National Spatial Reference System Adds Time to Spatial Measures
At the forefront of the changes in the National Spatial Reference System (NSRS) will be time-dependency, an issue the National Geodetic Survey (NGS) has not yet completely implemented. Beginning in 2022, points in the NSRS with defined coordinates will have epochs associated with them, based upon the time actual data were collected at those points. Such coordinates will be known as “Final Discrete” coordinates (if associated with finite timespans of data collection) or “Final Running” coordinates (if associated with continuous data collection). Consequently, passive control will have less reliability than active control, and NGS will treat the National Oceanic and Atmospheric Administration (NOAA) CORS Network as having the definitive, up-to-date coordinates within the NSRS.
A change of business will result: both leveling and classical surveys will require global navigation satellite system (GNSS) components to ensure coordinates computed in those surveys are up-to-date and are connected to the NSRS through the NOAA CORS Network (Continuously Operating Reference Station Network).
In order to bridge users into a time-dependent NSRS, NGS will also be estimating, and providing to the public, coordinates on points at five-year reference epochs. While such estimates will mimic the current status quo [the 2010.00 epoch of NAD 83(2011), for example], they will not be considered the “definitive” NSRS coordinates. Whereas users will have the option, via an updated Online Positioning User Service (OPUS), to take any campaign survey at any date and adjust their surveys to such reference epochs, NGS will not do this. Rather, if your survey data is submitted to NGS, it will compute Final Discrete coordinates at the epoch of your survey. Then, in the future, those Final Discrete coordinates will be used to estimate Reference Epoch coordinates.
NGS will be providing tools to users, under the catch-all name “OPUS,” for uploading, processing, analyzing, and submitting survey data of all types, such as: GNSS, RTK (Real Time Kinematic), RTN (Real Time Network), leveling, gravity, or classical. Additionally, OPUS will have tools for ingesting and analyzing continuous data (e.g. GNSS, gravity). The tool will be browser- based and will fully integrate all data types, whereby a single project, containing both GNSS and leveling could be uploaded and processed under the same project name.
Users processing their data in OPUS will always receive “Preliminary” coordinates from OPUS. NGS hopes to encourage users to submit that data so that NGS can provide quality control, internal national processing, and creation of Final Discrete coordinates from their data. Only data submitted to NGS will make it into the NSRS database and be processed and redistributed to the public using an updated Data Delivery System, previously known as “datasheets.”
The entire NOAA Technical Report NOS NGS 67 is available at: https://geodesy.noaa.gov/PUBS_LIB/NOAA_TR_NOS_NGS_0067.pdf
NAD 83 and ITRF
When originally conceived, NAD 83 and ITRF were essentially coincident. Over the years NAD 83 remained static while ITRF constantly moved as more information was gained about the center of the earth. The non-geocentric aspect of NAD 83 is not acceptable since we live in a dynamic world, and a static set of geometric coordinates are no longer appropriate. Making this change will remove ambiguities in positioning dependent on the reference frame. In order to make this happen a new terrestrial reference frame will be required. There are additional parameters being added to coordinate calculations for the new terrestrial frames. ITRF2014 will have increased density and the Euler Pole Parameter (EPP) will be included in the calculation. An in-depth discussion of the EPP can be found in Blueprint for 2022 Part 1: Geometric Coordinates.
Moving the reference frame will include a horizontal shift, a rotation, and a tilt which will change the vertical position. This will require a replacement of NAVD88 with a new datum for orthometric heights.
In order to reach the goal of a 1 cm geoid, the National Geodetic Survey instituted the program called GRAV-D. The GRAV-D project has an overall goal of a 2 cm orthometric height which includes 1 cm of uncertainty from GNSS and 1 cm of uncertainty from the geoid model. There are two aspects to the program, one is the airborne gravity survey of the entire country and its holdings and a second aspect is long-term monitoring of any geoid change. The GRAV-D program is progressing on schedule; however, completion is dependent upon funding.
Since 2014, NGS has sponsored annual crowd-sourced data collection campaigns called GPS on Bench Marks (GPSonBM) to help improve the accuracy and geographic coverage of GEOID18. For many of these years, NGS has worked with the National Society of Professional Surveyors (NSPS) to promote participation during National Surveyors Week each March. In 2018 alone, nearly 600 people and agencies from across the continental United States and Puerto Rico submitted over 3,800, 4-hour GNSS observations on about 2,500 bench marks. This additional data has significantly improved the model by closing data gaps and resolving conflicts in older data.
Most of the difference in this hybrid geoid model comes from the GPSonBM data pinning the model to the surface of NAVD 88, but there are also improvements in the underlying gravimetric geoid model. These improvements include:
- Better elevation data and improved digital elevation modeling techniques,
- New gravity data from satellite gravity missions,
- New airborne gravity data from the NGS GRAV-D program, and
- Improved geoid modeling techniques.
These improvements will be realized more directly in the future when NGS switches to a purely gravimetric geoid model with the new datums in 2022.
Blueprint for 2022 Part 3
Part 3 of the Blueprint, BP3, is the part that will mean the most to those who are working with the new datums. This part was presented by Dru Smith, the former Chief Geodesist for the National Geodetic Survey, who is now the NSRS Modernization Manager.
BP3 is critical reading for anyone who will be working with the new datums. Not only are there going to be new datums introduced but there will also be new terminology, new coordinate names, as well as new ways and time periods to process data. (See the Terminology Guide). As an example of the terminology change, ARP (antenna reference point) will still exist but may not necessarily be the point to which the CORS is referenced; that point will be called the GRP (geometric reference point). The ARP and the GRP may coincide, or they may not coincide.
There will also be five types of coordinate names:
- Reported: These are from any source where the coordinate is directly reported to NGS without the data necessary for NGS to replicate the coordinate.
- Preliminary: These are coordinates at survey epoch that have been computed from OPUS, but not yet quality checked and loaded into the National Spatial Reference System Database (NSRS DB).
- Reference Epoch: These are coordinates which have been estimated by NGS, from time-dependent (final discrete and final running) coordinates, and at an Official NSRS Reference Epoch (ONRE).
- Final Discrete: These are coordinates computed by NGS using submitted data and metadata, checked and adjusted and referenced to one survey epoch.
- Final Running: Of all types of coordinates on a mark, these are the only ones which will have a coordinate at any time.
These are just two examples of the types of changes that are coming soon. In addition to the above two examples, there will be a new name for and method of operating in the NOAA CORS Network (NCN), GNSS surveys will be done by “GPS Month”, there will be a new way for users to process GNSS projects and a new way for users to process leveling projects; therefore, it is important that all surveyors familiarize themselves with BP3.
NGS is in the process of building a suite of multi-constellation GNSS capable software to replace PAGES. The new software will accomplish baseline processing and orbit production with a planned release of June 2021.
OPUS uses the IGS08 reference frame and Geoid12B for processing data. Beta OPUS is available on the NGS website and uses the ITRF2014 reference frame and Geoid18. To align NAD83 with the IGS08 11 reference frame, NGS completed a multiyear CORS solution (MYCS1) that incorporated data from 1994 to 2010.
On January 29, 2017 (GPS week 1934), the International GNSS Service (IGS), released the new coordinates and corresponding antenna calibrations in the IGS realization of the ITRF2014 reference frame (hereafter, referred to as ITRF2014). As part of this transition, products in the IGS08 frame are no longer updated. Instead, all the updates will be in the ITRF2014 frame.
Although NGS did not participate in the 2nd IGS reprocessing campaign, they have completed the reprocessing of the CORS stations. The newly reprocessed CORS solution, called the MYCS2, is aligned to the ITRF2014 frame and supersedes the previous reference frame and realization, which was released in 2011 under the name MYCS1. See Multi-Year CORS Solution 2 (MYCS2) Coordinates for additional information.
For additional information, contact the Florida Geospatial Users Group:
The Florida Geospatial Users Group also has a You Tube video available:
NGS will not be a network RTK provider. However, to assist the surveyor NGS has completed studies on incorporating kinematic data from a real-time network into OPUS Projects. This is part of the OPUS for Everything initiative. The errors that were encountered during the studies were greater than what one would expect with a static session but were well within kinematic expectations.
To use real-time data in OPUS Projects one would process the static data in the normal fashion and then upload the RTN vectors prior to running the least squares adjustment. Eventually, data will be allowed into the NGS database using RTN vectors. This will make it much easier and faster to “Bluebook” a GNSS project.
In understanding the full report and throughout the discussion of the issues surrounding the NSRS changes, it is important to define many of the terms that are used, both for consistency and in their context of usage by the geodetic community, not necessarily within their broader usage within the English language. Those definitions follow:
Antenna Reference Point (or ARP): The antenna reference point (ARP) is the point on a GNSS antenna from where antenna calibration values are referenced. The ARP is preferably, but not always, an easily accessible point on the plane that contains the antenna’s lowest non- removable horizontal surface. The ARP could be physically identifiable on that (above- mentioned) surface of the antenna; or it may be the center of a mounting axis, and thus co- planar with that surface, without being on the surface itself.
The ARP can, but is not required to, coincide (in space) with the geometric reference point (GRP) when the antenna is mounted as part of a CORS. For this reason, NGS has for decades erroneously described the coordinates at a CORS as referring to the ARP, and not the GRP, a practice we ceased in 2019. Note that the ARP is a point that is part of an antenna, but it is not a point on a mark. Therefore, a CORS only has an ARP at those times when an antenna is mounted at it, whereas a CORS always has a GRP.
Bluebooking: A phrase used to describe how geodetic survey data were formatted and submitted to NGS using Input Formats and Specifications of the National Geodetic Survey Data Base (FGCS, 2016) so they could be checked and included in the National Geodetic Survey’s Integrated Database (NGS IDB). The term Bluebooking was derived from the original document that had been distributed with a blue cover.
Continuously Operating Reference Station (CORS): A station, composed of a variety of equipment, but usually including at least one mark (containing one geometric reference point, or GRP), as well as a GNSS antenna and receiver, as well as some source of power and communications. The purpose of a CORS is to continuously collect and distribute GNSS data so as to monitor the coordinates of the GRP. The term CORS, however, has grown to acquire a general use worldwide, therefore, there is no guarantee a station being referred to as a CORS is actually part of the NOAA CORS Network (plural: CORSs).
Also referred to as: Continuously Operating GPS Reference Station, Continuously Operating GNSS Reference Station, Active Control Station
Coordinate Function: A set of three piecewise continuous functions (one for each of the X, Y or Z coordinates with respect to time), fit to the daily or weekly coordinates implied by analyzing daily or weekly data collected at a CORS. Serves as the official time-dependent NSRS coordinates of the GRP of each CORS. Specific to CORS only, the coordinte function is identical to Final Running Coordinates.
Geometric Reference Point (or GRP): A unique point that is part of a particular station. The GRP is the point to which any coordinates of the station refer. The operator of each station identifies the GRP of that station. The GRP is sometimes independent of equipment, such as when it is contained within a mark at a CORS (and thus it exists even when the antenna is removed).
In other cases, such as with very long baseline interferometry (VLBI) and satellite laser ranging (SLR), the GRP is a point in space defined by the motion of the telescope, typically the intersection of the azimuth axis with the common perpendicular of the azimuth and elevation axis, and thus it only exists when that particular set of equipment is at that station.
Local Site Survey: A survey—often consisting of GNSS, leveling, and classical observations using survey-grade instruments—at one site. High-precision local tie vectors are determined between the site marker and the geometric reference points of co-located space geodetic technique (SGT) stations on that site so as to contribute to realizations of the International Terrestrial Reference Frame (ITRF).
GPS Month: Four consecutive GPS weeks, with the first week in the GPS month having a GPS week number that is a multiple of four. Thus, GPS month ‘zero’ is the consecutive period spanning GPS weeks zero, one, two, and three; GPS month ‘one’ is the consecutive period spanning GPS weeks four, five, six, and seven, etc.
Mark (or Marker): A physical structure of varying size or construction, attached to Earth’s crust in some way that is presumed to be stable throughout years (or decades) and whose function is to contain a single, unique, identifiable point in a stable location. Such points are often a small divot or cross on the top of the mark (though even the smallest divot is not zero-dimensional, so for highest accuracy, one must clearly identify which part of the divot is the point. For example, the point on the mark might be the bottom of such a conical divot). Common forms of a mark include:
- A metal (often brass or aluminum) disk (often about 3 inches in diameter but varying from 0.5 inches to more than 12) with a stem underneath which keeps it mounted in stone, masonry or concrete.
- A metal rod (usually 1-2 centimeters in diameter) driven into the ground and rounded on the top.
- When NGS refers to the “coordinates of a mark,” we are referring to “the coordinates of the point on the mark.”1
- Also called: Bench Mark, Control Mark(er), Disk, Geodetic Control Mark(er), Monument, Passive Mark(er), Physical Mark(er), Rod, Survey Mark(er)
NGS IDB (or IDB): The National Geodetic Survey Integrated Database. Prior to the modernization of the NSRS, the NGS IDB was the definitive storage place for all NSRS data. Datasheets were generated only from this database. It was “Integrated,” because two separate databases (one for horizontal and one for vertical) were combined into the NGS IDB in the 1990s.
The NOAA CORS Network: The name of the collection of CORSs whose data are collected and processed by the National Geodetic Survey. Note that many other countries and agencies around the world refer to their individual stations as being CORSs. This generic use of the term CORS does not, however, mean their stations are in the NOAA CORS Network.
NSRS Database (or NSRS DB): The official database built to house the modernized NSRS. Some information from the NGS IDB will be converted directly into the NSRS DB. For example, the Permanent Identifier (PID), of a mark. Other information, such as coordinates, will be recomputed from raw measurements using the modernized NSRS as their foundation.
PID: Abbreviation for ‘Permanent Identifier,’ the unique six-character alphanumeric code assigned to each point included in the NGS IDB or NSRS DB and residing on a mark.2
Point: A zero-dimensional location. Two points cannot exist in the same space at the same time. A point might be physically “touchable” (such as the bottom of a small conical divot on top of a mark) or it may not be (such as the location of an airborne gravimeter’s sensor at any given moment during a flight).
Redundancy: Taking the same measurement more than once, where each measurement is taken separately and independently of the other. Strictly speaking, this is impossible, as anything measurable in the universe changes to some degree or another from one moment to the next. However, in the context of this document/discussion, redundancy will generally mean “collecting GNSS data at a point during two different occupations within the same GPS month.”
Site: The smallest civil location name of the area where (one or more) stations are located. (Legal, i.e., recognizable by deed; national- or state-recognized city, town, village, or hamlet; or geographic feature). Multiple stations can be on one site. (Example: “MacDill Air Force Base” is a site, and it happens to contain two stations, which are the CORSs known as MCD5 and MCD6).
Site Mark(er): A single, unique mark, installed one per site. All vectors from the geometric reference points of every station on that site are tied to that single mark within a local site survey. Note that local site surveys often use many marks, and all may be located at a site (for the purpose of redundancy and to provide a backup of the site marker), but only one can be (and must be) designated as the site marker.
Station: A collection of equipment located at one site to collect one specific type of data for a particular geodetic purpose. Within the geodetic community there are many types of stations, and most common are:
- Continuously Operating GNSS Reference Station (CORS)
- Satellite Laser Ranging (SLR) Station
- Very Long Baseline Interferometry (VLBI) Station
- Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) Station
- Continuously Operating Relative Gravimeter Station
- Two or more stations located on the same site may share some pieces of common equipment, but at least one unique thing should distinguish one station from another.
- To that end, NGS plans to change our official policy (from an unofficial practice that has been in place for approximately 10 years) that all surveying to a mark, and all coordinates of a mark, should refer to one uniquely identifiable point on that mark. The policy will be necessary to undo the official policy from the NOAA leveling manual (Schomaker and Berry, 2001) that states, “Place the rod so that the exact center of the base plate rests on the highest point of the turning point or control marker.” Such a practice meant that, on any sort of tilted mark, the “highest point” might not be the same as the point at the center of the disk to which, say, a classical or GNSS survey might refer. Furthermore, as “depth of dimple” becomes an issue (particularly with using pointed fixed-height poles in GNSS surveys), the unique point of any given mark may need to be identified as the bottom of the dimple (or cross mark).
- Recall, points exist in the NSRS DB that are not on marks, such as the points an airborne gravimeter’s sensor may have occupied during a flight. As each mark should hold only one unique point, the PID of a point may equally be considered to be the PID of the mark upon which that point resides.