Lab 5 - Tessellation (fishnets) in PostGIS

You know those “aha..” moments? I totally had one in this lab on the 2nd deliverable when I had to connect to my database in PostgreSQL via ArcGIS Pro). That was so cool! But before I get ahead of myself, let me recap this week's lab assignment into 4 main points:

1. Organized and populated a database in PostgreSQL, which involved creating a new schema and then importing both spatial and non-spatial data of water quality sample properties at Bayou Texar in Pensacola Beach.

2. Connected to my PostgreSQL database from within ArcGIS Pro.

3. Went back to PostgresSQL (pgAdmin) to create a fishnet via the Postgres function ST_CreateFishnet. Then fitted my tessellated grid to the study boundary using a more complex SQL script.

4. Went back to ArcGIS Pro and created a simple little map of my results. Again, because I cannot provide screenshots of my scripts or SQL statements here in my blog because future students might cheat, instead see my final map below and read about my take on what tessellated surfaces of a feature can be used to show and how it is useful in analysis.

 

A tessellated surface of a feature, such as the one we created in this lab, can be used to show estimation of values per cell (Bolstad, 2005). There are many instances where such a representation can be useful in analysis. Specifically, one pertains to the datasets/case we were presented to in this lab: what if the Marine Science Program (whom tasks the high school students with collecting water quality samples at the four specific sites) wanted to conduct an inverse distance weighted (IDW) interpolation for analyzing the phosphorous and nitrogen levels in Bayou Texar based on the data collected at the sample sites? If so, the Program would need a surface across in which to interpolate to (Morgan, 2018). By doing so, the estimations illustrated would provide geo-location insights to marine scientists in trending or emerging hot spots, which as a result could fire-up environmental health initiatives and/or mitigation strategies.

Another great reason to create tessellations is to keep people’s personal information safe and private. Though mostly nowadays U.S. Census Blocks are used in spatial analysis and thematic/choropleth mapping to indicate attributes of those given administrative boundaries, sometimes for the safety of a public group, greater tessellations are used (or should be used) when representing data. For example, my first job in GIS involved working with child addresses (geocoded points) for a federally-funded agency — very, very sensitive data. When creating trend reports to be used by major stakeholders, the point data was essentially de-identified by distributing it/aggregating equally among a net of rectangular cells using the Fishnet tool in ArcGIS.

Other variations of tessellations include Voronoi diagrams, Thiessen polygons, Triangular Irregular Networks (TINs), and Digital Elevation Models (DEMs).

References

Bolstad, P. (2005). GIS fundamentals: A first text on geographic information systems. White Bear Lake, MN: Eider Press.

Morgan, J. D. (2018). Lab 5: Tessellations [PDF document]. Retrieved from University of West Florida, Spatial Data Management. Canvas: https://uwf.instructure.com/.