September 12, 2023

A first look at the 2022 Alpha State Plane Coordinate System

Written by: Sam Knight


The United States is quickly approaching the official adoption of a new national reference frame. The National Terrestrial Reference Frame of 2022 (NATRF22) has been long discussed as a part of the National Spatial Reference System (NSRS) Modernization project at the US National Geodetic Survey (NGS). This new datum, or “geometric reference frame” in the more modern geodetics lingo, has had much buzz around the surveying industry. For many, the practical use of NATRF22 will come from the projections used for flattening the earth to paper in the form of an updated State Plane Coordinate System (SPCS). 

What is the State Plane Coordinate System (SPCS)?

The current State Plane Coordinate System is not just one coordinate system; it is a large collection of coordinate systems that create a patchwork coverage over the entire US. The current, coming, and previous generations of SPCS each consist of one “layer” of projections covering the US with a non-overlapping collection of zones, varying from one to a small handful of zones for each state. These are all roughly similar in terms of mapping scales, and a lack of overlap. 

The new zones will exist at multiple scales instead of a single layer. Each state will have a single zone that can represent the entire state, as well as smaller zones that are somewhat analogous to the older zones. A third layer will be available at finer scales in more localized, low-distortion projections. An additional fourth grouping of “special use zones” will span some state lines, such as the area of the Navajo Nation, and the Gulf of Mexico offshore area. 

This release will increase the number of SPCS zones from 125 to 967 unique projected coordinate systems. The first time I saw 967 written down, it took a moment to sink in. There are currently about 6000 total coordinate systems in the Geographic Calculator database, which cover the entire planet. When the SPCS2022 goes live, this will be a massive update. Since the recent alpha release of these zone definitions, we at Blue Marble Geographics have been eagerly digging in to make sure everything is as expected.

Alpha Release of SPCS 2022

What exactly is an Alpha release of a collection of coordinate systems? Just like software releases, an Alpha is a functionally complete package and the first full attempt at what will become the official release. You can think of it as more than a rough draft, as the definitions are mostly complete. At the time of writing this, exactly one coordinate system out of the 967 is fully accepted, and the other 966 remain preliminary. The NGS has stated that they expect only minor changes to any definitions, but there is a fairly significant process needed before they will become official for use. It is important to note that these systems are preliminary only; they are not yet for production use or use in any official capacity. Even though only minor changes are expected, only the SPCS1983 systems are currently valid for official use until the NGS says “go.”

No surprise to US-based mappers, there are three projections involved: Transverse Mercator (TM), Lambert Conformal Conic (LCC), and Oblique Mercator (OM). These are the same projections that were involved in the previous generations of the SPCS. The notable variance is that the Oblique Mercator (sometimes also known as “Hotine Oblique Mercator” or “Rectified Skewed Orthomorphic”), which was used in only a single zone for the Alaskan panhandle, will now see significantly more action in 161 of the new zones. This will be new for many people in populated areas. The rest fall into the more familiar LCC and TM parameters. 

There are two variations within the LCC zones: the 1 Standard Parallel and 2 Standard Parallel versions. Mathematically, they can be used to the same effect, they are simply two ways of defining the orientation of the plane of projection (a cone) to the globe. For publishing the zone definitions, NGS has decided to standardize the the 1 standard parallel version. However, these definitions will be able to be input with alternate parameters into most modern GIS systems as a 2 standard parallel version as well. In modern geodesy, there isn’t much to say about the remaining TM projection, it is about as “plain vanilla” as it gets. If there is a projection that is universally compatible and understood across the industry, it’s Transverse Mercator.

Importing and Testing the New Coordinate Systems

The least exciting part of working with the new systems was the actual import of systems into Geographic Calculator’s database, and that’s a good thing. The NGS currently has a web page with all the parameters of each zone listed in a massive table. They were also kind enough to provide us with the zone parameters as a quickly digestible collection of PRJ files for testing. Importing these files into Geographic Calculator took mere seconds, and there they were 967 new coordinate systems. From there, the process of validating new systems is about like most people imagine. One by one, the definitions are opened, and the parameters are examined to make sure all the digits are where they are supposed to be. From origins to scales to skew parameters for the Obliques, the first sanity check is just making sure all the numbers are there.

One thing stood out when reviewing the new zone parameters: all of the false origins are defined such that they are whole numbers in both Meters and Feet. I know I previously said that exciting is bad in geodetics, but this simple, refreshing detail makes for very graceful numbers, no rounding errors, and no complications for using either of our standard units. The origins in Latitude and Longitude are all represented as whole values to the minute in Degrees Minutes notation, with the exception of county-level zones in Wisconsin where some zones go out to the five-place of decimal seconds. This makes quickly scanning the parameters very easy on the eyes. As an added convenience, the NGS table of parameters lists both +/-180° as well as 0-360° longitudes, as well as, Degrees Minutes, Seconds, and Decimal Degree notation for all values. This change simplifies interactions with the table lookup, whichever standard you happen to employ. 

Testing with Control Points 

Anyone in geodetics will tell you that a test point is critical with any coordinate system. If you’re catching on to the trend here, that’s 967 control points. In high style, NGS also has a dedicated page for those points. One for each zone, and importantly, a point that is not simply the origin of the zone. This table also includes additional information not always found with control points, such as a variety of formats of the latitudes and longitudes, Scale Factors, Convergence Angles, and Combined Factors. Our testing of the systems is ongoing, but the initial results were predictably uneventful. Using the control points, our tests here are matching exactly, which is just what you want in a control point. 

We work with new coordinate systems all the time here at Blue Marble, and rarely is there such comprehensive data to support new standards. The alpha release of SPCS2022 has so far been remarkably easy to work with, and that clearly isn’t an accident. NGS has been preparing for this for years, and as we draw into the end of this stage of the NSRS Modernization project, it’s time to make sure all that hard work is smoothly implemented. As we approach the coming release of NATRF2022 and SPCS2022, the groundwork is well prepared. 

We will not be including the preliminary zone definitions in release versions of our software until the NGS announces that they are ready. It isn’t a stretch to imagine the confusion that could arise from releasing this many new systems and then publishing later changes; it would invite chaos. When they are ready for release, you can rest assured that they will be found in our tools as soon as possible thereafter. With the comprehensive information provided for testing and validation, there is plenty of time for most to work out the kinks of their implementation. If you are looking to do any validation of systems that you might work with, one thing to take a closer look at is how your various software tools handle the Oblique Mercator zones. Historically, there have been several ways to handle the mathematics of the origins. This has largely been worked out across the industry over the years, but if you happen to be in an area that will be using this slightly less common system, it might be worth a look around in the other tools you work with. We were ready to have to examine discrepancies in our own implementations, whether in simple import or in mathematics. So far, we are quite pleased to say that it has been a very smooth experience, and we are looking forward to the full release beyond the alpha.

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