Questions and Answers from the March 8, 2018, NGS Webinar on State Plane Coordinate Systems |

Written by Michael Dennis, NGS | |

Tuesday, 10 April 2018 | |

Don't miss Part 2 of this webinar on April 12. You can register here:
There were about 200 pertinent questions and comments, depending on how they are counted, which includes some that were sent by email shortly after the webinar. Of those, 128 were classified as “actionable”, i.e., that required a response or some action or awareness on the part of NGS. In an attempt to make the task of responding more organized, the 128 questions and comments were grouped into 13 categories in the outline below. Many of the questions and comments were very good, and they most definitely will influence content and implementation of the April 12 webinar (you can register at https://geodesy.noaa.gov/web/science_edu/webinar_series/state-plane-coordinates-2.shtml). I have tried to answer every question and address every comment, apart from a few specific ones that require me to directly contact the individuals. If a question was not answered, or if the answer given still leaves you puzzled, there is a good chance it will be addressed in the April 12 webinar. And of course there will be the opportunity to post questions and comments at that webinar as well. In the meantime, if there is something you’d like addressed sooner, please feel free contact us at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it .
a. b. c. d. e. f. g. h.
a. b. c. i. “Map Projections ─ A Working Manual”, ii. “State Plane Coordinate System of 1983”,
a. b. c.
a. i. Projected lines intersect at the same angle as on the Earth. For example, projected meridians and parallels intersect at right angles. ii. The iii. Shapes are iv. Projected (grid) north and geodetic north differ by a single number (the b. c.
a. b. by assigning a numeric scale factor value to the projection axis (i.e., the central standard parallel). It is merely a convention, likely adopted because of a tendency in the pre-computer era to avoid fractional values, especially when represented as repeating decimal values of less than 1. Note that a 1-parallel LCC with a projection axis scale of less than 1 also has north and south “standard” parallels (i.e., parallels where the scale with respect to ellipsoid is exactly 1). However, these two parallels must be calculated, and so that can’t really be called “standard.” That is, they are not used to explicitly define the projection. explicitlyFor LCCs that are “non-intersecting” (not tangent or secant), only the 1-parallel definition makes sense (with the projection axis scale greater than 1). This type is usually used for LCCs where it is desired to reduce projection at the topographic surface, when that surface is significantly above the ellipsoid. c. i. Set the scale factor value on the projection axis (central standard parallel) to a number less than 1 (e.g., 0.9999 for a scale of 1:10,000). ii. Define two (north and south) standard parallels along which the scale factor is exactly 1. Then the scale factor of the projection axis (which is still the central parallel)
a. b. c. d. e. f. g.
a. i. The projections will be scaled (and the projection axis possibly shifted) to minimize distortion at the ii. LCC projections will be defined using the 1-parallel definition. iii. SPCS2022 coordinates will differ from SPCS 83, SPCS 27, and UTM by at least 10,000 meters. iv. A few zones may have their zone extents and/or projection types changed. v. Zones may be added to a few areas that do not have SPCS 83 zones (such as Washington D.C. and American Samoa). b. c. d. e. f.
a. b. c. SPCS 83-like zones, but they cannot have both. They will be able to have a statewide zone in addition to one set of non-overlapping subzones, which can be LDPs or something similar to traditional SPCS zones. LDPs are discussed further in the following question category.or
a. b. c. d. e. f. g.
a. b. c. d.
a. b. c.
a. b. i. State departments of transportation ii. State GIS or cartographer offices iii. State professional surveying and engineering societies iv. State GIS or other professional geospatial organizations v. Universities or other post-secondary educational institutions within a state that perform geospatial education or research. c. i. ii. iii. d. e. f. g.
a. b. There were several comments and questions about reference frames, datums, and passive marks. These do not directly relate to the SPCS2022 topic, but they are of course very relevant to the NGS mission and the products and services we are developing as part of the transition to the 2022 NSRS. Some are listed below (without answers), and resources are provided in the next item. i. ii. iii. iv. c. NGS has produced various presentations, webinars, and publications that provide information about the 2022 terrestrial reference frames and geopotential datum: i. Webinar on the 2022 terrestrial reference frames: “Blueprint for 2022, Part 1: Geometric Coordinates” (https://geodesy.noaa.gov/web/science_edu/webinar_series/blueprint-2022-geometric-coordinates.shtml). ii. Webinar on the 2022 geopotential (“vertical”) datum: “Blueprint for 2022, Part 2: Geopotential Coordinates” (https://geodesy.noaa.gov/web/science_edu/webinar_series/blueprint-2022-geopotential-coordinates.shtml). iii. Report on the geometric reference frame: iv. Report on the geopotential (“vertical”) datum: v. Various presentations listed in the NGS Presentations Library (https://geodesy.noaa.gov/web/science_edu/presentations_library/). A PDF of this document can be found HERE |