TAG EXTENT (GeM+)
Section [EXTENT] (REL file)

This section contains sufficient data to document the horizontal extent of the dataset. In future it should also include data about the vertical and temporal extents of the dataset.

According to the CEN this section is obligatory. Its contents are based on the ENV 12657, the ENV 12762 and the ENV 12160 European Standards. The American metadata standard CSDGM has also been taken into consideration.

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Name of GeM+ entry : Description
Name of REL file key: GeographicIdentifierCode

Description and considerations: General description of the spatial and temporal extension of the database. This entry in the metadata is taken from the ISO metadata standard which defines the extension (MD_Extent). The ISO standard considers this metadata entry obligatory if there is no other information on the extension. In the case of GeM+, as the envelope is always obligatory, this key is considered optional.

Follows the proposal of : ISO

Obligatory according to ISO: Yes, if no other information is available on the extension
Obligatory according to GeM+: No

Cardinality: 1

Key type: C

AppliesToLayer: Yes
AppliesToSerie: Yes

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Name of GeM+ entry: Calculated scene centre
Name of REL file key: (not stored in the REL file, it is dynamically deduced)

Description and considerations: The scene centre is the geographical coordinates (longitude and latitude) of the central point in the data base ( Xmin+(Xmin-Xmax)/2 , Ymin+(Ymin-Ymax)/2 ). This value is calculated dynamically from the extension of the layer and the transformation (if necessary) of the projection system of the database in the latitude-longitude projection system (maintaining the base datum). The box provides information on the identifier of the geographic reference coordinates (for example: lat/long-ED50).

If the coordinate system is local then it is not possible to calculate the scene centre in geographical coordinates.

Follows the proposal of: GeM+

Obligatory according to GeM+: It is deduced dynamically.

Cardinality: 1

Key type: C

AppliesToLayer: Yes
AppliesToSeries: Yes

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Name of GeM+ entry: Nominal scene centre
Name of REL file key: CentreEscenaNominal

Description and considerations: The user may wish to save a value for the scene centre different from the calculated value. For example, in the case of satellite image headers the original file formats (eg Landsat and CEOS format) usually store information on the nominal scene centre, which the import programs recover and document in the data set's metadata. It is interesting to keep this different scene centre value as the calculated value may vary depending on the type and quality of the georeferencing of the image, whilst the nominal centre is unique and theoretical for each image (based on information from the image producer).

Follows the proposal of: GeM+

Obligatory according to GeM+: No

Cardinality: 1

Key type: C

AppliesToLayer: Yes
AppliesToSeries: Yes

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Information related to all the keys Minimum X, Maximum X, Minimum Y, Maximum Y

The proposed European standard (developed by the CEN) defines the BoundingQuadrangle as the allowable range of coordinates in the geographic dataset. GeM+ conserves the information in the older vector and raster documentation files: MinX, MaxX, MinY, MaxY and that delimit the BoundingQuadrangle.

For a multi-band layer it is possible to document a value for each band, different from the general value for this key (see the Thematic Information section). It is for this reason that GeM+ has a radial button (when dealing with multi-band layers) that allows either the general data or the selected band data to be displayed (this choice is made using the pull-down menu and the forward and back buttons in the upper part of the GeM+).

To avoid incoherent values between different GeM+ keys and since there is an implicit relationship between the numbers of rows and columns, the cell size (in both directions) and the extent of the layers, these keys cannot be modified directly. In order to change them the "Change..." option must be selected which opens a dialog box for modifying these parameters together (changes in the numbers of rows and columns, cell size and extents).

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Name of the GeM+ entry: Minimum X
Name of the REL file key: MinX (see the key in case of raster and vector)

Description and considerations: The minimum X coordinate of the dataset. The information contained in the MinX entry of the older vector and raster documentation files is conserved. The data is given in the units given by the units key of the Horizontal Reference System section.

See complementary information, related to the keys: minimum X, maximum X, minimum Y, maximum Y.

Follows the proposal of: the CEN

Obligatory according to the CEN: Yes

Cardinality:1

Key types:F

AppliesToLayer: Yes
AppliesToSeries: Yes

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Name of the GeM+ entry: Maximum X
Name of the REL file key: MaxX (see the key in case of raster and vector)

Description and considerations: The maximum X coordinate of the dataset. The information contained in the MaxX entry of the older vector and raster documentation files is conserved. The data is given in the units given by the units key of the Horizontal Reference System section.

See complementary information, related to the keys: minimum X, maximum X, minimum Y, maximum Y.

Follows the proposal of: the CEN

Obligatory according to the CEN: Yes

Cardinality:1

Key types:F

AppliesToLayer: Yes
AppliesToSeries: Yes

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Name of the GeM+ entry: Minimum Y
Name of the REL file key: MinY (see the key in case of raster and vector)

Description and consideations: The minimum Y coordinate of the dataset. The information contained in the MinY entry of the older vector and raster documentation files is conserved. The data is given in the units given by the units key of the Horizontal Reference System section.

See complementary information, related to the keys: minimum X, maximum X, minimum Y, maximum Y.

Follows the proposal of: the CEN

Obligatory according to the CEN: Yes

Cardinality:1

Key types:F

AppliesToLayer: Yes
AppliesToSeries: Yes

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Name of the GeM+ entry: Maximum Y
Name of the REL file key: MaxY (see the key in case of raster and vector)

Description and considerations: The maximum Y coordinate of the dataset. The information contained in the MaxY entry of the older vector and raster documentation files is conserved. The data is given in the units given by the units key of the Horizontal Reference System section.

See complementary information, related to the keys: minimum X, maximum X, minimum Y, maximum Y.

Follows the proposal of: the CEN

Obligatory according to the CEN: Yes

Cardinality:1

Key types:F

AppliesToLayer: Yes
AppliesToSeries: Yes

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