Tile Scaling

Extension Title

Tile Scaling

Introduction

This extension defines tile scaling behaviors for tile tables. When tiles at a requested zoom level are not present, alternate zoom level tiles may be scaled in place. The extension configures the allowable zoom directions and distance from the requested tile.

Extension Author

National Geospatial-Intelligence Agency, author_name nga

Extension Name or Template

nga_tile_scaling

Extension Type

New Requirement

Applicability

This extension applies to the table names of tile user tables (2.2.8).

Scope

Read-write

Requirements

GeoPackage

Extension Table Entry

The Tile Scaling extension is registered into the gpkg_extensions table as follows:

`gpkg_extensions`
table_name	column_name	extension_name	definition	scope
user tile table name	null	`nga_tile_scaling`	URL to this extension description	read-write

Tile Scaling Table

The Tile Scaling Table contains tile table names and the desired scaling behavior configurations.

`nga_tile_scaling`
Column Name	Type	Description	Null	Key
`table_name`	TEXT	Tile Table name, foreign key to `gpkg_tile_matrix_set`	no	PK, FK
`scaling_type`	TEXT	The scaling behavior type: ('in','out','in_out','out_in','closest_in_out','closest_out_in')	no
`zoom_in`	INTEGER	Max zoom levels in (higher zoom level) to search from a requested zoom level, null for no limit, ignored when `scaling_type` is 'out'	yes
`zoom_out`	INTEGER	Max zoom levels out (lower zoom level) to search from a requested zoom level, null for no limit, ignored when `scaling_type` is 'in'	yes


CREATE TABLE nga_tile_scaling (
  table_name TEXT PRIMARY KEY NOT NULL,
  scaling_type TEXT NOT NULL,
  zoom_in INTEGER,
  zoom_out INTEGER,
  CONSTRAINT fk_nts_gtms_tn FOREIGN KEY('table_name') REFERENCES gpkg_tile_matrix_set('table_name')
  CHECK (scaling_type in ('in','out','in_out','out_in','closest_in_out','closest_out_in'))
);

Tile Scaling Type

The Tile Scaling Type defines the zoom search directions and order from a requested tile.

`scaling_type`
Name	Description
in	Search for tiles by zooming in
out	Search for tiles by zooming out
in_out	Search for tiles by zooming in first, and then zooming out
out_in	Search for tiles by zooming out first, and then zooming in
closest_in_out	Search for tiles in closest zoom level order, equal zoom level distance prefers zoom in before zoom out
closest_out_in	Search for tiles in closest zoom level order, equal zoom level distance prefers zoom out before zoom in

GeoPackage SQLite Configuration

None

GeoPackage SQLite Extension

None

Examples

The rivers GeoPackage has a rivers_tiles tile table that is configured for tile scaling.

The example GeoPackage has tile tables configured for tile scaling.