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1G492 GIS ApplicationsVector Data ModelsI. IntroductionA. GIS Vector Data Model1. points and x,y geocoordinates are used to constructure spatial featuresB. Point-Based Spatial Features1. points (uh, dah!)2. linesa. straight line segmentsb. polylines(1) multiple linked straight line segments3. "arcs" - any curved or straight line segment that connnects two pointsII. Geometric ObjectsA. points, lines, polygons (enclosed areas)B. polyline features1. nodesa. start node (beginning of line)b. end node (end of line)2. vertexa. points not at the start or end of a line, that for the basis for polyline segmentsb. vertices - series of points that form the shape of the line3. smooth curves vs. segmented polylines4. Line intersections, Line Joinsstraight line (arc) polyline contiguous polygons doughnut and island polygonsC. Polygon - enclosed areas in which the start node and end node are located at the same x,ycoordinate1. continguous polygons - mutually adjoining polygons with common vertices and/or nodes2. isolated polygonsa. islands - free standing polygons enclosed by another polygonb. doughnut-shaped polygonsD. Scale as a Controlling Factor of Geometric Map Feature1. e.g. on a 1:24,000 topographic map a first order stream = a polyline; a wide riverdownstream (e.g. Willamette) = a polygona. At 1:1,000,000 the Willamette River = a polyline2. e.g. the building we are in at 1:2000 = a polygon, the building we are as viewed fromouter space = a point.2III. Map Feature Attributes and TopologyA. Attributes - Characteristic of a geographic feature described by numbers or characters, typicallystored in tabular format, and linked to the feature.1. E.g. attributes of a water well, represented by a point, might includedepth, pump type,location and gallons-per-minute.B. Topology - an expression of the spatial relationships between map features1. Map Features in a Topological Modela. composed of points and directed lines(1) directed lines - lines drawn in a particular direction and order from point topoint(2) arcs = directed lines2. Fundamental Components of Topological Relationships Between Map Featuresa. Connectivity - arcs connect to each other by nodesb. Area Definition - an area is defined by a series of connected arcsc. Contiguity - arcs are drawn in a given direction, from node to node, with left andright polygons of defined attributes3. Methodologya. Nodes are assigned numbers (1 = first node, 2 = second node, etc.)(1) each node has an x,y georeference coordinate(2) Resulting arcs are assigned numbers, with left and right polygon attributesb. Vertex - points along an arc or polyline, that are not at the start or finish points (ornodes)c. Left-Right Polygon Lists(1) Each arc is identified and the attributes of the left and right polygonidentifiedC. Non-Topological Vector Data1. Problem with geometric features and topology - more data intensive for computer devicesa. simple geometric features will load and run faster than topologically based features2. Shape Files in ArcView - non-topologically based feature vector filea. points - i.d. by simple x,y coordinateb. lines - a series of pointsc. polygons - series of linesd. mutual polygons can have duplicates arcs overlapping one anothere. File Format(1) *.shp - geometry of shape file stored here(2) *.dbf - attributes of shape file(3) *.shx - index file that links the *.shp file to the *.dbf file3In Class Exercise - Geometric Elements and TopologyThe Figure at the right is a polygon map theme with polygons A, B,C, and D. The polygons are constructed from arcs 1 through 7. Thearcs are composed of Nodes N 11 through N 14. The topology ofthe map is built upon graphical analysis of the georeferencecoordinates of the nodes and the arcs/polygons that they build.The table below shows a typical topological framework for thespatial relations. The abbreviations are as follows:Fnode The node at the beginning or start of an arc, "From Node"Tnode The node at the end of an arc, "To Node"Arc# The internal number assigned to identify the arcLpoly Attributes of the Left Polygon while "driving" from theFnode to Tnode, along the arc.Rpoly Attributes of the Right Polygon while "driving from theFnode to Tnode, along the arc.Exercise to complete. Based on the answer model for the first rowbelow, complete the topological tables for the map to the right. Arc Node List Arc Coordinate List Arc Polygon ListArc# Fnode Tnode Arc# x,y Coordinates Arc# Lpoly Rpoly1 11 12 1 (0,9) (2,9) 1 Polygon D Polygon B2 ___ ____ 2 _____________ 2 ________ _________3 ___ ____ 3 _____________ 3 ________ _________4 ___ ____ 4 _____________ 4 ________ _________5 ___ ____ 5 _____________ 5 ________ _________6 ___ ____ 6 _____________ 6 ________ _________7 ___ ____ 7 _____________ 7 ________ _________________________________________________________________________________________________D. Topological Errors1. topological errors arise when nodes and arcs are not properly "snapped" to one another oraligned2. Error Typesa. dangling nodes - nodes dangle in space without being snapped to another nodeb. undershoots - nodes are short of being snappedc. overshoots - nodes are long on being snappedd. leaky polygons - polygons are not closed, nodes are not properly snappedSee diagram below for examples4IV. Map Scale, Spatial Resolution, and Spatial Data AccuracyA. Map scale is an indicator of map accuracy1. The smaller the scale, in general, the lesser the accuracy, and vice versaa. e.g. map accuracy at 1:100,000 scale is much less than 1:24,000 scaleB. Locations Accuracy and Topological Accuracy in GIS1. Location Accuracy - measures the error in the absolute position of a map point or featurerelative to real world, georeference coordinates.2. Topological Accuracy - a measure of the error in topology and attribute features of mapfeaturesC. USGS Map Standards for Accuracy1. USGS maps are tested and standardized so that there is no more than 10% of totalposition points can be more than 0.02 inches (0.5 mm) out of position at the prevailingmap scale.___________________________________________________________________________________In Class ExerciseAt a scale of 1:65,000, 0.02 inches on the map represents how much distance on the ground in meters? Show allof your work.Given a scale of 1:24,000, 30 m error on the ground would represent how many millimeters of error on the map? Show all of your work.Given a scale of 1:24,000 and a spatial feature resolution of 10 m, how many inches of resolution does


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WOU ES 492 - Vector Data Models

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