Geomatics for Urban and Regional Analysis PDF

GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli How geographical data...

GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli How geographical data Geomatics for Urban and Regional Analysis are stored and organized in a GIS VECTOR AND RASTER DATA STRUCTURES Data structures: vector and raster DICAM DICAM Università di Bologna How data are structured and stored in a GIS? Università di Bologna Data models: Vector & Raster (I) Vector data model y y y code code code x x x Point Line Area Raster data model Row Row Row Conversion possible but to be evaluated case by Column Column Column case DICAM Università Key for the vector models: vertex (intermediate intemediate point point) di Bologna node Vector vs Raster Vector vs Raster VECTOR RASTER ADVANTAGES ADVANTAGES – simple data structure – compact data structure – faithful representation of real phenomena and their dimensions – simple overlay between maps and other spatial analysis procedures – efficient coding of the topological relationships and therefore support to – immediate integration of remote sensing data or digital photogrammetry the operations that rely on them (network analysis, etc.) (orthophotos) – accurate graphic representation – efficient to describe data with high spatial variability and to perform – suitable for numerical cartography simulation and modeling operations (each spatial unit the same shape/size) rapidly developing technology with decreasing costs DISADVANTAGES DISADVANTAGES – complexity in the data structure – non-compact structure, it can produce large volumes of data also to – complexity in management software represent phenomena with a limited area of interest – complexity in carrying out overlay operations – less pleasant and less precise graphic representation (depending on the – difficulty or impossibility to carry out some analysis operations, also due resolution of the cells), incorrect calculation of the dimensions to the presence of entities with different topological characteristics DICAM DICAM – not suitable for managing topological relationships Università Università di Bologna di Bologna GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 1 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli Vector / Raster / Alphanumeric data Vector data vertex non- topological Alphanumeric Vector data Raster data model attributes vertexes arcs DICAM DICAM topological nodes Università Università model di Bologna di Bologna The topological relationships (vector) Errors and their correction in a vector environment pseudo nodes indicate overshoot missing arc Overshoot: line that ends beyond a node Undershoot: line that ends undershoot before a node knot overshoot Missing label: unassigned acceptable pseudo node From a topological point of view labels (e.g. centroids of overshoot A subway map is not a topographic (adjacency between areas) these areas) poorly digitized arc map (faithful to reality) but three representations are completely Missing arc: non-digitized duplicate arcs missing label overshoot expresses only the topological equivalent arc or slivers relationships between the nodes Sliver: spurious area derived from the Connectivity matrix Adjacency matrix duplicated digitization of Side from node to node Line left right an arch Non-closed polygon (two nodes instead of one pseudonode) DICAM DICAM Università Università Correction di Bologna di Bologna Management of topological relationships in a vector system ESRI Topological errors in vector GIS: (a) effects of tolerances on topological cleaning; (b) topological ambiguities in raster to vector conversion DICAM Università di Bologna GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 2 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli Link between graphic elements and editing operations in a vector environment alphanumeric attributes (I) Automatic closure of a polygon Interactive data entry “Dangling lines” for each graphic Undershoot element DICAM Insert text and automatic DICAM Università Overshoot Università labeling of objects di Bologna di Bologna Link between graphic elements and alphanumeric attributes (II) Raster data: examples Satellite Airborne or UAV Remote Sensing orthophotos DTM in grid format Import of an external database and association between a key attribute (e.g. ID) and Thematic map from Digital terrestrial images the same image classification Scanning of maps attribute stored in the graphic representation DICAM DICAM Università Università di Bologna di Bologna Raster data: the matrix Topological relationships (raster) vector raster high resolution raster medium resolution raster low resolution Raster data is generally less suitable than vector for maintaining topological information: for example, an increase in the size of the pixel can alter the situations of adjacency / proximity between different polygons. DICAM DICAM Università Università The raster data is normally not used for “network” type systems: road networks, di Bologna di Bologna hydrographic networks, etc. GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 3 G. Bitelli  GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) G. Bitelli Cell contents for raster information The “mixed pixel” problem A cell can have a “nominal” type value (eg thematic classification): eg. value 9 = urbanized area, value 5 = wooded area, etc. and in this case the numerical values could also be modified or inverted (5 = urbanized area, 9 = wooded area). There might be an external table (value attribute table, VAT) that link the numerical values with descriptive attributes to better explain their meaning. Water dominates Winner takes all Edges separate In remote sensing imagery (cell = pixel) the values (DN) are W W G W G G W E G integer and are related to reflectance W W G W W G W E G In other cases the values of the cells contain real data (eg the W W G W G G E E G value 22.5 in a cell could be the temperature, or the altitude) DICAM DICAM The interest is toward Majority is the Edge is a sort Università Università water presence criterion of neutral pixel di Bologna di Bologna Conversion raster vector Conversion raster vector Rasterization: the conversion vector raster (the simplest) vector-to-raster can be done by The conversion from raster to vector is the most superimposing a grid on the problematic one (it is as if we increase the data vector map and assigning a quality level, impossible!) value to the pixels based on the presence or absence of raster-to-vector vector entities in It is advisable, as far as possible, to keep the data correspondence in their original format, unless certain operations cannot be done if the conversion is not carried out Vectorization of a raster entity can occur along the outer edges or according to the pixel centerline. The result also changes a lot in terms of perimeter length or area extension DICAM DICAM Università Università di Bologna di Bologna GEOMATICS FOR URBAN AND REGIONAL ANALYSIS (2024/2025) 4 G. Bitelli