See also Overview of the Data Preparation Process and Generate Grid for general description of process and documentation of specific scripts and procedures used in this step.
Even though the study area was the four main counties of the central Puget Sound region (King, Pierce, Snohomish, and Kitsap), the grid created encompassed the greater Puget Sound region consisting of eight counties (Skagit, Island, Mason, Thurston, King, Pierce, Snohomish, and Kitsap). This larger grid was created to allow for possible expansion of the study area in the future. The eight-county grid was clipped down to the four central counties of the initial study area. The four-county grid was the only grid used in the data preparation process.
The image below shows the Green Lake area of Seattle with an overlay of 150 meter gridcells, with Traffic Analysis Zones (TAZ) and Forecast Analysis Zones (FAZ) superimposed to show the relative sizes of these geographies.
Areas outside the project boundary or covered by large bodies of water were clipped from the grid to make the database more compact. Water bodies within the study area such as Green Lake in the image above were kept in the grid.