Module 1.3: Data Quality - Assessment

For this week's lab assignment, we compared the total lengths of roads for two different road networks: TIGER Roads and Street Centerlines The TIGER Roads shapefile came from the US Census Bureau whereas the Street Centerlines shapefile came from Jackson county, Oregon. The objective was to determine the quality and completeness of the road networks. The total length was used as the simple measure of completeness, assuming that more roads means a more complete network.

The first step in the analysis was to determine the total lengths of the roads in each network for the entire county. I used the Project tool to project TIGER Roads into the same coordinate systems as the Street Centerlines shapefile. Then I used the Summarize tool  to determine the total length in each county. The TIGER Roads was found to be longer than the Street Centerlines, thus making it the more complete road network.

The second step of the analysis was to determine the total length of the roads within each grid for both networks. I performed the following steps in my workflow for this analysis: 

  1. I used the Clip tool to clip each road network to the grid. I did this because there were road segments that resided outside the outer boundaries of the grid polygons. To ensure these segments did not affect the length calculations, I clipped each to the actual study area: the Grid feature layer. 
  2. After each road network was clipped, I added a new field to the attribute table and used the Calculate Geometry tool for each road network. This step was necessary because the original geometry of the road networks was changed when clipped, thus changing the length of the networks. 
  3. I then used the Summarize Within tool with the Grid as the input and the length field for each road network. I used this tool because it broke each road network into segments and only calculated the lengths of the segments within each grid polygon. 
  4. I then used the Table to Excel tool to export each road network attribute table to Excel so that I could calculate the percent differences of the lengths of the segments within each grid polygon between the two road networks. For this step, I copied the length for each road network into a new Excel spreadsheet and created a new column where I used the equation provided in the lab to calculate the percent difference between the segments within each polygon of the two road networks. I used zero values for the calculations that had divisions by 0 (there were two). I saved the new Excel table and added it to the map. 
  5. I used the Add Join tool to join the new Excel table to the Grid feature layer. I used this toll so that I could use graduated symbology to visually represent the percent differences of the road segments within each grid polygon between the two road networks.
In this analysis, negative values mean the TIGER Roads had longer segments and positive values mean the Street Centerlines had longer segments.  
The map layout below is the result of the second analysis:



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