A GIS is used to study various phenomena based on the spatial relationships among features. The location information of every entity in a GIS must be in the same coordinate system. The data layer needs to be registered to the same reference coordinate system. Otherwise, errors will result that will lead to problems in interpretation and analysis during the later stages of GIS operations. Converting the coordinate system of individual data layers to a unified coordinate system is the first priority when developing a GIS database.
Current GIS software has the ability to unify the coordinate system of layers that contain spatial information (i.e., projection-on-the-fly). However, the projection-on-the-fly function still relies on the fact that each individual data layer must have an assigned coordinate system. Due to the differences of vector and raster data structures, the projection-on-the-fly function still has problems when performing GIS operations that involve interactions between raster and vector data. All the data layers in a GIS should be converted to the same coordinate system. This ensures less complications with GIS operations and eliminates data management issues due to different coordinate systems.
The differences between various coordinate systems and map projections should be understood so that a standard spatial reference system can be selected. Although a GIS is operational as long as the data layers share the same spatial reference system, the use of a standard coordinate system is highly recommended. Incompatibility problems that may be associated with an arbitrary coordinate system can be avoided by using a standard coordinate system.
There are two types of coordinate systems used in GIS: a geographic coordinate system and a plane coordinate system. Geographic coordinates are represented as latitude and longitude values. The units used in a geographic coordinate system are decimal degrees. This type of coordinate system is not equally spaced in the x,y direction, which is a result of the different x,y axis associated with the spherical shape of the earth. For the y direction, latitude values range from -90°S to 90°N and for the x direction, the longitude values range from -180°W to 180°E. This type of coordinate system is useful for locating the spatial position of features for a large area that considers the earth's curvature—a spherical coordinate system. The geographic coordinate system has been the primary coordinate system used in navigation and fundamental surveying applications. However, due to the different lengths of a degree for latitude and longitude, Earth features appear elongated in the x direction (longitude) or shortened in the y direction (latitude). This characteristic makes the geographic coordinate system unsuitable for remote sensing imagery that uses a fixed grid size in both the latitude and longitude directions.
A second type of coordinate system, the plane coordinate system (also called rectangular coordinate system), defines the position on a flat map representation instead of the curved surface of the earth. Currently, there are two commonly used plane coordinate systems used in the United States: the Universal Transverse Mercator (UTM) system and the State Plane Coordinate (SPC) system. Each uses different projection methods to project the earth's surface onto a flat surface. The UTM system is more suitable for larger areas (regional scales), whereas the SPC system is more suitable for smaller areas (local scales).
Because the earth is a sphere, and does not have a flat surface, the geographic coordinate system is used as a positioning guide, because it represents the curvature of the earth's surface. Due to the units (decimal degrees) associated with a geographic coordinate system, a transformation is required to convert the decimal degree units into another linear system, such as feet or meters. The technique used to transform the spherical earth into a two-dimensional plane is called a map projection. The control points to support the map projection are called a datum.
The map projection is a mathematical function to transform the curved earth to a flat map, and the datum is the reference point used in the transformation. When using two maps with the same map projection, but with a different datum, the results will not be the same. The map projection and map datum information should be carefully examined before performing a projection conversion.
The National Geodetic Survey developed a SPC system for each state. To convert a geodetic position to plane rectangular coordinates, the point needs to be projected. A mathematical process is used to project points from the earth ellipsoid to an imaginary developable surface, which is a surface that can be unrolled and laid out flat without any distortion of shape or size. Two commonly used map projections are the Lambert conformal conic, where the projection is made onto the surface of an imaginary cone, and the transverse Mercator projection, which uses a cylinder as the developable surface. The Lambert projection is typically used for mapping states that are narrow in the north-south direction, but are longer in the east-west direction. Examples include states like Kentucky, Montana, Pennsylvania, and Tennessee. The transverse Mercator projection is used for mapping states that are narrow in the east-west direction, but are longer in the north-south direction. Examples include states like Illinois, Indiana, and New Jersey. Figure 10.3 shows an agricultural test site in Ohio—the Management System Evaluation Area (MSEA) site located in Piketon—in three different standard map coordinate systems. Another common map projection system is the UTM system that is used worldwide. The UTM system was developed by the military and covers the earth from 84°N latitude to 80°S latitude. There are sixty zones, with each zone 6° wide in longitude. The x,y coordinates are in meters, in northing and easting values.
Selecting an appropriate coordinate system for a GIS requires several considerations, which include the data types, existing coordinate system in the data layers, and future data layers to be generated. The data types will often result in positioning accuracy issues during a coordinate system conversion. For example, converting a raster data layer from one coordinate system to another may require rotating, shifting coordinates, and a resampling process that may severely distort the
Sampling site layers:
- 1994 & 1998 plant sampling grid
- Soil sampling sites
Ground features & site plan
Ground features & site plan
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