A Look at Public Elementary Schools of Pierce County
An analysis of public elementary schools in Pierce County looking at the percentage of impervious surface, the relative sizes of school grounds, and urban versus non-urban designations.
It should be noted this analysis was completed excluding private schools and was done purely for educational purposes in learning GIS.
Pierce County base map, urban zones, and public elementary schools.
Starting Point
The beginning layers were a Pierce County base map, Pierce County school polygons, urban zones in the US, and an impervious surface (aka paved features) raster layer of Washington State. In order to set up for analysis, only the public elementary schools were extracted using "select by attribute", followed by clipping the urban zones to the Pierce base map and using "extract by mask" to do the same for the impervious surface raster. The raster was then symbolized on a red-green spectrum showing more impervious surface as red.
Impervious surface raster extracted by Pierce County mask.
New Fields and Classification
New attribute fields were created for area in acres, relative size of schools, and their urban/non-urban designation. The relative size was classified using quantiles (botton 20%=small, middle 60%=average, top 20%=large) while the urban designation was created using "select by location" on schools completely within the urban zone layer.
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Zonal statistics were then applied to the school grounds using the impervious surface raster data in order to find the mean percentage of paved surfaces for each school. This was then classified with standard deviation and symbolized again on a red-green spectrum.
Classification via standard deviation for school polygons and impervious surface percentage, symbolized on the red-green spectrum atop the urban zone layer.
Resulting polygon layer from the spatial join.
Conversion, Join, and More Classification
The impervious surface/schools raster data then needed to be converted from raster to point so a spatial join (using intersect) could be conducted between this new point layer and the original elementary school grounds data, creating a new polygon layer.
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This join allowed for a new attribute field to be created to rank the percentages of impervious surface. Again, quantiles were used to classify the percentages with the bottom 20% (lowest percentages of pavement) being rank 1, the middle 60% rank 2, and the top 20% (highest percentages of pavement) as rank 3.
Final Touches
The last steps included converting the new elementary school polygon layer into a point layer using the "feature to point" in the data management tools. Next, Thiessen polygons were applied to the extent of Pierce County using the new school point layer. Finally, a clip was performed of the Thiessen polygons to match the Pierce County base map and the polygons were symbolized on the red-green spectrum using the percent impervious surface field classified into quantiles.
Conversion of feature (polygons) to points.
Completed Thiessen polygons, symbolized by percentage of impervious surface.