Louisiana Administrative Code
Title 43 - NATURAL RESOURCES
Part XXVII - State Lands
Subpart 1 - Land Information Mapping and Map Records System
Chapter 13 - Digital Mapping Requirements
Section XXVII-1303 - Data Definitions

1. Layer. A layer can be either a complete digital map or any single component overlay for a map. Layers may contain one or more components. A planimetric map, a topographic map overlay, and a parcel map overlay would each be considered an aggregated map layer consisting of various layer categories, with each category consisting of designated component layers to allow for inclusion or deletion of specific feature elements. Sections 1309-1313, Appendices A, B, and C include a general listing of layer categories and feature elements to be included within the digital database framework for all planimetric, topographic and parcel maps to be developed under these standards.

2. Component. A component is a discrete type of feature element that, in combination with other feature elements, creates a layer. Examples of feature elements include highway route numbers, contour lines, parcel identification numbers, and map grid lines. Each feature element is composed of objects that graphically represent a component's extent (lines, configuration or area) or that present information (symbols or text).

3. Objects. Objects are the base graphic elements that represent map features. They are interrelated in a topological data structure that defines the relationship of the elements to one another. The primary objects used to create components are points, nodes, lines, and areas. Because geographic spatial data handling is still a relatively new science, a number of different terms have been used in the past to report these objects. The terminology used here draws on the current working documents from the National Committee for Digital Cartographic Data Standards. To facilitate understanding, the object definitions presented herein are simplifications of that framework (see §1315 A, Appendix D, Figure 1).

4. Topological Data Structure. Topology is the mathematical relationship of how points and lines on a map interact with one another to define geographic areas. The use of a topological data structure is essential for the effective creation and manipulation of geographic data. Topology allows the interrelationships of graphic objects to be specified. It also prevents the storage of repetitive data. Thus, one line can represent a stream and the boundary of two adjacent areas or parcels. This structure allows the generation of areas based on node and line information. Topological structures are necessary if analysis of lines or areas is undertaken. Thus, road networks stored in a format with topological structure can be used for data aggregation, maintenance inventories, school bus routing, or emergency service dispatching.

5. Digital Data Sets. Digital data sets are computer files that store geographic files that store geographic data. Each data set may contain a number of components. For example, a data set containing contour line components would also contain contour labels. A street and roads data set could include pavement extent, pavement and base course thickness, average daily traffic volume, right-of-way limits, route labels, and related information.

6. Digital Labeling. Important differences exist between the attribute points of lines and areas as opposed to label points. Attribute points and associated information are stored or linked to each line, area, or feature point. They allow users to select the specific objects that they wish to plot, report on, or analyze. Thus, all of the line segments that constitute Louisiana Highway No. 30 could be extracted and plotted. Label points are used to label an object or a series of objects on a map. Thus, two different label points may be used to identify Highway No. 30 as it crosses a map, where 13 lines, each with an associated attribute point, are used to plot Louisiana Highway No. 30 on the map (see §1315 B, Appendix D, Figure 2).

7. Edge Matching. Edge matching is the matching of lines and areas between map sheets. With digital data the match implies that each line ends with a node which is either the same node or has the same coordinates of the node of the line that continues onto the adjoining map sheet (see §1315 C, Appendix D, Figure 3).

8. Digital Mapping Product. The product of digital mapping will be topologically structured digital data sets that store map objects using Louisiana State Plane Coordinates and associated attribute information with each object. The digital mapping product that a parish or other user produces or receives may include a number of individual data sets, or all data may be integrated in a single database. Attribute data may be stored as part of the geographic data set or in an associated database. Each graphic object will be attributed according to a specified labeling scheme. Thus, a road could have only the road name stored as an attribute of a road line (e.g., Louisiana Highway No. 30), or it could have road name, type of road, road owner, right-of-way width, and other related information for each line that represents the road. Similarly, an area representing a parcel might only have the parcel identification number associated with it or it might have all information that the parish or other users maintain on that property. Associated information may be keyed in or merged into a data set from information that has already been stored in a computer readable format.

9. Coincident Features. Coincident features are those which are in common between two or more data layers. For example, if a political boundary is formed by a river bank, the river bank and political boundary are coincident features. Coincident features must be digitized only once. Regardless of the care taken in digitizing, slight differences are inevitable if a feature is digitized more than once. These differences lead to problems resulting in small "slivers" if the layers are topologically joined. Coincident features, digitized once, can be placed into master template coverages from which they can be retrieved when needed for incorporation into another layer, or they can be taken directly from the original coverage. In the above example, if the river was digitized first, the arcs necessary for the political boundary can be selected and placed in the new coverage.

10. Topological Editing. Topological editing builds polygon and arc node topology by identifying areas enclosed by arcs and creates a list of arcs which define each polygon boundary. During editing, two or more coordinates within a specified tolerance of each other (i.e., fuzzy tolerance) are snapped together and become the same coordinate point. No distinction is made between interior arc coordinates and nodes or between arcs. Because the fuzzy tolerance actually moves arc vertices, understanding its relationship to coverage resolution is important. A dangling node refers to the unconnected node of a dangling arc. Every arc begins and ends at a node point. So if an arc does not close properly, or was digitized past an intersection, it will register as a dangling node. During editing, all unacceptable dangling nodes within a specified tolerance must be removed. In some cases, a dangling node may be acceptable. For example, in a street centerline map, cul-de-sacs are often represented by dangling nodes.