How accurate is your map? How do you determine or define the accuracy of your map? There's an old adage: "The man with one clock always knows the time; the man with two clocks is never quite sure..." This reminds us of the simple truth that virtually every clock has some error, and is not perfectly accurate. So you could say in all honesty that virtually every clock is wrong.
In the same way, every map is wrong, and contains some degree of error, so the question becomes how to quantify that error and determine whether the map's accuracy is suitable for its intended purposes. In his POB article about how to determine accuracy in mapping, Mike Tulley gives a good explanation of the issue, including insights into why it's important to know the accuracy of your mapping products. This is especially true as today's mapping professionals are "swimming in a sea of data" with the abundance of products available from various sources. We've all seen Bing maps or Google Earth images, and used them regularly for certain tasks, but it is wise to be careful where those products may be put to use since there are no accuracy standards stated. Mike refers to the National Standard for Spatial Data Accuracy (NSSDA) as a statistically valid, consistent method for measuring and reporting the accuracy and related error of a geospatial dataset. He gets into the nitty gritty of how to specify or define the accuracy of a map, but this concept is important for anyone who may be held accountable for the accuracy of their mapping products.
So back to our clock... What time is it? Are you sure??