discussed the preliminary results of NGS’ second Multi-Year CORS Beta Solution of the National CORS. The CORS coordinates will be updated to be consistent with the latest International Terrestrial Reference Frame of 2014 (ITRF2014). NGS has provided preliminary information from the new MYCS at the following website: ftp://www.ngs.noaa.gov/cors/coord/beta_coord_14. It was noted that these values are considered “Beta” so they are not final. It was emphasized that they may change before they are adopted by NGS for publication. This column provided potential changes in ellipsoid heights based on the updated beta CORS coordinates (downloaded on January 11, 2019). Future column will provide more details after NGS completes their analysis and adopts the final coordinates for the MYCS.