located in the middle of lakes (see image).
"Consider this increasingly common spatial data
processing dilemma. An important project requires
that the locations of certain public facilities be
plotted onto road maps so service providers may
quickly and easily drive to each point. Global Positioning
System receivers use state-of-the-art
satellite technology to pinpoint the required locations. To provide context, these facility locations
are then laid over a digital base map containing
roads, lakes and rivers. A plot of the results reveals
a disturbing problem: some facilities appear to be
located in the middle of lakes (see image).
Which data set is correct: the base map or the facility locations? No information about positional accuracy was provided for either data set, but intuition would lead us to believe that GPS points are much more accurate than information collected from a 1:100,000-scale paper map. Right? In this case, wrong. The GPS receivers used for this study were only accurate to within 300 feet. The base map was assumed to be accurate to within 167 feet because it complied with the 1947 National Map Accuracy Standards. In reality, the base map may be almost twice as accurate as the information gathered from a state-of-the-art network of satellites. But, how would a project manager ever be able to know this simply by looking at a display on a computer screen?"