![]() There are two main types of VCS: Ellipsoidal systems measure z-values from a mathematically-defined three-dimensional ellipsoidal surface. A VCS defines vertical linear units of measure, the origin of z-values, and whether z-values are "positive up" (representing heights above a surface) or "positive down" (representing depths below a surface). Vertical coordinate systems (VCSs) are significant when working with 3D data. ![]() ![]() Because the relationship of a local coordinate system to another coordinate system cannot be established easily, these are sometimes referred to as unknown coordinate systems. The origin and the x,y coordinate units correspond to a local point of reference. Local coordinate systems are often unrelated to any other coordinate system. It also defines a PROJECTION (Mercator_Auxiliary_Sphere) which defines how to project the 3D GEOGCS coordinates into 2D coordinates. You can see that EPSG:3857 references the same DATUM, SPHEROID, PRIMEM and UNIT fields as the 4326 projection. You can see these three parts in the well-known text for the 4326 spatial reference in the DATUM, PRIMEM and UNIT sections: A datum, which is an ellipsoidal (spheroid) model of the EarthĬommon datums include WGS84 (used in GPS, WKID=4326) and NAD83 (used in surveying and mapping in North America).There are three parts to a geographic coordinate system: Typically GCSs use latitude and longitude specified in degrees. These coordinates are based on angles from the center of the Earth to the surface. Horizontal coordinate systems locate data across the surface of the earth, and are split into categories:Ī geographic coordinate system (GCS) uses a three-dimensional ellipsoidal surface to define locations on the Earth. Coordinate systems and projectionsĭata is defined in both horizontal and vertical coordinate systems. Not all coordinates and their associated coordinate systems are alike: they can use different units (for example degrees minutes seconds, decimal degrees, or meters) and they can be based on different types of models.ĪrcGIS uses mathematical transformations to reproject coordinates from one coordinate system to another.Ī spatial reference provides all the information needed for reprojection. This is done using coordinates, which are expressed relative to a coordinate system, or an accepted frame of reference around a model of the Earth's surface. To integrate spatial data together into a map or when performing analysis, ArcGIS must know where things are located on the Earth's surface. Many governments, militaries, and other organizations mandate that a specific one be used. Why spatial references are importantĪrcGIS allows you to use thousands of different spatial references and offers many tools for working with spatial references. ArcGIS Server handles reprojecting the data (converting it to the requested spatial reference) 'on the fly'. 102003 - Albers Equal Area 3857 - Web Mercator The data for both these maps comes from the same source.
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