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Data types represented in Queensland Government’s Topographic Data set, comprises of vector data models containing objects in point, line, and polygon format. Attributes are associated with these geometric objects to provide additional information. The vector data model is suitable for representing discrete, well-defined features and is often used for mapping, cartography, and spatial analysis.
Features
Most items within the Digital Topographic Data are known as features. Features are objects of the real-world with a spatial and geographical component.
Feature Class
A "feature class" is a fundamental concept used to organize and manage geographic data. Feature classes are part of the geodatabase structure and represent a collection of geographic features with the same geometry type (point, line, or polygon) and the same attribute schema.
A feature class is a collection of feature types. A feature class can consist of a single feature type or multiple feature types. Metadata at the feature class level describes the accuracy of the data and where that data was obtained from: that is, the lineage of the data. Feature classes can also be known as datasets.
Derived Datasets
A derived dataset is a subset of a larger dataset. The purpose of these derived datasets is more efficient and focused datasets for specific needs e.g., emergency services for disaster management without the need to duplicate capture and maintenance in multiple datasets. They are also viewed/portrayed as a feature class.
Within the Digital Topographic Data, the following datasets are derived datasets:
· Community Facility Points
· Emergency Services Points
· Homesteads
· Shorelines
Feature Structure
In the vector data model, spatial features are represented as discrete geometric objects. These objects include points, lines, and polygons.
· Point Features: Represent singular, discrete geographic locations, or objects, encompassing specific landmarks like population centres, wells, tourist points, helipads, or monitoring stations. Point features are typically defined by a single set of x, y (and optionally, z) coordinates.
· Line Features: Portray linear or one-dimensional geographic elements such as railways, rivers, pipelines, or coastline. These features are characterized by a sequence of interconnected points.
· Polygon Feature: Capture areas or regions on the Earth's surface enclosed by one or more closed boundaries. Examples include land parcels, built-up areas and islands. Polygons are typically defined by a series of interconnected points, forming a closed shape.
· Multi-part Features: Constitute a distinct category capable of representing intricate or multipart geometries. They involve multiple interconnected points, lines, or polygons. For instance, a multi-part polygon might symbolize an island group, where each island serves as a component of the entire feature.
Feature Geometry
All features are defined spatially by their geometry type and vector coordinates. The following geometry types are supported.
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· Points - Defines point features or area features represented by a point that are displayed in mapping processes by a symbol.
· Multi-points that are two or more points that are treated as one feature are supported.
· Line/Polylines - A chain consisting of a sequence of non-intersecting line segments. A vertex is a change in direction along the length of a line segment or at the end of the line string.
· Multi-polylines are two or more polylines, not adjoining each other and treated as one feature.
· Polygons - A bounded continuous region consisting of an interior area.
· Area-connected planar polygon with one external boundary are polygons that do not completely cover the interior area of the polygon. A void area within the polygon is not part of the same feature.
· Multi-polygons are two or more polygons, not adjoining each other and treated as one feature. Multi-polygons can consist of polygons and holey polygons together.
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Table 1 - Feature geometry |
· All polygon boundaries must be closed.
· Polygon features may contain an inner set of holes or voids that can be assigned to any feature class. For example, a Lake may have in the middle of it an area of dry land (terrestrial island).
Feature Capture
Digital Topographic Data has been collected from numerous sources over a long period of time with differing levels of accuracy and capture specifications. Historically data was captured through photogrammetry for most relief and hydrological features. More recently, data for features have been collected through 2D digitising off existing mapping, photography, satellite imagery or manipulation of existing datasets to create new geographical dictionary or directory used to provide information about places, locations, and geographic features. The origin and accuracy of the source data is recorded in metadata and feature attributes.
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· Connector – An artificial line placed down the centre of a waterbody area (such as a lake) and connects to the watercourse lines on either end of the waterbody area. Forming a continuous linear hydrographic network used in spatial analysis and automated mapping processes. For further information on Connectors and their capture, refer to appendix B
· Coastline Junction – An artificial line separating mainland or marine island waterbody areas from the sea or openings to the sea. They assist in polygon generation for automated mapping processes and to generate a continuous coastline when required.
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· Stream ordering – Stream ordering values are assigned to identified watercourses lines in a stream network. Forming a hierarchy of the streams from the top to the bottom of the catchment by assigning numerical values (1-9). This order is a method for identifying and classifying types of streams based on their numbers of tributaries. For further information, refer to Appendix C. .
Attribution
Spatial data attribution is integral to spatial data analysis, as it provides the context and information needed to make informed decisions. When working with spatial data, users need to understand the meaning and limitations of attribute data, perform data validation and quality checks, and use appropriate methods to extract meaningful insights from the combined geographic and attribute data.
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