Section 2: Implementation of DLG-E
2. Implementation of DLG-E
This section will discuss how National Mapping Division (NMD) of the USGS is approaching the implementation of the DLG-E model. The product based on this model will be the next generation of NMD's topological-vector-based digital product. The information in this section provides a context for the mapping decisions in Part 2 and Part 3. In describing a mapping, invariably aspects of the environment surrounding the production and management of datasets has an impact. All of the content to satisfy the SDTS requirements is not contained in the dataset. Viewing the environment in which the dataset is produced helps identify the other sources of information that are needed to contribute to a "self-contained" transfer.
The information in this section will illustrate why the mapping to the SDTS is a prototype mapping and likely to change.
The DLG-E Production System is being developed to implement the DLG-E model. One of its main objectives is for the collection software to be content-independent. The addition of a new feature type or of an attribute should not cause a change in the software. This objective has been successfully met.
This is not at all obvious from the user-interface perspective. It is implemented using X and the menus are filled with content-specific choices. So where do the content-specifics come from? There is an on-line Content Standards Database that provides the information to the collection software (see 2.2). Although the content is not "hardcoded" into the software, it has access to it and uses it to assist the cartographic technician in the data collection process.
Another objective of the implementation is to make the software portable between hardware platforms. This principle applies to portable source code as well as "data files". The DLG-E Portable Data File (DPDF) is the file set that was designed to move the internal data structures of the processing software between hardware platforms. The description of the DPDF was useful in identifying other information that would aid in populating a DLG-E/SDTS transfer. (In Part 2, these DPDF elements are often treated as attributes of the objects they describe.)
The DLG-E Production System is still under development. However, there were enough pieces in place to produce some prototype DLG-E datasets in 1993. These will be the first DLG-E/SDTS datasets released to the public.
2.2 Content Standards Database
The DLG-E Content Standards Database (DSDB) is the repository of NMDs' content specification for the DLG-E product. The objective in developing the DSDB is to support all aspects of DLG-E data collection. This includes generation of data collection instructions (i.e. NMD's "templates"), supplying content specifics to the collection software, providing a basis for on-line help, supporting interactive training tutorials to help employees learn the content, and other related activities.
The Content Standards Database contains (or will eventually contain) the following types of information:
The information in the DSDB is a valuable source in meeting the requirements of the SDTS Data Dictionary.
The Content Standards Database design is still under development. And the content with which the database is populated is still evolving. However, there were sufficient portions of the DSDB in place to support a prototype release of a production system (Dec 1992.) The content-population reached a sufficient level to perform data collection for a prototype DLG-E product in 1993.
This section contains some examples of features, attributes, relationships and representation rules to illustrate the contents of the DSDB.
Consider a feature type of "lake/pond." It is described by attributes "hydrographic category", "impoundment status", "elevation", "name", and "water characteristics." The domains for the attributes when applied to feature type "lake/pond" are in the following table.
Feature Type: Lake/Pond a standing body of water with a predominantly natural shoreline surrounded by land.
-------------------------- -----------------------------------------------------------------------------------
Attribute Definition Domain HV Attribute Definition Doamin HV
======================================================================================================================
Hydrographic portion of Intermittent N -- -- -- --
Catagory the year the Perennial
feature Unknown
contains
water.
----------------------------------------------------------------------------------------------------------------------
Impoundment Whether an Impounded N -- -- -- --
Status area is Unspecified
atrificially
impounded or
natural
----------------------------------------------------------------------------------------------------------------------
Elevation The vertical (integer) Y Stage height of Average Water Elevation N
distance from water Date of Photography
a given datum surface High water Elevation
Normal Pool
Spillway elevation
----------------------------------------------------------
Source derivation Interpolated N
n of the Non-interpolated
value
-----------------------------------------------------------------------------------
unspecified -- -- -- --
----------------------------------------------------------------------------------------------------------------------
Name proper name, (alphanumeric T -- -- -- --
specific ) Unspecified
term, or epression
----------------------------------------------------------------------------------------------------------------------
Water distinctive Salt N -- -- -- --
Characteristic properties of Unspecified
the water
======================================================================================================================
For the feature type "lake/pond", the table lists the assigned set of attributes, their domains, and whether single or multi-valued. Both "elevation" and "name" can have multiple values, with the others having only single values. The "lake/pond" also has attributes on one of its attribute values. When the attribute value for "elevation" is numeric, it has attributes "stage" and "source" when applied to feature type "lake/pond."
In the NMD content, each feature type has rules for which feature objects may be used to represent it and within which relationships it can participate. A "lake/pond" is represented as either a 0-D BFO or a 2-D BFO. A 2-D BFO "lake/pond" can participate in a bounded-by relationship with a 1-D BFO "shoreline." 0-D or a 2-D BFO "lake/pond" can participate in network relationships (flows to, flows from, and connects to). As another example, "stream/river" is represented as either a 1-D BFO or a 2-D BFO. A 1BFO "stream/river" can participate in network relationships (flows to, flows from, connects to), but not in boundary relationships (bounds, bounded by) or vertical separations (above, below). As a 2-D BFO, it can only participate in network relationships and the boundary relationship "bounded by."
NMD content requires every feature object to belong to at least one theme. Feature types "lake/pond" and "stream/river" have their instances associated with the "hydrography" theme. Some instances may belong to more than one theme. Consider a "lake/pond" instance that is in both the "hydrography" theme and the "land use/land cover" theme. Some feature instances belong to a theme depending on the value of an attribute. For example, an instance of feature type "well" with value "water" or "heat" for attribute "product" belongs to the" hydrography" theme, otherwise it belongs to the "built-up" theme.
The DLG-E model establishes the framework within which the NMD content is defined. The distinction between content and model can be difficult to discern as both are still evolving. The model mapping to the SDTS reflects the NMD content restrictions in some cases. This keeps the complexity of the mapping to a minimum while still preserving the DLG-E model constructs.
2.4 DLG-E Development Continues
This document captures a snapshot of the development work relating to DLG-E. Development continues in all aspects. The DLG-E Model, NMD's implementation of it, and NMD's content specification for a DLG-E product are all still evolving.
Some of the data model issues that have come up are multi-tile support, a seamless database, and multi-level compound objects. Some of the issues that are in the gray area between model and content are the handling of void areas, tracking of historical change, and the encoding of time for temporal analysis.
The NMD is continuing to define its own specific content with regards to the DLGmodel. Currently, NMD content does not require the full flexibility of the DLG-E model. For example, relationships are allowed to have attributes, but NMD content does not use this option. The DLG-E compound feature object can be composed of other CFOs, but NMD content restricts a CFO to be composed of only basic feature objects. Some of these restrictions have affected the mapping to the SDTS. The mapping reflects some of the "grayness" between the evolving definition of the model and the evolving definition of content.
This discussion serves to illustrate the point that the mapping to SDTS is a prototype mapping and likely to change.
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