SDTS Transfer Overview

A DLG-3 SDTS transfer is created from one or more Digital Line Graph, option 3 (DLG-3) files. Each DLG-3 file is converted to one SDTS two-dimensional manifold, the SDTS's term for a topologically structured planar graph with two-dimensional objects. What a user will see is a list of many files created by the conversion of one DLG format file to an SDTS format transfer.

One or multiple 2-D manifolds may be included in one transfer. Multiple 2-D manifolds are included in a transfer for (1) DLG-3 Roads and Trails, Railroads, and Pipelines which are combined into a "transportation" transfer or (2) 1:100,000-scale DLGs where a transfer consists of 7.5 or 15 minute partitions of a 30 minute by 30 minute area (transportation themes are also combined into one transfer in this situation.) In other cases, a transfer will consist of one 2-D manifold (one DLG-3 file.)

Modules Which Apply to an Entire Transfer : Except for Attribute Primary modules, there will be exactly one of each of the following type of modules. Examples in () are from a hydrology module.

Modules Which Apply to a Particular 2-D Manifold : These modules will be repeated for each 2-D manifold (corresponding to one DLG-3 file). Examples in () are from a hydrology module.

The last two characters of the module name (01 in the examples above) will be the same for a particular 2-D manifold and will correspond to the Aggregate Object subfield value in the Catalog/Spatial Domain module. The first 2-D manifold will be "01"; all digits 0-9 and all upper-case letters A-Z are possible in these two characters.

Modules Included in Master Data Dictionary Transfer : To avoid duplication, the data dictionary is transferred in a master data dictionary and includes the files listed below.

Module names are shown in the previous tables. These are standardized by the Topoligical Vector Profile (TVP). In cases where there is a potential for multiple instances of a module type within a transfer, some characters of the module name will vary. These are indicated by dashes in the names. The TVP standardizes file names to be a 4-character transfer base name followed by the module name. The extension will always be DDF.

For 1:24,000 & 1:100,000-scale DLG-3's, the transfer base name is an abbreviation for the theme contained in a transfer followed by an arbitrary two characters. For example, in a transfer of the hydrography theme, the Identification module file might be named HY01IDEN.DDF.

For 1:2,000,000-scale DLG-3's, the transfer base name is the two letter state abbreviation followed by the theme abbreviation. For example, in a transfer of the hydrography theme of the state of Virginia, the Identification module will be named VAHYIDEN.DDF.

The following abbreviations are used in file names for themes:

The SDTS model of spatial phenomena describes the real world as consisting of entities which are characterized by attributes which have attribute values.

DLG-3 does not explicitly use this entity-attribute-attribute value model. Instead it assigns any number of "attribute codes" to spatial objects. Looking more closely at these DLG-3 attribute codes, it can be seen that there are codes which identify real world entities and codes which represent attributes of those entities. The SDTS implementation of DLG-3 identifies which DLG-3 attribute codes identify entities and which identify attributes of those entities.

There are two ways in the SDTS of identifying the entity type for a particular object in an SDTS transfer. One method is to create a separate Attribute Primary module for each entity type; in this method the entity type is assigned to a particular attribute and Attribute Primary module through the Data Dictionary/Schema module. The second method is specified in the TVP: a generic ENTITY_LABEL attribute is specified by the TVP, its value identifies the entity type. This method does not require separate Attribute Primary modules for each entity type. It is this second method which is used in DLG-3 transfers.

In most cases a DLG-3 spatial object will have one code which identifies a real world entity. This DLG-3 code is used as the value for the ENTITY_LABEL attribute in the SDTS transfer. The codes which identify entities are listed for each theme.

Other DLG-3 codes are converted to appropriate values for other attributes. These attributes may have true/false values, enumerated values, or quantitative values obtained from DLG-3 codes. Attributes will vary with the DLG-3 theme.

There will always be at least one Feature Attribute Primary module (transfers of transportation will include a separate Feature Attribute Primary module for each of the three transportation themes). The Feature Attribute Primary module contains the ENTITY_LABEL attribute which contains the 7-digit DLG-3 attribute code which identifies the real world entity. Additional attributes provide additional descriptive and parameter information about the entity. These additional attributes will differ with theme.

The name of the Feature Attribute Primary module will vary with theme:

AHPF - Hypsography
AHYF - Hydrography
ASCF - Vegetative Surface Cover
ANVF - Non-vegetative Features
ABDF - Boundaries
ASMF - Survey Control and Markers
ARDF - Roads and Trails
ARRF - Railroads
AMTF - Pipelines, Transmission Lines, and Miscellaneous Transportation Features
AMSF - Manmade Features
APLF - U.S. Public Land Survey System

Each transfer may contain one Coincidence Attribute Primary module, named ACOI. This contains records indicating that a feature is coincident with a feature in another DLG-3 theme. For example, roads are frequently coincident with boundaries and the Public Land Survey. This is a separate table because there may be multiple values of the COINCIDENT attribute (coincident with multiple themes) for one spatial object. The TVP specifies that attributes which can have more than one value be separated into their own Attribute Primary module. A particular spatial object will have an attribute record for each value: a particular object may have no, one, or more attribute records to which it points.

ACOI - Coincidence for all themes

Depending upon the theme there may be additional Attribute Primary modules to handle other attributes which can have multiple values for one entity instance: elevation on Hypsography, federal agency on Boundaries, and route numbers on Roads and Trails.

ABDM - Agencies for Boundaries
ARDM - Route Numbers for Roads & Trails
AHPF - Elevation in Feet for Hypsography
AHPR - Elevation in Meters for Hypsography

Attribute Secondary Modules

Attribute Secondary modules are only included in transfers created from 1:2,000,000-scale DLG-3's. These DLG's store information in companion text files which serve as input to the conversion process. The information in these text files serve to further describe attribute major minor pairs contained in the DLG file. As mapped into SDTS, this additional information is placed in Attribute Secondary modules. Each record of these Attribute Secondary modules is linked to records in other attribute modules by relational key. Attribute module subfields that participate in these linkages are identified as a foreign or primary keys in the Data Dictionary / Schema module. When attribute records in two separate modules reference each other by relational key, the relationship between the modules is recorded by a Catalog / Cross-Reference module record.

The attribute secondary modules included in transfers for 1:2,000,000-scale DLG-3's in SDTS carry information on federal land grant names, state and county names by FIPS codes, populated place names along with the respective Census population values, and airport names. The attribute secondary names are as follows:

BFPC - County Names by FIPS Code
BFPS - State Names by FIPS Code
BMSP - Populated Place Names with Populations
BMTA - Airport Names
BGRL - Land Grant Names

Attribute Modules by Theme : The following table summarizes which Attribute modules are used for transfers of each theme.

* -- Indicated modules may be absent
** -- Indicated modules are only used for 1:2,000,000-scale DLG-3's
*** -- Indicated modules may be absent and are only used for 1:2,000,000-scale DLG-3's

For additional information regarding this Web page, contact Deb Southworth Green, in the Division of Information Technology Management, at Deb_Green@fws.gov

Return to the GIS Activities Home Page
Visit the U.S. Fish and Wildlife Service Home Page

Keywords=GIS, data, metadata, steering, standards
Last Modified January 04, 2001 08:37 AM