Geoscience Reference
In-Depth Information
10.4 CoMMon CooRdInAte SySteMS
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 differ-
ences 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 geo-
graphic 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 coor-
dinate 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 sys-
tem 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 projec-
tion. 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.
