Many survey markers in the U.S. were set over 100 years ago. There was also a surge in creating these marks in the U.S. from about 1930 to 1955, in conjunction with the expansion of map-making activities across the country. In the U.S. some of these marks (triangulation stations and GPS points, loosely also referred to as "benchmarks") have precise "adjusted" coordinates (latitude and longitude). The "ADJUSTED" coordinates are precise to sub-centimeter accuracy, while others, typically true elevation Bench Marks, have only coordinates scaled from a map. "SCALED" coordinates were read from a topographic map, rather than being surveyed. Many are accurate to 100 feet but some few are as much as 3,000 or 4,000 feet distant from the mark to which they refer, rendering a handheld GPS unit of little use in locating them. Most marks have clear "to-reach" descriptions, but some lack complete descriptions, or changes in the surrounding buildings, roads, or terrain over decades make the descriptions obsolete. Marks may have been removed by construction or buried.