Modernization of the National Spatial Reference System (NSRS) is driving the adoption of the State Plane Coordinate System of 2022 (SPCS2022). This is big news for surveyors because 48 states already have a codified SPCS.
Since the days of Ptolemy nearly 2,000 years ago, how to represent the round Earth on a flat surface while minimizing the inevitable distortions in the shape and size of areas has been a perennial challenge for geodesists, surveyors, and cartographers. The short answer, of course, is that the best way depends on the intended uses of a map or chart.
In the 1930s, to support surveying, engineering, and mapping activities in the United States, the National Geodetic Survey (NGS) created the State Plane Coordinate System (SPCS), which achieves a high level of accuracy by using relatively small zones. It is used mostly for land surveying, engineering, and local mapping.
SPCS was first conceived to provide a way to perform geodetic surveys using plane trigonometry and as a practical way to access the National Spatial Reference System (NSRS). While computers have made that original purpose moot, SPCS use has grown due to the widespread adoption of computer aided drafting and design (CADD), geographic information systems (GIS), and global navigation satellite systems (GNSS).
For more than 200 years, NGS and its predecessor agencies, in collaboration with public and private organizations, have established survey marks at precisely determined locations across the United States. There are now more than 1.5 million of them. Additionally, in recent decades, NGS has helped to develop a network of continuously operating reference stations (CORSs) consisting of highly accurate GNSS receivers placed in very accurately surveyed locations. On this basis, NGS defines, maintains, and provides access to the NSRS, a consistent coordinate system that defines latitude, longitude, height, scale, gravity, orientation, and shoreline throughout the United States and includes:
- geodetic positional coordinates
- acceleration of gravity
- deflection of the vertical
- the official national shoreline
- GNSS orbits
- the information needed to describe how these values change over time
- models, tools, and guidelines
In the NSRS, the geodetic positional coordinates — which consist of latitude, longitude, and ellipsoid and orthometric heights — are in the official U.S. datums, currently, the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88). NSRS also provides the orientation, scale, and offset information to relate NAD 83 to the International Terrestrial Reference System (ITRS).
It should be noted that while NGS is very much focused on GNSS, a big part of what it does also involves gravity, as well as support for “classical” (optical) methods, especially leveling.
“Our goal is to provide a geospatial framework that supports all positioning technologies,” says Michael L. Dennis, Ph.D., RLS, PE, a geodesist and NGS’ SPCS2022 project manager.
Modernizing NSRS and Launching SPCS 2022
NGS is currently working to modernize the NSRS and will soon replace NAD 83 and NAVD 88 with the 2022 Terrestrial Reference Frames (2022 TRFs) and its State Plane Coordinate System of 1983 (SPCS 83) with the State Plane Coordinate System of 2022 (SPCS2022).
“We did something similar when we switched from NAD 27 to NAD 83,” says Dennis. “At the minimum, it helps avoid confusion by generating projected coordinates that differ substantially (by many miles) between the versions of state plane referenced to different datums/reference frames.”
In SPCS2022, most states will have multiple layers. Some states are leveraging this concept to best serve their needs. An increasing number use Low Distortion Projections (LDPs), for example Oregon. LDPs are not part of the existing SPCS 83, but they will be included in SPCS2022 for at least two dozen states.
“I expect having NGS-sanctioned LDPs could be very popular across the country,” says Scott P. Martin, assistant division chief of the Division of Right of Way & Land Surveys and chief of the Office of Land Surveys of the California Department of Transportation..
Additionally, to provide national uniformity in the measurement of length, as of December 31, 2022, NGS and the National Institute of Standards and Technology (NIST) will deprecate use of the U.S. survey foot. Beginning on January 1, 2023, the U.S. survey foot will be superseded in all applications by the “international foot” definition (i.e., one foot = 0.3048 meter exactly), which is currently used throughout the United States for a large majority of applications and is typically referred to as simply the “foot.”
What Will Change
The major difference between SPCS 83 and SPCS2022 is that the zones in the latter will be defined to minimize map projection linear distortion at the topographic surface, rather than at the ellipsoid surface. This reduces linear distortion, i.e., the difference between “grid” and “ground,” Dennis explains. “People work on the ground, not on the ellipsoid, so why would you not design for the best performance at the ground surface?” Other differences include the use of zone “layers.”
SPCS2022, by definition, computes coordinates from 2022 TRF latitude and longitude values. “Although the algorithms could compute coordinates from any latitude and longitude, the idea here is that when you see SPCS2022 coordinates, you know that they are based on the 2022 TRFs,” Dennis says. “The same is true of SPCS 83, which is based on NAD 83 everywhere, and SPCS 27, which is based on NAD 27 in most areas, except for various island datums. There is always a specific datum/reference frame associated with a projected coordinate system (PCS). That’s part of what makes it a coordinate system.”
“SPCS2022 will be released at the same time as the 2022 TRFs and geopotential datum, likely in 2024 or 2025,” Dennis predicts. However, he expects NGS and state stakeholders to have all preliminary zones designed by mid-2021. Then, after a brief public comment period, NGS will finalize the designs by late 2021 or early 2022 and distribute them to the public, enabling software companies and others to get the definitions into their systems in time for the official release.
“Although the zone designs will be available,” he cautions, “they should only be used with the 2022 TRFs, not with NAD 83.”
In SPCS2022 every state and U.S. territory will have a single statewide zone covering the entire state. For 10 states and six territories, this will be their only zone, while all other states will have one or two multiple-zone layers.
“At this point, it looks like 26 states will have one multiple-zone layer, and 12 will have two multiple-zone layers,” Dennis says. “It appears that 29 states will have a multiple-zone layer that consists of LDPs.”
Currently, he points out, Kentucky already has “layered” zones in SPCS 83: a single statewide zone, plus a layer with the two traditional zones. In SPCS2022, the state’s two-zone layer will be replaced with a seven-zone LDP layer.
Why Use NSRS and SPCS
While a boundary survey for a lot can be completely “local” and make no use of the NSRS at all, Dennis says, many other applications require a much larger view.
“For example, most developments must account for flood risk, and the elevations used for that risk assessment are based on the NSRS.” However, even a lot survey should be referenced to the NSRS, he argues. “There really is no reason not to do that, because it is now so easy. It makes the data more useful and valuable and allows use of other publicly available data (such as imagery and digital elevation models).”
The many local projected coordinate systems that exist around the country are almost always based on the NSRS geodetically (for latitude, longitude, and height). Additionally, the NSRS provides the foundation for the datasets used in consumer-grade GPS-based navigation devices.
“GNSS, CADD and GIS completely changed how SPCS is used,” Dennis says.
“Before GNSS, it was difficult to use SPCS, because you had to find and occupy NGS control and bring it to your site with conventional instruments. GNSS has made that much easier to do.”
Additionally, he says, SPCS is perfect as a projection to make the output of GNSS receivers useful and provide digital coordinates well suited for making plans in CADD and maps in GIS. Therefore, these technologies have made SPCS more relevant than ever.
“Use of SPCS for state transportation projects has been the norm for decades,” points out Martin. “Technology has made it easier to use them, in the sense that all our data collection platforms/tools and geospatial software have the intelligence to work in SPCS. In our SPCS training course, we no longer teach the tedious hand computations to go from geodetic to SPCS and back. We focus more on concepts, best practices, and lessons learned to raise awareness of steps in which errors tend to be made.”
Low Distortion Projections
Mathematically, Dennis explains, LDPs and “traditional” zones are the same thing, in that both are designed to reduce the difference between “grid” and “ground,” but the latter typically cover much larger areas, therefore causing more distortion.
“Surveyors get around that,” he says, “by ‘scaling’ State Plane ‘to ground.’ Since they are used to doing that, some do not see a need for LDPs. However, there are other problems with scaling to ground.”
In 1997, Montana surveyor Wallace “Wally” J. Gladstone, LS, PE co-founded a surveying and engineering firm to improve the human and physical infrastructure on reservations. He now works with the Shoshone, Arapaho, Crow, Assiniboine, Sioux, Gros Ventre, Chippewa, Cree, and Blackfeet tribes.
By 2009, Gladstone recalls, GPS and robotic total stations had eliminated the traditional path to surveying, which now required a surveyor to be half geodesist. Creating a new path for future surveyors and enabling more tribal members to survey tribal lands required simplifying and standardizing the survey process. Gladstone and some colleagues discussed this with Montana’s NGS advisor, Curt Smith, who referred them to Dennis. At the time, Dennis had just begun an LDP project with the state of Oregon. He tutored Gladstone and his colleagues about LDPs and they followed his and Oregon’s lead.
Today, Gladstone and other tribal surveyors routinely use tribal LDPs in Montana and Wyoming. Gladstone considers it a major accomplishment that they will be recognized in SPCS2022. The Rocky Mountain Tribes and the Rocky Mountain Tribal Leaders, he points out, lobbied for that, recognizing the benefits it would have for the tribes — including the fact that tribal LDPs will be embedded in major GIS and survey software; NGS datasheets will include tribal LDP coordinates, simplifying survey checks; and the next generation of tribal surveyors will have a formal process, accepted by the NGS, that will not change with the next tribal council.
Ten years ago, Gladstone says, there was tension between LDPs and traditional zones. In 2009 and 2010, he recalls, when he shared with fellow surveyors his dream of doing surveys on the Crow Reservation in one coordinate system with less than 20 ppm distortion between communities separated by up to 50 miles, they dismissed it as impossible. After numerous discussions, the Montana Department of Transportation and the tribes agreed to disagree.
Today, however, this tension has vanished because LDPs exist on the Blackfeet, Fort Belknap, Fort Peck, Crow, and Wind Rivers reservations and are routinely used by the local surveyors because they have simplified and standardized the survey process. These LDPs have been accepted by the European Petroleum Survey Group (EPSG) Geodetic Parameter Registry, which is a commonly used world repository for coordinate system definitions (including projections). Therefore, they are embedded in Esri and AutoCAD software.
They are also recognized by the Montana Association of Registered Land Surveyors, which hosts information on the tribal LDP system on its website and has created a committee to devise more LDPs in Montana to supplement the planned tribal LDPs for SPCS2022. The existing State Plane distortion in Helena, Montana’s capital, is up to 4.5 feet per mile, Gladstone points out, whereas that achieved for tribal communities is typically less than 0.1 feet per mile.
“Using LDPs,” Gladstone says, “surveyors who excel in research, finding evidence, and following in other’s footsteps but who are weak in geodesy can use GPS units with ease and confidence.”
California’s Special Challenges
California faces special surveying challenges because of the state’s northwest movement due to plate tectonics, which requires frequent epoch updates, and the steep rise in elevation from the coast to the Sierra Nevada Mountains.
“We have areas in California that are moving northwesterly at about five centimeters a year,” says Martin. “Those areas have moved well over a foot since the latest NGS 2010.00 epoch. We have a statewide NAD 83 2017.50 epoch, produced by the California Spatial Reference Center, for this reason.”
The large relief across a zone, from the coast across the mountain ranges to the east, Martin points out, induces rapid distortion across relatively short distances.
“This is why NGS proposed using the average topographic surface (elevation) for the projection surface of a zone. However, in California most of our population (and work) is along the coast, at lower elevations, so we requested that they consider using a population weighting algorithm to determine the height of the projection surface, which NGS accommodated.”
CalTrans, California’s department of transportation, ties all its survey control references to the NSRS. However, Martin says, it does all its mapping, design, and construction in SPCS, because it is simpler. “The trade-off is managing the distortion and understanding the relationship between grid and ground coordinates.”
“I see the inclusion of a statewide layer — in addition to our smaller, lower distortion layers — as a great improvement for statewide datasets,” says Martin, “rather than working with zones that were not designed to optimally fit the state or using non-conformal projections.” Surveyors, he cautions, will need to be careful to build the change in the foot definition into their workflows for setting up new projects and bringing legacy data into the new SPCS.
“That, however, is manageable and an overinflated fear, in my opinion. Regardless of what people have said, it will have no negative impact on boundary surveying.”