The grid consists of a triangular array of points that cover the United States, combined with several types of hexagons associated with the grid points. The grid has a base density of approximately 640 sq km per point with a spacing of approximately 27 km between points. This density may be increased or decreased while maintaining a triangular structure.
Hexagons. At any given density, "tessellation" hexagons completely fill the space between the points (but do not connect the points) such that they form the Voronoi (or Thiessen) polygon network with respect to the points. Unrandomized tessellation hexagons pertain to the grid before randomization and randomized tessellation hexagons afterward. "Landscape" hexagons are the 40 sq km hexagons (40-hexes) surrounding the base density grid points (and as such are not tessellation hexagons) that were originally proposed for landscape characterization.
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Placement of base grid. We positioned the base grid arbitrarily to fit well about the conterminous U.S. and neighboring areas of Canada and Mexico. The bounding shape of the base grid is a hexagon from the truncated icosahedron (soccer ball) model that has an edge length of approximately 2600 km. The hexagon is rotated about the center (which is at about 41.89 degrees north latitude and 97.75 degrees west longitude) such that one radius from the center to a vertex is 10 degrees clockwise from geodetic north.
Construction of base grid. We constructed the base grid by first projecting the bounding hexagon from the earth's surface to the plane of an equal area map projection. The grid then consists of a set of points derived by dividing each side and radius into 96 equal parts and connecting the points. An alternate way to view this is to consider the six vertices and the center of the original hexagon as a triangular point grid, and then to enhance (add points to) this grid by regular factors in stages to arrive at the base grid. The base grid of points has a companion areal structure called a tessellation. The base tessellation hexagons constitute this tessellation. In other words, surrounding each grid point is a hexagon that defines the area within which all points are closer to this grid point than to any other, and the set of hexagons defined this way completely and mutually exclusively covers the space of the grid. The spacing between points of the base grid is approximately 27 km; the tessellation hexagons are approximately 640 sq km in area. Over the entire truncated icosahedron hexagon there are 27,937 base tessellation hexagons. There are approximately 12,000 base tessellation hexagons covering the conterminous U.S.
The coverage described above was clipped using Arc/Info. All hexagons that were inside or intersecting the boundary of Pennsylvania were selected.
Area Perimeter Pahex# (Arc/Info default) Pahex-ID (Corresponds to EPA hexagon code) Stgrid-id Usgrid-id (EPA hexagon code) Ha (area of hexagon in hectares)