Coverage in GML The coverage model and representations

202 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. urn:x-ogc:specification:gml:schema-xsd:coverage:3.2.1 All schema documents are listed in Annex C.

19.1.4 Relationship with ISO 19123

The coverage components of the GML schema specified in this clause provide a conformant, partial implementation of the ISO 19123 coverage schema. The relationship is discussed in detail in D.2.11. The ISO 19123 coverage types implemented in GML are specified in ISO 19123; additional constraints specified in ISO 19123 for these types are also constraints on the coverage components of the GML schema.

19.2 Grids schema

19.2.1 Overview

An implicit description of geometry is one in which the items of the geometry do not explicitly appear in the encoding. Instead, a compact notation records a set of parameters, and a set of objects may be generated using a rule with these parameters. This clause provides grid geometries that are used in the description of gridded coverages and other applications. In GML two grid structures are defined, namely gml:Grid and gml:RectifiedGrid.

19.2.2 Grid

gml:Grid implements ISO 19123 CV_Grid see D.2.11 and ISO 19123:2005, 8.3 and is defined as follows: element name=Grid type=gml:GridType substitutionGroup=gml:AbstractImplicitGeometry element name=AbstractImplicitGeometry type=gml:AbstractGeometryType abstract=true substitutionGroup=gml:AbstractGeometry complexType name=GridType complexContent extension base=gml:AbstractGeometryType sequence element name=limits type=gml:GridLimitsType choice element name=axisLabels type=gml:NCNameList element name=axisName type=string maxOccurs=unbounded choice sequence attribute name=dimension type=positiveInteger use=required extension complexContent complexType The gml:Grid implicitly defines an unrectified grid, which is a network composed of two or more sets of curves in which the members of each set intersect the members of the other sets in an algorithmic way. The region of interest within the grid is given in terms of its gml:limits, being the grid coordinates of diagonally opposed corners of a rectangular region. gml:axisLabels is provided with a list of labels of the axes of the grid gml:axisName has been deprecated. gml:dimension specifies the dimension of the grid. In GML the gml:limits element contains a single gml:GridEnvelope, in accordance with the following schema definitions: Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. 203 complexType name=GridLimitsType sequence element name=GridEnvelope type=gml:GridEnvelopeType sequence complexType complexType name=GridEnvelopeType sequence element name=low type=gml:integerList element name=high type=gml:integerList sequence complexType The gml:low and gml:high elements are each gml:integerLists, which are coordinate tuples, the coordinates being measured as offsets from the origin of the grid along each axis, of the diagonally opposing corners of a ―rectangular‖ region of interest. EXAMPLE The following example illustrates a simple Grid. gml:Grid dimension=2 gml:limits gml:GridEnvelope gml:low0 0gml:low gml:high3 3gml:high gml:GridEnvelope gml:limits gml:axisLabelsx ygml:axisLabels gml:Grid In this example the Grid has posts points at locations 0,0, 0,1,1,0,1,1 through to 3,3. When a grid point is used to represent a sample space e.g. image pixel, the grid point represents the center of the sample space see ISO 19123:2005, 8.2.2.

19.2.3 RectifiedGrid

A rectified grid is a grid for which there is an affine transformation between the grid coordinates and the coordinates of an external coordinate reference system. It is defined by specifying the position in some geometric space of the grid ―origin‖ and of the vectors that specify the post locations. gml:RectifiedGrid implements ISO 19123 CV_RectifiedGrid see D.2.11 and ISO 19123:2005, 8.9 and is declared as follows: element name=RectifiedGrid type=gml:RectifiedGridType substitutionGroup=gml:Grid complexType name=RectifiedGridType complexContent extension base=gml:GridType sequence element name=origin type=gml:PointPropertyType element name=offsetVector type=gml:VectorType maxOccurs=unbounded sequence extension complexContent complexType Note that the grid limits post indexes and axis name properties are inherited from gml:GridType and that gml:RectifiedGrid adds a gml:origin property contains or references a gml:Point and a list of gml:offsetVector properties specified using gml:VectorType as its data type as described in 10.1.4.5.