Areas (or polygons)
Areas (or polygons) are either stored as raster files comprised of a collection of continuous cells in the shape’s interior, or as vector files, which use a mathematically connected series of points that define the shape’s bounding lines.
Pertaining to cartography, the practice or study of
charts and maps.
A continuous, imaginary ground-line signifying a set elevation above or below a specified datum.
A datum can be described as the model selected to convert ground locations into points for visual representation on a map. Map positions are established in relation to “known” points. Early maps were created when survey teams with transits measured distances between the “known” positions and selected features; however, this method could not be used to span continents and frequently could not cross political borders. Today these problems have been remedied by utilizing a space-based surveying system with standardized datum based on the center of the Earth.
Maps showing geographic coordinate systems, such as Latitude/Longitude or Universal Transverse Mercator, should list the applied datum at the edge of the map.
Most United States Geological Survey (USGS) maps are based on the North American Datum of 1927 (NAD 27), although this datum was updated in 1983 and now more accurate coordinates can be measured. GPS units sometimes subdivide NAD 27 datum into different datums. For example, the continental United States uses the NAD 27 CONUS datum.
The Global Positioning System is based on an earth-centered datum known as the World Geodetic System, created in 1984 (WGS 84). WGS 84 derives from and is an extension of the North American Datum of 1983 (NAD 83). For all practical purposes, there is no difference between WGS 84 and NAD 83.
DEM (Digital Elevation Models)
DEM is a continuous raster layer in which data file values represent elevation. It is also a GIS raster layer representing terrain relief. This is a common GIS product created at a variety of scales. DEMs are available from the USGS at 1:24,000 and 1:250,000 scale.
The Universal Transverse Mercator projection is used, with the cylinder in 60 positions. This creates 60 zones around the world. Positions are measured using Eastings and Northings, measured in meters, instead of latitude and longitude. UTM easting coordinates are referenced to the zone’s middle line, known as the central meridian. The central meridian is arbitrarily given an easting value of 500,000 meters; consequently, eastings sometimes are referred to as “false eastings.”
Lines are either stored in raster format, as a series
of connected cells, or in vector format as a set of
mathematically connected points.
It is important that data is accompanied by written documentation to enable accurate assessment of their reliability. Metadata (data about data) describe the content, quality, condition, source, history, and other characteristics of a dataset. Because it gives information pertaining to the collection and modification of the collected data, metadata retains data value for those not directly involved in the process. With metadata, prospective users can determine what data exist, the applicability of these data for their applications, and the conditions for accessing these data.
A raster image is a representation of energy that is either emitted or reflected off the visualized object and manifested in a matrix of row and column data points. These values, or pixels, can be viewed on a monitor as either a black-and-white or color image.
UTM northing coordinates are measured relative to the Equator. The Equator is assigned a value of 0 meters North for positions on the Northern Hemisphere. To avoid negative numbers, all points on the Southern Hemisphere are found in relation to the Equator, which is assigned an arbitrary value of 10,000,000 meters North.
An orthophotograph is an aerial photograph that has
been corrected so that scale variation and image displacement
are no longer distorted. The resulting product shows
terrain features in actual detail with true distances,
angles, and areas.