GML 3.1.1 change request - Correct grid cell terms

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CR-Form-v3

CHANGE REQUEST

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Proposed change affects:  AS Imp Spec Best Practices Paper X Other

Title:  GML 3.1.1 change request - Correct grid cell terms

Source:  Arliss Whiteside, Abstract Specification Topic 2 change request OGC 08-010r1

Work item code:  Date:  2008-07-29

Category:  C

Use one of the following categories: F (Critical correction)

A (corresponds to a correction in an earlier release) B (Addition of feature),

C (Functional modification of feature) D (Editorial modification)

Reason for change:  Abstract Specification Topic 2 change request OGC 08-010r1 has corrected the terms for “grid cell” in Tables 33, 39, and 40 to be consistent with ISO 19123 terms. Now the corresponding changes need to be made in GML 3.1.1.

Summary of change:  Correct descriptions of ImageDatum and pixelInCell XML elements

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Consequences if 

not approved:

Significant discrepancy between GML 3.1.1 and corrected Abstract Specification Topic 2

Clauses affected:  11.5.1.14, 11.5.4.3, 11.5.4.4, datums.xsd

Other specs  X Other core specifications  grid CRSs profile, WCS 1.1

Affected: Abstract specifications

X Best Practices Papers Definition identifier URNs in OGC namespace [OGC 07-092r1]

Supporting Doc. 

Other comments: 

Status 

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Edit Clauses 12.5.1.4, 12.5.4.3 and 12.5.4.4 of OGC 03-105r1 as follows:

12.5.1.4 Image datum

An image datum is used in a local context, to describe the origin of an image coordinate reference system. The image (coverage) grid is defined as the set of lines of constant integer ordinate values. The term “image grid” is often used in other standards to describe the concept of Image CRS. However, care shall has to be taken to correctly interpret this term in the context in which it is used. The term “grid cell” is often used as a substitute for the term “pixel” , but that is not correct based on ISO 19123 [6]_{. Each pixel is centred at a grid}

point, which is the corner of (up to) four coverage grid cells.

The origin of the image pixel (coverage) grid is at an image pixel identified by the anchorDefinition. This pixel is usually either at the centre of the image or is the first-recorded corner of that image.

The grid lines of the image may be associated in two ways with data values of the pixel or grid cell (ISO 19123). The data values of the image usually represent an average or integrated value that is associated with the entire pixel.

a) An image grid can be associated with data values such that the grid lines run through the centres of the pixels. The cell centres will thus have integer coordinate values. In this case the “pixel in cell” will have the value “cell centre”.

b) Alternatively, the image grid may be defined such that the grid lines define the cell or pixel corners rather than the cell centres. The cell centres will thus have non-integer coordinate values, the fractional parts always being 0.5. ISO 19123 calls the grid points in this latter case “posts” and associated image data: “matrix data”. The “pixel in cell” will now have the value “cell corner”.

The exact origin of the image coordinate system may be associated in two ways with a pixel or grid point data. In the most frequent case, the grid is associated with the pixel data in such a way that the grid lines run through the centres of the pixels, producing grid points at the centres of the pixels. The grid points and (recorded) pixels will thus have integer coordinate values. The image coordinate system origin is then the anchorDefinition grid point recorded for that image, at the position of that recorded image pixel (coverage range) value. In this case, the attribute “pixel in cell” will have the value “cell corner”.

Alternatively, the exact origin of the image coordinate system may be defined such that the grid lines are associated with pixel corners rather than the coverage grid cell corners. In this case, the extent of each pixel value is crudely approximated by a cell that is offset from the coverage grid cells, so that the pixel cell corners lie at the centres of these offset grid cells. The grid cell corners and pixel locations will thus have non-integer coordinate values, the fractional parts always being 0,5. The image coordinate system origin is then the centre of the grid cell whose inside corner is the anchorDefinition grid point recorded for that image, with this corner at position of the anchorDefinition recorded image pixel (coverage range) value. The attribute “pixel in cell” will now have the value “cell centre”.

This difference in perspective has no effect on image interpretation, but is important for coordinate transformations using coordinates in involving this image CRS.

12.5.4.3 gml:ImageDatum

</extension>

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</complexContent> </complexType>

An image datum defines the origin of an image coordinate reference system, and is used in a local context only. For an image datum, the anchor is usually either the centre of the image or the corner of the image, as indicated by the anchorDefinition. As indicated by the pixelInCell, the exact origin may be at a grid point (the centre of a pixel or the corner of a grid cell). However, the exact origin may alternately be the centre of a grid cell (half way between grid points or the centres of pixels). For more information, see Subclause 12.5.1.

12.5.4.4 gml:pixelInCell

</simpleContent> </complexType>

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Edit fragment of the datums.xsd file for GML 3.1.1 as follows:

<documentation>An image datum defines the origin of an image coordinate reference system, and is used in a local context only. For an image datum, the anchor definition is usually either the centre of the image or the corner of the image, as indicated by the anchorDefinition. As indicated by the pixelInCell, the exact origin may be at a grid point (the centre of a pixel or the corner of a grid cell). However, the exact origin may alternately be the centre of a grid cell (half way between grid points or the centres of pixels). For more information, see OGC Abstract Specification Topic 2. </documentation>

</annotation> <element name="pixelInCell" type="gml:PixelInCellType"/>

<complexType name="PixelInCellType">

<documentation>Specification of the way an image coverage grid is associated with the image data coverage range (pixel) valuesattributes. </documentation>

</annotation> <simpleContent>

<documentation>Reference to a source of information specifying the values and meanings of all the allowed string values for this PixelInCellType. </documentation>

</annotation> </attribute> </restriction> </simpleContent> </complexType>