feet but some 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. Useful tools In addition to the survey data sheet for a mark being hunted, many hunters bring along a digital camera to take close-up and area pictures of the survey mark (a disk, a cross-cut in a rock, an old copper bolt, etc.). These photographs, as well as a current description of the mark by the person who finds ("recovers") the location of the mark, can then be uploaded to a website as proof of a find. Surveyors use the term "recovering" as a synonym for "finding" a mark. This does not mean that the found mark should be disturbed in any way. On the contrary—disturbing a survey mark in even a small way often destroys its usefulness to surveyors and others. In the U.S., benchmark hunters often file two different reports on marks they find. One, less formal, is made to the website at Geocaching.com. The other more formal type is made by more experienced and careful hunters to the NGS database itself and describes the mark's found/not found status, current condition, and updated directions to reach it so that others (particularly surveyors) can more easily re-find the mark. Benchmark hunters also frequently carry a compass (to follow directions gleaned from the datasheet), a probe (like a long-bladed screwdriver) to search for buried marks, a trowel (or a small shovel) to uncover buried marks, a whisk broom (to clear away debris), and one or more tape measures of various lengths, used in taping out referenced distances found on the data sheets. Handheld GPS receivers are often used to get within a few yards probable error of a mark that has adjusted coordinates. Metal detectors are useful for finding marks that have become deeply buried over time. Use of several of the tools in combination is sometimes required, but many marks are set on the surface of sidewalks, buildings, walls, boulders, or monuments and can often be found without special tools. Description of markers Survey markers vary considerably from one country to another. In the