The DIgital Geographic Exchange STandard
There are two basic types of spatial data supported by DIGEST. These are spatial data defined in terms of boundaries and spatial data defined in terms of sets of attribute values distributed over an area by a “coverage” function. Spatial data defined in terms of boundaries are commonly called “vector” data. Spatial data defined by the distribution of attributes over an area can be further subdivided primarily into “raster” data and “matrix” data, where raster data addresses the distribution of radiometric information over an area such as imagery, and matrix data addresses the distribution of non-radiometric information such as elevation values over an area. Other spatial data defined by special coverage functions such as Triangular Irregular Networks (TINs) are not presently addressed by DIGEST.
These types of data are listed below:
- vector data (at various levels of topology);
- raster or image data;
- matrix data.
These types of data are complementary in that they provide different methods of representing information about the same geographical area.
Vector data represents geographic information in terms of the boundaries and attributes of individual features. Features are real items that can be identified on the earth, such as a river or a tower, or they are abstract items such as political boundaries. Attributes may be ascribed to features, which describe what type of item that they are and also provide information with respect to the particular instance of the item. For example, a feature code might describe that a particular item is “bridge” and an attribute code may describe that the bridge is capable of supporting a specified load. Features may be either of Point, Line or Area type or be of Complex type composed of other Point, Line, Area, Text or other Complex features. The spatial extent of features is described in vector data in terms of Node, Edge and Face elements. These primitive elements carry positional attributes. The relationship between these primitive spatial elements is optionally described in terms of topology. For example, whether a point feature is contained in a particular area feature is described in terms of the containment of the corresponding entity point (node) element within a face element. DIGEST permits topological relationships to be described in a vector dataset to four different levels of sophistication. DIGEST provides a single vector data model, which can carry any of the four identified levels of topology
- Level 0 has a post spacing of approximately 900 meters.
- The most basic vector dataset (level 0 topology) does not carry any topological relationship information. This is known as “Spaghetti” data.
- The next level of data is “Chain-Node” (level 1 topology) where all edges terminate on nodes. This describes the connectivity of the dataset, but edges are permitted to cross without intersecting.
- The next level of vector data is “Planar Graph” data (level 2 topology) where a node must exist at the intersection of each edge. Adjacency information is carried at this level of topology.
- The highest level of topology, currently supported by DIGEST vector data, is termed “Full Topology” (level 3 topology) where the dataset forms a mathematical 2D manifold, complete with faces. Containment information is carried at this level of topology.
The DIGEST spatial schema for vector data is aligned with the ISO / TC211 Spatialschema work (as represented in ISO FCD 19107). The DIGEST vector geometry and the four levels of topology correspond with equivalent structures within the ISO standardization work. In addition, the DIGEST Feature model for vector data corresponds to the General Feature Model defined in the ISO / TC211 work on Application Schema (as represented in ISO FCD 19109).
Raster or image data represents geographic information in terms of an array of picture elements, corresponding to the individual colours or greyscale for each small (raster) segment of a map or image or other geographic information. A raster file is defined as a distribution of picture elements together with a traversal order, which is normally a set of rows and columns of picture elements (pixels). Each pixel may take on a given colour (or greyscale). Colour may be described either as a set of Red, Green, or Blue values for the individual pixel, or as a reference to a colour lookup table which indirectly describes the Red, Green, or Blue colour values (or greyscale). A significant amount of support metadata is carried along with the set of pixels to put the raster data set in context. If the raster dataset was created by scanning a paper map or chart, the projection, map grid, reference points, and other information about the scanned source paper map is carried. If the data is from some other source, information about the source is carried. Matrix data represents geographic information as a set of values measured on a grid of points. For example, a Digital Terrain Elevation Model consists of a set of elevations measured on a regular grid of points.
The DIGEST schema for raster and matrix data is aligned with the work in both ISO / TC 211 Geographic Information / Geomatics and ISO JTC1 SC24 Computer Graphics and Image Processing. ISO / TC211 is defining general Coverage Geometry and Functions (as represented in ISO WD 19123) and other aspects of Imagery and Gridded Data Components (as represented in ISO RS 19124). ISO JTC1 SC24 has developed the Basic Imagery Interchange Format (BIIF) in ISO 12087-5. DIGEST Part 2 Annex D is closely aligned with BIIF.
Theoretically, raster and/or matrix data can provide the geometry definition for attributed features, however, this is currently not addressed in DIGEST.
Note: The reference to ISO WD (Working Draft), FCD (Final Committee Draft), and RS (Review Summary) are valid as of the date of publication of DIGEST 2.1. These ISO documents are evolving and DGIWG will ensure that future versions of DIGEST reference (and are aligned where possible) the most current applicable ISO standards.
DIGEST 2.1 Part 1 General Description
DIGEST 2.1 Part 2 Theoretical Model Exchange Structure and Encapsulation Spec
DIGEST 2.1 Part2A ISO 8211 Encapsulation Specification
DIGEST 2.1 Part 2B ISO 8824 Encapsulation Specification
DIGEST 2.1 Part 2C Vector Relational Format
DIGEST 2.1 Part 2D Image Interchange Format
DIGEST 2.1 Part 2D Image Interchange Format Errata
DIGEST 2.1 Part 2E Standard ASCII Table of Contents
DIGEST 2.1 Part 3 Codes and Parameters
DIGEST 2.1 Part 4 Annex A Feature Codes
DIGEST 2.1 Part 4 Annex B Attribute and Value Codes
DIGEST 2.1 Part C Alphabetized Content Listing of all Features and Attributes
DIGEST 2.1 Part 4 FACC
DIGEST 2.1 Support Document 1 – Geodesy Background Notes
DIGEST 2.1 Support Document 2 -Table of Encoding Options for Geographic Entities
DIGEST 2.1 Support Document 3 – The ARC System
F E I S (FORMAT D’ECHANGE D’IMAGES SAIM) – Profil d’implementation de Digest Image Profile (DIP) 1.1
F E I S (FORMAT D’ECHANGE D’IMAGES SAIM) – ANNEXES