Tiled Gridded Coverage Data ¶

The “GeoPackage Extension for Tiled Gridded Coverage Data” extension (previously titled Elevation Extension) defines how to encode and store tiled regular gridded data, such as a digital elevation model, in a GeoPackage. The tiles contain values, such as elevation, temperature or pressure, and are stored in one of two encodings:

The PNG encoding uses PNG files to store 16-bit integer values and a scale and offset may be applied to fine-tune the coverage range.
The TIFF encoding uses TIFF files to support 32-bit floating point data. To simplify development, this encoding constrains many of the TIFF options to the minimal set needed to meet the floating-point requirement.

gpkg_extensions ¶

To use this extension, add the rows to this table as described by Table 1.

To use this extension, add the following rows to this table as described in Table 1.

Table 1: `gpkg_extensions` :widths: 20 15 15 40 10 :header-rows: 1¶
`table_name`	`column_name`	`extension_name`	`definition`	`scope`
`gpkg_2d_gridded_coverage_ancillary`	NULL	`gpkg_2d_gridded_coverage`	http://docs.opengeospatial.org/is/17-066r1/17-066r1.html	read-write
`gpkg_2d_gridded_tile_ancillary`	NULL	`gpkg_2d_gridded_coverage`	http://docs.opengeospatial.org/is/17-066r1/17-066r1.html	read-write
tile pyramid user data table name	`tile_data`	`gpkg_2d_gridded_coverage`	http://docs.opengeospatial.org/is/17-066r1/17-066r1.html	read-write

gpkg_contents ¶

Like any other content type, add a row for each coverage, using a data_type of “2d-gridded-coverage”.

gpkg_spatial_ref_sys ¶

Like any other content type, the SRS for your contents must be registered in this table. Much like GeoPackage provides a default SRS (EPSG::4326) for normal use, when this extension is in use, EPSG::4979 (WGS-84 with height above ellipsoid) is provided as a default. However, any valid SRS may be used.

gpkg_2d_gridded_coverage_ancillary ¶

Add a row to this table for each coverage as described in Table 2.

Table 2: `gpkg_2d_gridded_coverage_ancillary` :widths: 20 80 :header-rows: 1¶
Column	Value
`id`	Primary key
`tile_matrix_set_name`	tile pyramid user data table name
`datatype`	“integer” or “float”
``scale``*	Scale as a multiple relative to the unit of measure (Default 1)
``offset``*	The offset to the 0 value (Default 0)
`precision`	The smallest value that has meaning for this dataset
`data_null`	The value that indicates NULL
`grid_cell_encoding`	“grid-value-is-center” (default), “grid-value-is-area”, or “grid-value-is-corner”
`uom`	Units of Measure for values in the grid coverage (see UCUM)
`field_name`	Type of Gridded coverage data (default is “Height”)
`quantity_definition`	Description of the values contained in the gridded overage (default is “Height”)

* Only used for “integer” datatype

gpkg_2d_gridded_tile_ancillary ¶

Add a row to this table for each coverage tile as described in Table 3.

Table 3: `gpkg_2d_gridded_tile_ancillary` :widths: 20 80 :header-rows: 1¶
Column	Value
`id`	Primary key
`tpudt_name`	tile pyramid user data table name
`tpudt_id`	Foreign key to `id` in tile pyramid user data table
``scale``*	Scale as a multiple relative to the unit of measure (Default 1)
``offset``*	The offset to the 0 value (Default 0)
``min``**	Minimum value of this tile
``max``**	Maximum value of this tile
``mean``**	The arithmetic mean of values in this tile
``std_dev``**	The standard deviation of values in this tile

* Only used for “integer” datatype

** These values are natural, not scaled or offset

Using Scale and Offset¶

Integer values may be scaled and offset in order to make more efficient use of 16-bit integer space available in PNG files. The scale and offset may be applied to the entire coverage and/or the individual tile. The scale and offset do not apply to the data_null value.

Actual cell values are be calculated by:

Multiplying the stored value by the gpkg_2d_gridded_tile_ancillary_table.scale value and then adding the gpkg_2d_gridded_tile_ancillary_table.offset,
Multiplying that value by the gpkg_2d_gridded_coverage_ancillary.scale value and then adding the gpkg_2d_gridded_coverage_ancillary.offset.

In pseudo-code, this conversion would look like:

elevationInUnitOfMeasure = (SomeGrid_RegularCoverage.tile_data→pngpixels[i] *
gpkg_2d_gridded_tile_ancillary.scale + gpkg_2d_gridded_tile_ancillary.offset) *
gpkg_2d_gridded_coverage_ancillary.scale + gpkg_2d_gridded_coverage_ancillary.offset;

PNG Encoding¶

Encode integer data using the PNG format. Pixel values are 16-bit unsigned integer (single channel - “greyscale”).

TIFF Encoding¶

Encode floating point data using the TIFF format. Tiles are encoded as a single-band image using one 32-bit floating point component per pixel. Each TIFF encoded tile is a baseline TIFF as defined in the Part 1 of the TIFF standard.

This implies a number of constraints which are listed below:

Only one band per TIFF tile, i.e., for any pixel in the TIFF tile, there SHALL be only one component
All pixels in the tile SHALL be set with a valid component value
Other TIFF tags are derived from the other constraints in this extension
No multi-image/tiling extensions

The only allowed encoding extensions are:

IEEE floating point
LZW compression

Tiled Gridded Coverage Data¶

gpkg_extensions¶

gpkg_contents¶

gpkg_spatial_ref_sys¶

gpkg_2d_gridded_coverage_ancillary¶

gpkg_2d_gridded_tile_ancillary¶