# User:Mav/Introduction to GIS notes by maveric149/Week 4

Data Structures

• Vector data structure: Based on use of Points and their x, y coordinates to make spatial features . Objects are treated as discrete features.
• Raster data structure:

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US Censu Bureay TIGER (topolofically integated georpahic encoding and feferencing) file

• Each line is a directed line froma stating point to ending point with explicit lefta and rigt

ESRI Topology

• Files called "coverages" in ArcINFO are topological vector data files
• Three topological realtionships are supported: Connectivity (Arcs connect to each ofther, Area definiton (area is defined by a series of connected arcs), Continuity, *arcs have directions and left and right bordering polygons) see box pg 35

Topology - It is the mathematical relationship between line, point and polygon features Edited by A.Narayana Murthy(20-06-05)

Application of Topology

• point (or nodes) are coded with their ID numbes (coordinates) which refer to 3.6 page 36 (points are separeated so no topology.)
• Lines (or arcs) are connected to two nodes (starting or "from" a node and and ending or "to" node. Figure 3.7 p. 37
• Areas are defined by listing the bounding ars and by specifying the polygons on either side of each arc; Figure 3.8 p. 38

Non-topological vector data

• Takes less time to display than topological files
• ArcView's shapefiles are non-topological files
• Shpefiles sce ponts as coordinate pairs, lines as a series of point, and a polyfona as series of lines.
• Adjacent shpaefile polygons have separate arcs from their shared boundaries which may ovelap or duplicaed on another

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OAN example of higher level vecor objects : TINs

• Triangulated Irregular Networks -- used to represent surfaces. And entire surface is broken up inot triangles each with a constant gradient. Fig 3.1 p. 41
• Each trianlge has three coordinates wiht w2, y, and z values.
• Portions of the surface where there is little cariation in elevation are repesied ... gap in notes

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Additional higher level objects: Regions Dynamich Segamationa .... gap in notes

Spatla data Concepts

• Accuracy v. Precision

Accuracy is how close te apparent locaiton of a map feature is to its tre location on the ground precision is th exactness with which the location is reocrded (number of sign figs)

Scale, resolution, and acuuracy

• Map scale influences the repesenttion of spatioal feature.
• Map scale influences the level of detail on a ap. he smaller teh mapscale, the less the detail and more gneralization.
• Data entry methods based on non-map resourcse (GPS and remote snesing imagery) bypass the map scale asn ... gap in nots

Locaion Accuracy v. topological accuracy

• LA = accuracy of spatioal fearu ovton,; determined by map scale and the data entry provess
• Topolofical acuuracy = how well topological rerlationships between features are maintained; determined by data entry, error, error detection (by software) and data editing.