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Projections

A projection is a system that is used to reference geographic locations.  Projection systems, sometimes called coordinate reference systems (CRS) are typically comprised of the following components:

 

Projection (Type): a coordinate system that transforms the 3D surface of the earth into a  2D coordinate system. Some of the more commonly used projections include:

othe so-called geographic system which consists of a longitude (east/west x-coordinate) and a latitude (north/south y-coordinate);

oUniversal Transverse Mercator (UTM), and

oLambert conformal conic projection which is used by the U.S. National Geodetic Survey for the State Plane CRS's.

 

Horizontal Datum: a reference frame that includes a reference ellipsoid or geoid that models the shape of the Earth, an origin tied to a know location, and associated control points. Two of the more commonly used datums include:

oWGS-84 used for global maps and by many national jurisdictions

oNAD-83 used in the United States and Canada

 

Units: measurement units for the coordinate system. Typically, decimal degrees, meters, or feet.

 

There are hundreds of well-known CRS that are identified using numeric EPSG codes developed by the European Petroleum Survey Group and maintained at https://spatialreference.org/. For example, EPSG:4326 represents a CRS with a geographic system, using the WGS-84 horizontal datum and coordinates in decimal degrees.  Many geographic information system (GIS) applications store the CRS information in projection (.prj) files that accompany spatial layers such as those used in the Map Viewer Module (e.g. shapefiles and images).  When creating a map, the map and each layer within it must either have a common CRS or each component and the map itself must have a specified CRS that allows the map to correctly position each layer.

By default, maps in your Map Viewer maps will use the CRS defined when you create a new project or import an existing project from AquaChem. 2014.

For more information, please refer to: https://docs.qgis.org/3.16/en/docs/gentle_gis_introduction/coordinate_reference_systems.html

 

 


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