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.
Data capture processes include:
Photogrammetry (historical) – Photogrammetry was the traditional method of topographic capture. Originally data was collected by an analogue method where the captured topographic features were plotted on atmospherically stable film. This method captured only relief and hydrography, with relief being the contour and spot elevations recorded manually, and hydrography stored as 2D. In 1995, this method advanced to a full 3D digital capture program of all topographic features including Relief, Drainage, Transport, Culture and Vegetation. The current method of photogrammetry is the production of high accuracy digital elevation models (DEM)’s from aerial photography. These digital elevation models (DEM)’s consists of elevation points and 3D break lines. Hydrographical features are captured as break lines in this process but are labelled separately for feature identification. Contours are auto generated from these digital elevation models (DEM)’s using terrain generation software. the elevation of
Topographic Repromat (historical) – Repromats are the plotted analogue data obtained in the earlier photogrammetric capture process. These sheets have now been scanned and digitised using various scanning software. This process has lent to some spatial degradation within the positional accuracy of the data. This data e.g., relief (contours) and hydrographic (watercourses and waterbody areas) features.
Cadastre – Features are captured from cadastral boundaries held within the Queensland Spatial Cadastral Database. This includes features such as golf courses, schools, hospitals etc. Data captured by this process is stored as 2D.
Imagery – Features are identified and digitised from orthorectified aerial photography and satellite imagery. This media is also used to assist in spatial validation of features captured by other processes. Data captured by this process are stored in the database as 2D.
Existing mapping – Features identified from existing mapping and not captured by other processes are digitised and stored as 2D features within the database. This data can come from numerous sources with various degrees of accuracies. Metadata stored with the features at the dataset level will define the spatial accuracies of the data.
Other digital datasets – Features captured by other organisations using alternate processes and available for use via licensing arrangements are validated and stored within the database. In most cases these features will be in stored as 2D.
Individual feature capture process is described in more detail later in the document.
Artificial Features
To allow spatial analysis, polygon construction and automated mapping, artificial feature types are present in the drainage network. These feature types do not exist in the real world but are required in the dataset to form the connected (concatenated and segmented) drainage network where possible.
These artificial features are crucial for geospatial data management, analysis, and modelling, as they enable a more accurate representation of the real world and facilitate the extraction of valuable information from geospatial datasets. They allow for the efficient processing of data in a way that's meaningful and useful for various applications related to drainage networks and environmental studies.
Such features include but not limited to:
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.
DEM Connector – An artificial line placed when the watercourse feature disappears beneath the landscape or land development. No visual evidence via image interpretation can been detected so the DEM Connector is placed along the route considered most likely for water flow and connects to watercourse lines at either end. DEM Connectors can be used to enforce Digital Elevation Models.
Shoreline Junction – An artificial line between two adjoining waterbody areas
Underground Connector – An artificial line placed when the watercourse feature disappears underground through pipes, tunnels, or other structures where the approximate position is known but not visible. The Underground Connector is placed along the route considered most likely for water flow and connects to watercourse lines at either end.
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.