GIS Blog: Melanie A. Sutton

Estimating the values for chlorophyll a across the tessellated grid using inverse distance weighting (IDW) involved examining the average measurements of chlorophyll a taken from the four sample sites over a two-month window and then weighting these values based on the distance of these sample sites from each grid cell (Bolstad, 2005). For this case study, I selected parameters for the tessellated surface to create square-shaped grid cells (creating a more uniform visualization for the grid). IDW is a spatial interpolation method well-suited to this application due to the ease of implementation, the speed of the calculation, and the assumption within this case study that the neighborhood of known sites captures relevant local information that should impact each grid cell in the tessellation (i.e., we are assuming no significant outliers). In this example, all sample sites are also in the same waterway, and we are relying on the assumption that there are no barriers to water flow that

  The final map I created in this lab includes some preliminary analysis of the differences in average dissolved oxygen levels as a side-by-side comparison for January 2014 versus February 2014. In this example I used graduated symbols colored red (dangerous level), yellow (warning level), and green (acceptable level). This provides visual cues on areas within Bayou Texar that may be at risk for water quality issues. A tessellated surface for this feature would be very useful to show the scope of the impact of the change in values from the northern to southern regions of this waterway. For example, for the results from January 2014, the tessellated cells colored near the northernmost site could provide guidance on how far in this area community groups could explore for potential contributing causes. Tessellated surfaces would also be beneficial for examining temperature-related data for this case study, as well as other environmental impact case studies where water temperature/quality