The American Indian population percentage in Montana correlates with relativity to reservations.

The U.S. Census Grids (Summary File 1), 2010 data set contains grids of demographic and socioeconomic data from the year 2010 in ASCII and GeoTIFF formats. The grids have a resolution of 30 arc-seconds (0.0083 decimal degrees),

or approximately 1 square km. The gridded variables are based on census block geography from Census 2010 TIGER/Line Files and census variables (population, households, and housing variables).

Data can be found at: https://sedac.ciesin.columbia.edu/data/set/usgrid-summary-file1-2010