Spatial Data Collection Lecture Notes PDF

Kwame Nkrumah University of Science & Technology, Kumasi, Ghana Lecture 3 Spatial data collection and management Ing. G. Ashiagbor Dept. Wildlife and Range Management Facu...

Kwame Nkrumah University of Science & Technology, Kumasi, Ghana Lecture 3 Spatial data collection and management Ing. G. Ashiagbor Dept. Wildlife and Range Management Faculty of Renewable Natural Resources E-mail: [email protected] Office # : 115 FRNR Two fundamental ways of representing geographic data 1. Raster (fields): represents the real world as a finite number of variables, each one defined at each possible position. ¥ f ( y) = ò f ( x , y )dx x = -¥ Continuous surface f(x,y) y x 2. Vector (Discrete objects): represents the real world as objects with well defined boundaries in empty space. (x1,y1) Points Lines Polygons www.knust.edu.gh Raster and Vector Data Raster data are described by a cell grid, one value per cell Vector Raster Point Line Zone of cells Polygon www.knust.edu.gh Vector and Raster Representation of Spatial Fields Vector Raster www.knust.edu.gh  Raster Representation A grid defines geographic space as a matrix of identically-sized square cells. Each cell holds a numeric value that measures a geographic attribute (like elevation) for that unit of space. www.knust.edu.gh The grid data structure Grid size is defined by extent, spacing and no data value information Number of rows, number of column Cell sizes (X and Y) Top, left , bottom and right coordinates Grid values Real (floating decimal point) Integer (may have associated attribute table) www.knust.edu.gh Definition of a Grid Cell size Number of rows NODATA cell (X,Y) Number of Columns www.knust.edu.gh The grid data structure Points as Cells Line as a Sequence of Cells Polygon as a Zone of Cells www.knust.edu.gh  GIS data types GIS technology utilizes two basic types of data 1. Spatial data: describes the absolute and relative location of geographic features. 2. Attribute data : describes characteristics of the spatial features. These characteristics can be quantitative and/or qualitative in nature. Attribute data is often referred to as tabular data. The coordinate location of a forestry stand would be spatial data, while the characteristics of that forestry stand, e.g. cover group, dominant species, crown closure, height, etc., would be attribute data. spatial data models have evolved for storing geographic data digitally. These are referred to as: Vector and Raster/Images www.knust.edu.gh  Attribute data models A separate data model is used to store and maintain attribute data for GIS software. These data models may exist internally within the GIS software, or may be reflected in external commercial Database Management Software (DBMS). The most common are: 1. Tabular 2. Hierarchial 3. Network 4. Relational 5. Object Oriented www.knust.edu.gh GIS Data entry Spatial data can be obtained from various sources. 1. Using direct spatial-data acquisition techniques, i. survey data ii. remotely sensed images 2. Indirectly by making use of existing spatial data collected by others. i. Printed maps ii. Existing digital data sets. www.knust.edu.gh Spatial data Sources One way to obtain spatial data is by direct observation of relevant geographic phenomena. This can be done through ground-based field surveys or by using remote sensors on satellites or aircraft. Data that are captured directly from the environment are called primary data. E.g.: 1. Aerial surveys and satellite remote sensing 2. Terrestrial/land surveys 3. Field surveys 4. Mobile GIS www.knust.edu.gh Spatial data Sources spatial data that is captured/sourced indirectly is known as secondary data. E.g.: 1. data derived by scanning existing printed maps, 2. data digitized from a satellite image, 3. processed data purchased www.knust.edu.gh Digitizing Digitizing in GIS is the process of converting geographic data either from a hardcopy or a scanned image into vector data by tracing the features. During the digitizing process, features from the traced map or image are captured as coordinates in either point, line, or polygon format. There are two forms of digitizing: 1. On-tablet : In on-tablet digitizing, the original map is fitted on a special surface (the tablet) 2. Heads up digitizing/On-screen manual digitizing: in on-screen digitizing, a scanned image of the map (or some other image) is shown on the computer screen. www.knust.edu.gh Digitizing www.knust.edu.gh Digitizing In both of these forms, an operator follows the map’s features with a mouse device, thereby tracing the lines, and storing location coordinates relative to a number of previously defined control points Automated digitizing involves using image processing software that contains pattern recognition technology to generated vectors. www.knust.edu.gh Vectorization The process of distilling points, lines and polygons from a scanned image is called vectorization. As scanned lines may be several pixels wide, they are often first thinned to retain only the centreline. The remaining centreline pixels are converted to series of (x; y) coordinate pairs, defining a polyline. Subsequently, features are formed and attributes are attached to them. This process may be entirely automated or performed semi- automatically, with the assistance of an operator. www.knust.edu.gh Obtaining spatial data elsewhere Over the past two decades, spatial data have been collected in digital form at an increasing rate and stored in various databases by the individual producers for their own use and for commercial purposes. Some data are freely available, yet other data are only available commercially, as is the case for most satellite imagery. High quality data remain both costly and time consuming to collect and verify, as well as the fact that more and more GIS applications are looking at not just local, but national or even global, processes. As a result of this increasing availability, we have to be more and more careful that the data we have acquired are of sufficient quality to be used in analyses and decision-making. www.knust.edu.gh Data formats and standards An important problem in any environment involved in digital data exchange is that of data formats and data standards. Different formats have been implemented by various GIS vendors, and different standards came about under different standardization committees. “data standard” refers to an agreed way, in terms of content, type and format, of representing data in a system. Several meta-data standards for digital spatial data exist, including those of the International Organization for Standardization (ISO) and the Open Geospatial Consortium www.knust.edu.gh Rasterization or vectorization Vectorization produces a vector data set from a raster i.e the production of a vector set from a scanned image. Another form of vectorization is used when we want to identify features or patterns in remotely sensed images i.e. feature extraction and pattern recognition Rasterization is the conversion of vector data sets to raster data. www.knust.edu.gh Organizing and managing spatial data The main principle of data organization applied in a GIS is that of spatial data layers. A spatial data layer is either a representation of a continuous or discrete field, or a collection of objects of the same kind. Usually, the data are organized such that similar elements are in a single data layer. For example, all telephone booth point objects would be in one layer, and all road line objects in another. A data layer contains spatial data—of any of the types discussed above—as well as attribute (i.e. thematic) data, which further describes the field or objects in the layer. www.knust.edu.gh Organizing and managing spatial data Most GIS software organizes spatial data in a thematic approach that categorizes data in vertical layers. The definition of layers is fully dependent on the organization's requirements. Typical layers used in natural resource management agencies or companies include forest cover, soil classification, elevation, road network (access), ecological areas, hydrology, etc. The clear identification of the requirements for any GIS project is necessary before any data input procedures, and/or layer definitions, should occur. www.knust.edu.gh Spatial data layers Data layers can be laid over each other, inside a GIS package, to study combinations of geographic phenomena. www.knust.edu.gh  Kwame Nkrumah University of Science & Technology, Kumasi, Ghana THANK YOU