Lab 3 [GIS5935]: Data Quality Assessment

 


The goal of the accuracy assessment summarized in the map above was to determine quality of road networks provided by Jackson County GIS compared to a data set provided by the U.S. Census Bureau. One measure of such quality is the completeness of the data set, or how comprehensive the coverage of the data set is. As noted by Haklay (2010, pp. 690-692), one method to make this comparison between two data sets of road networks involves overlaying a grid over the study area and then calculating the difference in the total length of line segments in each cell. To get a percentage, one of the data sets serves as the base (in this case, the Jackson County GIS data set served this purpose). The calculated percentage can then be positive or negative based on this formula:

     % difference = 100% * (Jackson County GIS - U.S. Census Bureau)/Jackson County GIS

A positive value indicates better coverage by Jackson County GIS in the underlying grid cell (noted by shades of blue). Alternatively, a negative value indicates better coverage the U.S. Census Bureau in the underlying grid cell (noted by shades of red). In the map above, the darker the color, the greater the percentage difference between the data sets, and notably only a handful of cells fall into the outermost percentage ranges. Incomplete data for this map represents either cells with no road networks or cases where the denominator would lead to a division by 0 error (i.e., there was no road segment in the grid cell for the Jackson County GIS data set).

Also noted in the map description above the legend, across the 296 grids spanning this area containing road networks from at least one of these data sets, the U.S. Census Bureau road networks were more complete in 55% of the 5x5 km grid  cells, based on a comparison of the total length of road segments within each grid.


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