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State Plane Coordinate System of 2022  

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Shawn Billings
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With the new geometric datum of 2022 coming, the need for new State Plane Coordinate System definitions is also on the horizon. Every surveyor who uses State Plane should get involved in this process. While it is still almost four years away, the changes are both technical, political and legislative, each requiring time to maneuver. 

 

Each State Plane Zone will be redefined for SPCS2022. The projection types used will be the same as used in SPCS83: Lambert Conformal Conic, Transverse Mercator, and Oblique Mercator. The Lambert Conformal Conic will be a single parallel rather than two parallel definition. From a functional standpoint, there is no difference between them. The single parallel is simpler to define, but can still be a secant (cutting into the ellipsoid) system if desired by making the k0 (the scale factor at origin) less than 1.

 

SPCS27 zones were designed such that the grid factor would not exceed 1:10,000 (or 100 ppm). In the SPCS83 most zones inherited the same feature, although many States opted to reduce the number of zones which caused the zones to cover a larger geographic area with the consequence of higher grid distortion. It should be noted that this 1:10,000 distortion is only for the difference between ellipsoid distance and grid distance. Typically we do not work at either the ellipsoid or the grid, we work at the topographic surface (ground). The new zones will be designed to minimize linear distortion (the difference between grid and ground). There will still be distortion and for most, the distortion will need to be accounted for by users, but it should be less than it currently is. 

 

The NGS is also allowing "stake-holders" within a State to decide if they would prefer something other than a slightly enhanced version of the current SPCS, namely low distortion projections (LDP). Stake-holders are the State DOTs, State societies of surveyors and engineers, State-level GIS professionals. The stake-holders must unanimously decide to implement something other than the current SPCS or the NGS will implement the default slightly enhanced SPCS we currently have. 

 

In an NGS webinar by Michael Dennis on April 12, 2018, I was surprised when a poll was taken by participants of the webinar that asked what approach to the coordinate systems they preferred. If I recalled correctly, only about 25% of participants were interested in a low distortion projection approach. 

 

In my opinion we stand at an historic opportunity. We all know the difficulties that come with using the State Plane Coordinate System as it is currently defined. All projections have linear distortion. The SPCS often involves linear distortion that cannot be ignored. The remedies for this difference between a grid distance and a ground distance are seldom handled properly and require some overhead. 

 

Low distortion projections are designed such that the linear distortion between a grid inverse distance and a horizontal surface distance are within +/-25ppm and usually much better. This translates to a difference of only 0.025' in 1000'. In many cases this difference can be ignored. In cases in which this difference cannot be ignored, the same process for dealing with grid and ground distances can still be applied to LDP zones.

 

The greatest argument against the use of LDP's has been that the parameters for homegrown LDP's are not published. If the next State Plane System zone definitions are LDP's this becomes moot as the definitions will be codified. Survey software developers will have these definitions pre-loaded in their software. Likely, these LDP's, when properly designed, will cover several counties and follow county boundaries just like SPCS does now. If you know what county you are working in, you will know what LDP to select (just like you do now). 

 

I've been engaged with my State society for several months now regarding this and hope to encourage members to advocate for LDP for 2022. I encourage you to do the same in your State. If you are a surveyor in Texas, I'd love to hear from you, whether you agree with me or not.

 

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R.J. Schneider
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'If you are a surveyor in Texas, I'd love to hear from you, whether you agree with me or not.'

 

No .. just the field crew. Why are we doing this again ? Is this in regards to the tectonic plate shift those people on the left coast have to compensate for ? and why did we pick the planar instead of secant model ? 

If this does go through, is it possible TPTB include a wholly recognizable new false N & E so we don't have to play a bunch of buckethead coordinate games with downloaded coordinate files ?

 Honestly Respectfully,

~the field crew~

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Shawn Billings
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Posted by: R.J. Schneider

'If you are a surveyor in Texas, I'd love to hear from you, whether you agree with me or not.'

 

No .. just the field crew. Why are we doing this again ? Is this in regards to the tectonic plate shift those people on the left coast have to compensate for ? and why did we pick the planar instead of secant model ? 

If this does go through, is it possible TPTB include a wholly recognizable new false N & E so we don't have to play a bunch of buckethead coordinate games with downloaded coordinate files ?

 Honestly Respectfully,

~the field crew~

The reason, as I understand it, is so that the new datum will at a particular epoch be identical to ITRFxx. Currently, in 3D, there is about a 2 meter shift between NAD83 and ITRF08. This is largely unnoticed when using differential GNSS positioning, but gets to be more problematic with correction services. WAAS for example is based on ITRF. In its early days the difference was within the noise of the position accuracy, but as improvements have been made, users of the WAAS should account for the transformation to get the most out of the correction.

 

I am sure that if the SPCS zones remain the same (not replaced by LDP) that the grid origin will be assigned a new value just as was done between SPCS27 and SPCS83.

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Gene Kooper
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Posted by: Shawn Billings

SPCS27 zones were designed such that the grid factor would not exceed 1:10,000 (or 100 ppm).

I do not ever recall seeing this as a design factor in the NAD27 SPC definitions.  Not that I doubt you, but can you provide a reference?

What I do recall is that for the UTM system, the scale factor of the Central Meridian was chosen to be 0.9996 to keep the grid distortion below 1:10,000 within each zone.

 

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Glenn Borkenhagen
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Posted by: Gene Kooper

I do not ever recall seeing this as a design factor in the NAD27 SPC definitions.  Not that I doubt you, but can you provide a reference?

Check NOAA Manual NOS NGS 5 State Plane Coordinate System of 1983 by James E. Stem, a PDF of which is available for download from the NGS web site, specifically the pages numbered 1 and 2 on the bottom.

GB

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MightyMoe
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I recall it being .9994 for my WYEC (TM) or something like that. I can look it up, but then I'd have to look at that blue book again.

 

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JaRo
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I want two things.

1) Grid North is still Grid North

2) None of the coordinates go over 9,999,999.99

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Shawn Billings
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Posted by: JaRo

I want two things.

1) Grid North is still Grid North

2) None of the coordinates go over 9,999,999.99

Both are technically possible, James, but would add a significant additional effort to the design of the LDP (at least regarding Grid North).  I've heard this request before and the reason given was so that coordinates from SPCS83 could be translated without the need for rotation. Is this your reasoning as well?

 

What many of the State-wide LDP designers are doing is giving a grid origin (i.e. False N, False E) value that identifies the particular zone the coordinate belongs to. So if you have 20 zones for a State, the Northing of Zone 1 would be in the 1,000,000's, and the Northing of Zone 13 would be in the 13,000,000's.

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