A Single Zone Coordinate System for Idaho
There are two single zone projected spatial reference systems commonly used in Idaho referred to as IDTM27 and IDTM83.
The original Idaho Transverse Mercator coordinate system (IDTM27) was defined with the North American Datum of 1927. In 2003, it was redefined to use the North American Datum of 1983 creating IDTM83. House Bill No. 425 (2010) modified Title 55, Chapter 17, Idaho Code to include The Idaho Coordinate System of 1983, single zone formally recognizing IDTM83. IDTM83 was incorporated into the EPSG in 2018 and assigned the EPSG Code 8826.
The majority of the data served publicly by IDWR is projected using IDTM83. For a comparison of IDTM27, IDTM83, and other coordinate system parameters, visit here.
The Need for a Single Zone Coordinate System
The commonly used spatial reference systems of Universal Transverse Mercator (UTM) and Idaho State Plane (ISPCS) divide Idaho into numerous zones. This was particularly undesirable before modern GIS software had the ability to accommodate display of multiple projections in a single map view. For display and analysis of large datasets, the data would need to be reprojected from one zone into another, leading to inconsistencies and at times degrading the positional accuracy of data. There were also cartographic difficulties of using multiple zone data in a single map. These difficulties inspired the development of the Idaho Transverse Mercator projected coordinate system. To review a discussion by Wayne H. Valentine, LS, PE, about his original proposal for IDTM27, visit here.
Even with software packages being better able to use data in multiple projections, a single zone is still needed today. Location data for natural resource uses are often collected regionally and rarely is a UTM or ISPCS zone used as a project boundary. This need has been recognized by the National Geodetic Survey (NGS). In developing the State Plane Coordinate System of 2022 (SPCS2022), based on the North American Terrestrial Reference Frame of 2022 (NATRF2022), the NGS and the Idaho Geodetic Control Technical Working Group, developed a single “statewide” zone for Idaho. In anticipation of changes resulting from NATRF2022, Senate Bill No. 1011 (2021) was introduced modifying 55-1701 Idaho Code to define Idaho Transverse Mercator (IDTM) to be the statewide mapping projection, and any successor system, specific to the state of Idaho maintained by the NGS.
As of April 2023, NGS has yet to release SPCS2022 and the associated new statewide single zone system. For updates on the status of SPCS2022 and NATRF2022, please visit the NGS New Datums website.
