This week's lab assignment was to assess the accuracy of two different sets of street data against reference points taken from high-resolution orthoimagery. The first step was to establish test points according to the National Standard for Spatial Data Accuracy (NSSDA): at least 20 points at well-defined locations (in this case, street intersections) with at least 20% of the points located in each quadrant of the study area and a distance between points of at least 10% the diagonal length of the study area.
Screenshot of test point distribution, with street data:
XY coordinates were obtained at each test point for each of the datasets to be assessed as well as for the actual location of the intersection based on the orthoimagery. Then the errors statistics for each point and the RMSE and NSSDA accuracy statistic were calculated for each dataset.
Result for the first dataset, from the City of Albuquerque: Tested 23.94516 feet horizontal accuracy at 95% confidence level.
Result for the second dataset, from StreetMap USA: Tested 184.40877 feet horizontal accuracy at 95% confidence level.
In other words, 95% of the data is expected to fall within 23.94516 feet or 184.40877 feet, respectively, of its true location.
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