Last month’s column highlighted GEO-ESCON and how it supported the advancement of the science of geodesy. That said, the National Geodetic Survey (NGS) has been working to improve the National Spatial Reference System (NSRS) by replacing the North American Datum of 1983 (NAD 83) frame and all vertical datums, including the North American Vertical Datum of 1988 (NAVD 88), with four new terrestrial reference frames and a geopotential datum. Many of my previous GPS World columns have addressed various phases of the project.
Recently, NGS has developed an Alpha site to enable users to preview preliminary NSRS products and services. I mentioned the Alpha site in my July column, in which I highlighted NGS’s presentations on the new NSRS at the International 2023 FIG Working Week.
Alpha preliminary products page. (Image: NGS)
The concept of the Alpha site is to provide examples of the content, format, and structure of data and products that NGS plans to release as a part of the modernized NSRS.
NGS highlights that these products are for illustrative purposes only and do not contain any authoritative NGS data or tools. It states that they are under active development and are subject to change without notice.
That said, NGS would like everyone to try the Alpha products and provide feedback to NGS. The first two Alpha products are State Plane Coordinate System of 2022 (SPCS2022) and NGS Coordinate Conversion and Transformation Tool (NCAT). On July 20, NGS held a webinar previewing the Alpha site. Readers can download the powerpoint and video of the presentation here.
Webinar on preview of SPCS2022. (Image: NGS)
As usual, Michael Dennis of NGS did a great job of describing the new SPCS2022, and the differences between the State Plane Coordinate System of 1983 (SPCS83) and SPCS2022. I have included a few of his slides that highlight the SPCS2022.
First, SPCS2022 has significantly more zones than the current SPCS83 zones. Second, SPCS83 map projections were designed to minimize linear distortion at ellipsoid surface, whereas the SPCS2022 map projections were designed to minimize linear distortion at topographic surface. The purpose being to reduce the difference between projected “grid” and “actual” ground distances.
Number of SPCS2022 zones. (Image: NGS)
Linear distortion of SPCS2022. (Image: NGS)
Dennis described NGS’s distortion design performance as seen in the image below. He explained that the performance is a range of +/- distortion for a zone, such as +/-50 ppm. The analysis involved determining parameters where the range includes 90% of the population, 75% of the cities and towns, and 50% of the total area. He highlighted those zones designed by NGS that where typically limited to +/- 50 ppm design criteria, but many low distortion projections (LDP) zones designed by stakeholders consisted of +/- 20 ppm design criteria.
Distortion design performance. (Image: NGS)
Dennis provided a slide depicting SPCS2022 linear distortion for all CONUS zones with a 50 ppm distortion increment as seen below. As indicated on the slide, green is +/- 50 ppm. The distortion performance is +/- 45 ppm.
All CONUS SPCS2022 zone layers. (Image: NGS)
As a comparison to the existing SPCS83 zones, he provided a similar slide for the CONUS SPCS83 zones. See below. As in the previous slide, green represents +/- 50 ppm. The distortion performance is +/- 159 ppm.
All CONUS SPCS83 zone layers. (Image: NGS)
Now, let us look at the Alpha products. First, all zone information can be found here.
SPCS2022 zone information. (Image: NGS)
Users can click on the image below for a table of all zone definitions. The table provides the type of projection, if it was designed by NGS or the state, and the zone definition.
Online interactive table of zone definitions. (Image: NGS)
By clicking on the image below, users can obtain information for a point in a particular zone. The table provides northing and easting (meters and feet), scale factor, linear distortion, and convergence angle for a specific coordinate in a particular zone. It should be noted that all values that are provided in feet will be international feet units (ift).
SPCS2022 example of coordinates and distortion values. (Image: NGS)
The Alpha page provides an online option to look at all maps. The arrow in the image below highlights the link to access the online interactive maps.
Alpha page for SPCS2022. (Image: NGS)
When users click the link on the page, they are directed to an ArcGIS NOAA web map viewer.
Alpha SPCS2022 experience. (Image: NGS)
To access the online map function, users need to click one of the Alpha SPCS2022 zone options.
Alpha web maps. (Image: NGS)
Once users click on one of the web map buttons, another map page with a map icon appears on which users will need to click to get to the map of zones.
Alpha SPCS2022 all zone web map. (Image: NGS)
After users click on the map icon, they will get another web page that contains the map zones based on their selection. In my example, I selected “all zone web map.” Once users get to this page, they can zoom into any area to find a particular zone.
All zone web map. (Image: NGS)
I zoomed down until I located North Carolina’s map zone. The web page provides access to various layers and information. First, if users move their curser over the layer button, a list of layers pops up. Next, select one of the layers, such as Statewide Zones, then the properties of the map are placed on the map. Finally, when readers click on the map itself, the information about the SPCS2022 zone appears on the map.
Alpha North Carolina Statewide Zone web map. (Image: NGS)
North Carolina is a state that elected to have a single statewide zone. Some states decided to design several LDPs that cover certain areas or cover the entire state. Ohio is a state that designed 89 LDPs that cover the entire state. Again, by selecting the layer button, users have an option to select multizone complete zones, the properties appear on the map, and finally clicking on the map provides the zone information for that zone. In this example, I clicked on Columbus, Ohio, which is in the Ohio Franklin Zone.
Alpha Ohio multizone complete zones web map. (Image: NGS)
Users can obtain specific information for a coordinate located in the Ohio Franklin Zone by clicking on the online interactive table of coordinates. Note that the distortion is 6.725 ppm at the coordinate in the zone. As previously stated, this was a user–defined LDP zone.
SPCS2022 example of coordinates and distortion values in Ohio Franklin Zone. (Image: NGS)
Another Alpha site available for users to evaluate is the NGS Coordinate Conversion and Transformation Tool (NCAT). NCAT is probably the tool that most surveyors will be interested in using and providing feedback to NGS. Users can access NCAT on the Alpha SPCS2022 webpage or by clicking here.
Alpha NCAT button. (Image: NGS)
Alpha NGS Coordinate Conversion and Transformation Tool. (Image: NGS)
The Alpha NCAT website has a note about the coordinates that users should provide as input to the routine. The bottom line is that the input coordinates need to be in ITRF2020 (epoch 2020.0), or readers may not get their desired zone. NGS recommends that users convert the coordinates to ITRF2020 (epoch 2020.0) using the Horizontal Time-Dependent Positioning (HTDP) tool.
Users can access HTDP here. I provided an example of HTDP for a CORS in North Carolina. I used the NAD 83 (2011) [epoch 2010.0] published coordinates of the CORS as my input values.
Example of a HTDP computation. (Image: NGS)
Output of a HTDP computation. (Image: NGS website)
After using HTDP to transform the coordinates from NAD 83 (2011) to ITRF2020, I used the Alpha NCAT tool to compute the SPCS2022 values for the mark. I provided an example of the Alpha NCAT routine using the coordinates of the North Carolina CORS NCMR. The program defaults to horizontal only, so I changed it to the horizontal-height option. The user then enters the latitude, longitude, and height of the mark. Lastly, the user has an option to select the SPC zone or the program will select the zone based on the coordinates of the mark. In my example, I selected the auto pick option.
NCAT input for MONROE CORS (NCMR). (Image: NGS)
The image below provides the output of NCAT. I have highlighted a few items in the image. First, the program selected North Carolina’s Statewide Zone, the distortion is -54.554 ppm at this mark, and the UTM zone selected is Zone 17. The output also provides the scale and combined factors.
NCAT output for NCMR. (Image: NGS)
North Carolina is a state that elected to have a single statewide zone, but, as previously mentioned, some states decided to design their own LDPs. Again, Ohio is a state that designed LDPs that cover the entire state. Once again, I entered the coordinates into the input boxes and selected the auto pick (default zone) option. As indicated in the converted coordinates section, the program selected the OH FRA-391025 zone based on the coordinates of the mark. Notice that the distortion is only +8.024 ppm.
NCAT results for Columbus CORS (COLB). (Image) NGS)
The user has the option to select a different zone than the default zone. The image below provides the SPC values for the COLB mark when selecting the Ohio Statewide Zone. Notice that the distortion value changes from +8.024 ppm to -168.316 ppm. Also, as expected, the UTM and X, Y, and Z values have not changed.
NCAT results for COLB selecting Statewide Zone. (Image: NGS)
One last option to highlight is that the user can change the default UTM zone by clicking on the up or down arrows under the UTM column. In my example, I changed the UTM zone from 17 to 16. Obviously, the values under the UTM column changed.
Option to change default UTM zone. (Image: NGS)
The concept of the NGS’s Alpha site is to provide examples of the content, format, and structure of data and products that NGS plans to release as part of the modernized NSRS. NGS states that these Alpha products are for illustrative purposes only and do not contain any authoritative NGS data or tools. It states that they are under active development and are subject to change without notice.
That said, NGS would like everyone to try these Alpha products and provide feedback to NGS, so that they can improve their products and services. I would encourage readers to try these Alpha sites and provide comments and suggestions to NGS.
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