Lab 2 - Determining Quality of Road Networks

This week’s lab was about testing the horizontal accuracy of road networks while following the procedures of the National Standard for Spatial Data Accuracy (NSSDA).  After downloading the data and creating street networks for both the City and the StreetMap USA data sets, I used the Sampling Design Tool addin for ArcGIS to create 100 random junction points, starting with the City data. I wanted a total of 20 “well-defined” points, which in this case are road intersections. I picked 20 intersections in a reasonably well-distributed manner on the City dataset; I then turned on the StreetMap USA data layer and matched the junctions on that map. Below is a screenshot of my 20 points using the City data.

I created new layers from these selections, so that I now had layers showing the 20 points I was using to calculate the positional accuracy. Then I created a blank reference data layer, in which I used the orthophotos provided to create 20 reference points, again matching the intersection (I used the center of the intersection for each point). When looking at the two datasets as well as the orthophotos, some of the StreetMap USA roads went through buildings, and the road network seemed incomplete in places. The City data seemed much more complete and seemed to go down the middle of the roads indicated on the orthophotos, so the City data appeared more accurate. At this point, I used the Add XY coordinates tool for all 3 layers, and imported the data into 2 Excel tables – one for the City data and one for the StreetMap USA data. Using the NSSDA worksheet provided, I quickly calculated the sum, average, RMSE, and the NSSDA accuracy.

Using the NSSDA procedures, I was able to create an accuracy statement for both datasets:

City Data: This data has been tested to meet 19.4 feet horizontal accuracy at the 95% confidence level.

StreetMap USA Data: This data has been tested to meet 686.7 feet horizontal accuracy at the 95% confidence level.

As was suggested by the map appearance when overlaid over the orthophotos, the City data is much more positionally accurate than the StreetMap USA data.