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Unformatted text preview: To position this on the landscape all you need is... Origin, cell resolution, metadata on projection and datum Features Objects Vector Model
Entities: (described by coordinates) & Attributes: (descriptions) Bolstad 2002 The Vector Model
s s s s s A vector data model uses points stored by their geographic coordinates (e.g., UTM, latitude/longitude). Lines are built from sequences of points stored in order. Lines often have a direction to the ordering of the points. Polygons can be built from points or lines. Topology can be stored explicitly. Vector representations are widely used and seem to conform to the way that people construe the world. Is the way that people construe the world an accurate representation of the world? People are good at simplification, abstraction, and classification. Vector data structures have dimensionality (a variety of terms used to
describe the same thing) Point Line Area 0-d 1-d 2-d 0-cell 1-cell 2-cell http://www.opengeospatial.org/ http://mcmcweb.er.usgs.gov/sdts/ VECTORs s s Early vector-based GISs used spaghetti data structures. Vector data evolved the chain/node model in the 1960s (Dual Independent Map Encoding- DIME, Spatial data transfer standards (SDTS). Open Geospatial Consortium Spaghetti- a brief history lesson
Spaghetti model gets its name from the idea of a bunch of unstructured (spatially independent) noodles dumped on a plate.
In this case polygon 63 is treated as if it is completely independent of 64 even though they share a common border. What are the problems associated with the spaghetti approach? The spaghetti data structure and dual line encoding:
1. Internal borders are stored twice. 2. Easy for each copy to be different! Slivers This happens because polygons must be retraced and it is almost impossible to duplicate the line exactly. Sliver Spatial search operations Point Dictionary
Point dictionary adds structure and reduces redundancy, but it is still not useful for editing, error checking or for "localizing" geographic information. Spatial Data Transfer Standard In the chain/node model, a polygon consist of lines and a line consists of points. Points, lines, and polygons can each be stored in their own files, with links between them. The topological vector model uses the chain as a basic unit. Polygons are built up from chains. The endpoints of a chain are called nodes. Chain junctions are always at nodes. Stored with the chain is topology (i.e. the connecting nodes and left and right polygons). Spatial Data Transfer Standard chain chain
chain network chain G-Ring G-Polygon chains http://mcmcweb.er.usgs.gov/sdts/SDTS_standard_nov97/part1b10.html http://mcmcweb.er.usgs.gov/sdts/SDTS_standard_nov97/part1b10.html Rules associated with topological spatial data structures
Chains must begin at a node (the from node) and end at a node (the to Node). If applicable, information about left and right bounding polygons are stored with the chains Nodes occur at all intersections. Polygons must "close" and contain one and only one "label point." TOPOLOGY
s s s s s s Topological data structures dominated GIS software in the 90s (now often topology on the fly). Topology allows automated error detection and elimination. Rarely are maps topologically clean when digitized or imported. GIS software is able to build topology from unconnected arcs. Nodes that are close together can be snapped. Slivers due to double digitizing and overlay can be eliminated. Topological Model Topological model is widely used. It adds information about "connections". These things need to be linked Non-Topological Topological Bolstad 2002 Topology would have to be rebuilt Bolstad 2002 Topological invariance; all 3 are topologically equivalent. Complete topological structure Networks are special cases... Networks can become quite complex because of relationships that must be encoded; modified directed graph used. Rule: Polygons must "close" and contain one and only one "label point." Visual inspection for closed polygons can be difficult. Automation is helpful. Unsnapped node Unsnapped node, snapped Topology Matters
s s s The tolerances controlling snapping, elimination, and merging must be considered carefully, because they can move features. Complete topology makes map overlay more efficient. Topology allows many GIS operations to be done without accessing the point files. Open Geospatial Consortium (http://www.opengeospatial.org/standards) Aggregation Inheritance (kind of relation) ESRI help files Topological rules Must not overlap: Requires that lines not overlap with lines in the same feature class. This rule is used where line segments should not be duplicated; for example, in a stream feature class. Lines can cross or intersect but cannot share segments. http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=Topology_rules Multiple topological rules can be set for all types of geometry (points, lines and polygons) http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=Topology_rules FORMATS
Most GIS systems can import different data formats, or use utility programs to convert them. s Data formats can be industry standard, commonly accepted or standard.
s Vector Data Formats for Graphical and Geographical Data
DLG and TIGER, which have topology. s Spatial data transfer standard s ESRI Geodatabase, Shapefiles, Coverages
s TIGER: The abridged version Raster Data Formats
s s s s Most raster formats are digital image formats. Most GISs accept TIF, GIF, JPEG, and Mr. Sid which may or may not be georeferenced geoTiff and Mr Sid files can be associated with a world file that provides the data needed to integrate these images with others. DEM, Imagine, Thematic mapper and MODIS. EXCHANGE
s s s s Most GISs accept many formats which they can convert to one of their own data structures (proprietary versions of raster, vector, or TIN). If a GIS supports many data structures, changing structures becomes the user's responsibility. Changing vector to raster is "easy"; raster to vector is hard. Data also are often exchanged or transferred between different GIS packages and computer systems. Raster Vector and Vector Raster transformations Many forms of format conversion will introduce error. ...
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- Fall '11