I am working with some sample GIS data for Idaho. The data is in geographic coordinates but I want it in projected coordinates.

I am unsure how to address the issue of projection as Idaho crosses UTM zones 11 and 12.

Should I:

  • apply UTM 11 to all?
  • apply UTM 12 to all?
  • apply another system completely to all?
  • filter the data into those that fall into 11 and 12 respectively and work with them separately?

2 Answers 2


I would advise against using coordinates outside UTM boundaries as this can cause unforeseen problems further down the line when others use the data and may be unaware of this. I would consider it bad practice unless no other alternatives exist.

This has been recognized as a problem in Idaho and a solution exists so why not use it? The Idaho Transverse Mercator (IDTM) statewide protection was built for this purpose. It is SR-ORG:6677.

Your data is unprojected and you need to select a projected system. Why select one that does not cover the state when a perfectly good one exists and was designed just for this purpose?

  • advise = advise against?
    – nmpeterson
    Jan 11, 2016 at 18:20
  • ESRI has designed code Nr 12605 for IDTM.
    – AndreJ
    Sep 3, 2018 at 6:05
  • The projection is now included in the EPSG registry as EPSG:8826 NAD83 / Idaho Transverse Mercator. It will take some time until it finds its way in the GDAL release.
    – AndreJ
    Sep 11, 2018 at 17:28

I think that it is acceptable to use one UTM zone for all for the sake of analysis. For longevity, a continental-scale projection (like Albers Equal Area in North America) would be more appropriate (especially for published data). We used to use UTM Zone 17 for all of West Virginia when I worked in WV even though the eastern panhandle is in zone 18. Same for western Maryland, with most of MD being in Zone 18 but western MD being in zone 17. Idaho seems to be fairly evenly split between the two. I would probably use Zone 11 if I were in your position. I would not split the layers up if analysis needs to be done for the entire state.

A potentially better recommendation would be to use a State System like the Idaho Transverse Mercator Projection (IDTM). I think that would actually be your best bet if you don't mind units in feet (as described in the link). http://idwr.idaho.gov/GIS/IDTM/

  • 1
    I would second @jbchurchill's recommendation of using a statewide system. For Idaho, it is NAD83 ID-TM. I would edit this answer as best practice is using a statewide projection over using a UTM zone for the entire state.
    – Jeff May
    Jan 11, 2016 at 14:56
  • Its already in the answer. No edit needed. Thanks for your vote! Jan 11, 2016 at 14:57
  • I agree with part 2 (IDTM) but I disagree with part 1. Jan 11, 2016 at 16:29

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