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American National Standard for
Information Systems

Spatial Data Transfer Stan
dard - Part 2, Spatial Features

1 Introduction

This part of the standard responds to the need for common definitions of spatial features in the process of data transfer. The conceptual model and defined terms are the foundation for exchanging data between existing digital cartographic systems and may also be used as a viable approach to creating a new spatial feature file.

Traditional map symbols and their digital code equivalents often refer to real world features which have been defined only in the mind of the cartographer responsible for compiling the data. Even when agencies make an effort to define systematically the features portrayed in their data bases, these definitions are likely to vary from one organization to another. Thus, agreement on a common format for data exchange is not sufficient to ensure that the information provided by the supplier organization will be meaningful or useful to the receiving organization.

In order to facilitate transfer of data that is meaningful both to exporters and to importers, a common bridge of well understood feature definitions is required. The feature and attribute lists and definitions contained in the annexes to part 2 are a first step in this direction. These lists and definitions are the product of several years of effort during which approximately 2,600 definitions of geographic features were compared and distilled into the initial list of 200 entity types. The definitions of these entity types encompass the meanings found in the definitions of over 1,200 alternative terms.

1.1 Purpose

The purpose of part 2 is to provide a common classification and meaningful definitions for the spatial features identified in a transfer.

1.2 Objectives

Based on the sources consulted at the time of the preparation of this part of the standard, existing hydrographic and topographic features could, with few exceptions, be encoded to one of the 200 defined standard entity types, thereby facilitating transfer between organizations that might otherwise differ on the nuances of their particular term for, or definition of, essentially the same real-world phenomenon.

This list and these definitions are not presented as a definitive product, but as a living compendium subject to future amendments and extensions to meet the diverse needs of the spatial data community.

The primary focus of this initial effort is on topographic and hydrographic features, using sources available at the time of preparation. Among the extensions anticipated are (a) updating to accommodate new topographic and hydrographic products and (b) addition of features from other disciplines concerned with the exchange of spatially referenced geographic information.

Because all spatial data are likely to be exchanged in the future, organizations considering the development of new data bases may use these standard categories as the basis for classifying phenomena to be entered into such data bases.

2 Conceptual Model

2.1 Definitions

Spatial features are defined entity types and the representation of instances of those types by spatial objects, as specified in section 2 of part 1.

Spatial features shall be described by the concepts: Entity Type, Entity Instance, Attribute, and Attribute Value; and by the terms Standard and Included. These are defined as follows:

2.1.1 Entity Type - the definition and description of a set into which similar entity instances are classified (e.g., bridge).

2.1.2 Entity Instance - a spatial phenomenon of a defined type that is embedded in one or more phenomena of a different type, or that has at least one key attribute value different from the corresponding attribute values of the surrounding phenomena (e.g., the 10th Street Bridge).

2.1.3 Attribute - a defined characteristic of an Entity Type (e.g., composition).

2.1.4 Attribute Value - a specific quality or quantity assigned to an attribute (e.g., steel) for a specific Entity Instance.

2.1.5 Standard Term - primary label of an Entity Type or Attribute (listed in annex A and B).

2.1.6 Included Term - nonstandard label of an Entity Type or Attribute that is cross-referenced to a Standard Term of an Entity Type or Attribute. Included Terms used in more than one context may be cross-referenced to more than one Standard Term. (See annex C.)

2.2 Conceptual Approach

Many feature classification schemes are based on a hierarchical system. This standard specifically avoids adopting any particular hierarchy, in part because the higher level classifications differ from one organization to another. Hierarchical classification, if desired, may be constructed using standard attributes (such as air/land/water, or manmade/natural) or through user-defined non-standard attributes.

Network, relational, and object-oriented structures of feature data may also be constructed from this non-hierarchized list of standard features and attributes.

The features presented are defined independently of map or geographic scale. Scale independence implies that no specific spatial object (such as a point, line, or area representation) is necessarily used to represent a given Entity Type. Further, the list includes features defined at any level of geographic interest, from mountain ranges and oceans to sidewalks and trees.

3 Scope and Field of Application

3.1 Scope

This part of the standard applies to the terms and definitions used for Entity Types, Attributes, and Attribute Values in a transfer. The terms may appear in the Data Dictionary/Definition, Data Dictionary/Domain, Data Dictionary/Schema, Attribute Primary, and Attribute Secondary module types as specified in part 1. Definitions included in part 2 need not be repeated in a Data Dictionary/Definition module in a given transfer, provided that the Standard Term is used with its standard meaning.

The definitions of standard Entity Types are mutually exclusive, but not exhaustive. Future extensions are anticipated.

3.2 Field of Application

There are two intended applications of this part of the standard for systems employing spatial data in digital form: a) new users will have a model and the definitions needed for creating a features set, and b) transfer of the meaning of feature information between existing systems will be facilitated.

4 Relationship to Other Standards

Federal Information Processing Standard codes exist for the instances of certain Entity Types, for example states of the United States (FIPSPUB 5-2). By implication, the Entity Type for which these codes are provided (i.e., states and outlying areas of the United States) has already been recognized as a standard Entity Type.

Many federal agencies have agency standards for the classification and definition of features within their particular realms of responsibility. This standard is not intended to replace existing agency standards, but to provide a basis whereby existing data can be shared with other organizations that do not necessarily accept the same terms and definitions for similar real-world phenomena. Depending on the conformance level selected and the functional requirements of a particular transfer, each agency may find it useful to develop a cross-reference between its internal feature and attribute classification system and the Standard Terms and definitions in annexes A and B.

5 References

Selected references used in the preparation of part 2 of the standard are included in annex F of part 1. The following is a short list of the authors included in these references:

• Canadian Council on Surveying and Mapping
• Defense Intelligence Agency
• Defense Mapping Agency
• Monkhouse, F.J.
• Moore, W.G.
• Stamp, D.
• U.S. Geological Survey, Geographic Names Information System
• U.S. National Ocean Service

This list will be updated and expanded as additional feature lists and definitions are submitted to the maintenance authority.

6 Conformance

All entity type, attribute, and attribute value terms and (or) codes used in a transfer shall be defined. Non-standard Entity Type and Attribute terms shall be specified through the use of the Data Dictionary/Definition module. Attribute Value terms shall be defined in the Data Dictionary/Domain module. See also part 1, 4.1.3.6 (Attributes).

The mandatory Features Level subfield of the Identification Module specifies the conformance level of a transfer with respect to feature definitions (see part 1, 5.2.1.2.3). Use of conformance levels 2, 3, or 4 for features limits the scope of transfer and are discouraged. Recurring needs to resort to non-standard terms and definitions should be referred to the maintenance authority, for possible amendment and extension of the standard.

7 Status of Annexes

Annex A contains definitions for standard Entity Type terms. Annex B contains definitions for standard Attribute terms. Annex C contains cross-reference lists between Included Terms and Standard Terms.

These annexes are normative in that they shall be used for transfers of feature conformance levels 1 and 2, as described in part 1, 5.2.1.2.3. However, use of the terms and definitions contained in these annexes is optional for feature conformance levels 3 and 4.