Adventures in Metadata

This week's exercise in Data Quality includes examination of various data standards, namely the National Standard for Spatial Data Accuracy (NSSDA) and the older National Map Accuracy Standards (NMAS), which indicate the data set's accuracy, and are typically found in its metadata.  The actual procedure described in the NSSDA involves taking sample points from a test data set and a known, highly accurate reference value, and produces a value that indicates the tested data's horizontal or vertical accuracy at a 95% confidence level.

Above is a map, which includes 20 sample points, taken to be compared with a reference data set, which was created using aerial imagery.  The sample location distances from the reference points are used to calculate an error statistic, which is typically included in a data set's metadata.  The statistical and testing method prescribed in the NSSDA allows for a statement on the likely amount of error in the data, like the one for the map above-
tested 2492.12642 feet horizontal accuracy at 95% confidence level.  This indicates that a horizontal position taken on the map will generally be within about 2,492 feet of its actual location on the ground, as accurate as true location is, 95% of the time.