Coordinate systems |
This International Standard uses two principal classes of Coordinate Systems: a Map CS applicable to the map portrayal generated by the WMS, and a Layer CRS for a Bounding Box applied to the source data. During a portrayal operation, a WMS converts or transforms geographic information from a Layer CRS into a Map CS. In addition, a Layer may have an associated vertical, temporal or other coordinate system.
A Map CS is a coordinate reference system for a map produced by a WMS. A WMS map is a rectangular grid of pixels displayed on a computer screen (or a digital file that could be so displayed). The Map CS has a horizontal axis denoted i, and a vertical axis denoted j. i and j shall have only nonnegative integer values. The origin (i,j) = (0,0) is the pixel in the upper left corner of the map; i increases to the right and j increases downward.
The WIDTH and HEIGHT parameters used in the GetMap request and by inclusion in the GetFeatureInfo request correspond to i and j as follows:
The I and J parameters used in the GetFeatureInfo request denote integer values along the i and j axes, respectively, of the Map CS.
A Layer CRS is a horizontal coordinate reference system for the geographic information that serves as the source for a map. A Layer CRS appears in the following entities relevant to the WMS:
A WMS must support at least one CRS, and maps from multiple servers may be overlaid only if all the selected servers have at least one CRS in common.
Every Layer CRS has an identifier that is a character string. Two types of Layer CRS identifiers are permitted: "label" and "URL" identifiers.
The Layer CRS has two axes, denoted x and y. The x axis is the first axis in the CRS definition, the y axis is the second axis. Depending on the particular CRS, the x axis may or may not be oriented West-to-East, and the y axis may or may not be oriented South-to-North. The WMS portrayal operation shall account for axis order, origin and direction in the Layer CRS when projecting geographic information from a Layer CRS to the Map CS.
Bounding box values specify the portion of the Earth to be mapped through two pairs of coordinates in a specified Layer CRS. The first pair specifies the minimum coordinate values in the Layer CRS, the second specifies maximum coordinate values. Although for most CRSs with axes incrementing to the east and north this would be the lower left and upper right corners of the area of interest, the minimum and maximum values might be at other points in some instances. For example, when using geographic coordinates to describe an area over a pole, or when the layer CRS axes increment in directions other than east and north. The order in which ordinates in each pair are listed shall be as defined by the Layer CRS; x corresponds to the first axis in the Layer CRS and y to the second. This order may not coincide with the Map CS axis order i, j. The bounding box coordinate values shall be in the units defined for the Layer CRS.
Bounding Box values appear in the following entities relevant to the WMS:
A Bounding Box shall not have zero area.
EXAMPLE 1: A <BoundingBox> metadata element for a Layer representing the entire Earth in the CRS:84 Layer CRS would be written as in the CRS:84:
<BoundingBox CRS="CRS: 84" minx="-180" miny="-90" maxx="180" maxy="90">.
A BBOX parameter requesting a map of the entire Earth would be written in this CRS as:
BBOX=-180,-90,180,90.
EXAMPLE 2: A <BoundingBox> representing the entire Earth in the EPSG:4326 Layer CRS would be written as:
<BoundingBox CRS="EPSG:4326" minx="-90" miny="-180" maxx="90" maxy="180">.
A BBOX parameter requesting a map of the entire Earth would be written in this CRS as:
BBOX=-90,-180,90,180.
Some geographic information may be available at multiple elevations (for example, ozone concentrations at different heights in the atmosphere). A WMS may announce available elevations in its service metadata, and the GetMap operation includes an optional parameter for requesting a particular elevation. A single elevation or depth value is a number whose units, and the direction in which ordinates increment, are declared through a onedimensional vertical CRS. Depending on the context, elevation values may appear as a single value, a list of values, or an interval
A server may declare at most one vertical CRS for each layer. For the purposes of this International Standard, the horizontal and vertical CRSs are treated as independent metadata elements and request parameters.
A request for a map at a specific elevation includes an elevation value but does not include the vertical CRS identifier (the horizontal CRS, which is included along with the horizontal bounding box in the request parameters). When providing elevation information, a server should declare a default value in service metadata, and a server shall respond with the default value if one has been declared and the client request does not include a value.
Two types of Vertical CRS identifiers are permitted: "label" and "URL" identifiers:
Some geographic information may be available at multiple times (for example, an hourly weather map). A WMS may announce available times in its service metadata, and the GetMap operation includes a parameter for requesting a particular time. Depending on the context, time values may appear as a single value, a list of values, or an interval. When providing temporal information, a server should declare a default value in service metadata, and a server shall respond with the default value if one has been declared and the client request does not include a value.
Some geographic information may be available at other dimensions (for example, satellite images in different wavelength bands). A WMS may announce available sample dimensions in its service metadata, and the GetMap operation includes a mechanism for requesting dimensional values. Each sample dimension has a Name and one or more valid values.