Module 1.3 Data Quality - Assessments

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This week's lab objective is about data quality assessments by evaluating accuracy/completeness by road networks. We were given two (2) road networks in Jackson County, Oregon named TIGER Roads from the 2000 census data and street centerlines data created by the county itself. 

First, we were asked to compute the total length of each road networks in kilometres, both data should have the same projection, to know which is more complete. With this method we used the Calculate Geometry Attributes to convert the shape length from feet to kilometres and then used the Summary Statistics to compute the sum. The result for this method was that TIGER roads was more complete than the centerline roads with the following values: 

Street Centerline: 10805.8 km

TIGER Roads: 11382.7 km

The second method involved having a grid data which method was based on a study comparing OSM and Meridian networks in England (Haklay, 2010). The goal here is to determine the length (in km) for TIGER and Centerline roads in each grid and then apply Haklay (2010) methodology to have an accurate summary of results.

  1. Make sure that both roads in comparison have the same projection and length are already calculated in kilometers (as requested in the module).
  2. Using the Clip tool, clip both road networks separately to remove any excess road lines, using the Grid layer, in your study area.
  3. Use the Summarize Within tool to get the length of each road network per grid polygon. Do this for both TIGER and Centerline roads.
  4. Then use Join Features tool to join both roads using Grid Code to link them together. After that you can calculate the percent difference but making sure you exported the newly joined feature as a feature class.
  5. Calculate %percent difference by this formula: % 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒= (𝑡𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑒𝑛𝑡𝑒𝑟𝑙𝑖𝑛𝑒𝑠−𝑡𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑇𝐼𝐺𝐸𝑅 𝑅𝑜𝑎𝑑𝑠)(𝑡𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑐𝑒𝑛𝑡𝑒𝑟𝑙𝑖𝑛𝑒𝑠) ×100%
  6. Create an SQL expression to assign TIGER or CENTERLINE to you new field to show whether a grid is more complete in TIGER or CENTERLINE road networks.
  7. Then use that to create a summary statistic on how many grids are more complete on TIGER Roads and Centerline Roads.
In conclusion, TIGER was more complete in 162 of the 297 grids, while centerline was more complete in 134 of the 297 grids. There is one empty grid square that was omitted as neither roads were existent in that grid. There’s also another grid that was included in the TIGER summary though its percent difference is 0. That 2nd grid has no centerline roads, only TIGER roads, so it didn’t proceed with the calculation. See the map below for visual reference.



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