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DePaul GEOG 458 - Logical Consistency

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Logical consistencyOutlinesSlide 3Attribute consistencySlide 5Slide 6Slide 7Temporal consistencyPossible temporal relationships between entitiesSpatial consistencyTopological rules within a layerSlide 12Possible spatial relationships between entitiesTopological rules between layersDetecting topological errorsEditing topological errors: automated vs manualEditing topological errors: depending GISystemLogical consistency and object-oriented databaseLogical consistency test: general proceduresDocumenting logical consistency in the metadataLogical consistencyFebruary 24, 2006Geog 458: Map Sources and ErrorsOutlines•What is logical consistency?•Testing spatial consistency–Attribute consistency–Temporal consistency–Spatial consistency•Documenting logical consistency in metadataLogical consistency•Lack of contradiction in a database•Fidelity of relationships encoded in the data structure of the digital cartographic data (SDTS)•Ensures logical consistency of operations performed on data•Deals with logical rules applied to space, time, and attribute•Attribute consistency draws upon database integrity rules (database consistency)•Spatial consistency refers to the conformance to topological rules based on graph theory (topological consistency)Attribute consistency•Key constraints–Primary Key (PK: attributes of a class that uniquely identify an instance of the class) should be unique•I.e. should not be repeated–PK should be Not Null (if it’s missing, how would you identify an instance uniquely?)•Referential consistency constraints–Foreign Key (FK: attributes of a class that establish relationships among tables) should correspond to PK in related tables, or ensure the existence of related tables–No value should be inserted in a table as FK without a corresponding column in the related tables (i.e. they should be updated simultaneous whenever update occurs)Attribute consistency•Domain consistency rules (consistency within column values)–Attribute values must fall within certain ranges or may assume only certain pre-defined values•E.g. the value for the day may only be in the range 1 and 31•E.g. area or length cannot be negative•E.g. a location in geographical coordinates can only in a certain range–Domains can be composite•E.g. a location in Degree Minute Second•E.g. a day in Day Month YearAttribute consistency•Consistency between column values–A given attribute value must comply with other attributes when its value is derived from the attributes•5 digit FIPS code for county should be consistent with values for state and county–A given attribute value must also comply with the value that can be derived from the metric characteristics of spatial objects•The area of a parcel stored in a DB must be consistent with the computed are based on the stored geometry–Whenever column values are changed, whose values are derived from that column, should be updated accordinglyAttribute consistency•Broadly defined, conformance to logical rules applied to attributes–E.g. If the study area is the conterminous U.S., longitude should be negative (since it’s in the western hemisphere) •Usually, database management system enforces integrity rules automatically –Automatic update, NotNull enforcement•If GIS is not implemented as DBMS, you should check them manually–Use of summarize tool for checking key/domain constraints–Compute metric values; Compare metric values with stored values–Table join with master table; compare the master value to stored valuesTemporal consistency•No violation of temporal topological rule•Mostly temporal information is treated as attributes in common DB system•For example,–Individual travel survey data: one person can exist at one point in time and at one point in space–Traffic accident data: accident time should occur before dispatch timePossible temporal relationships between entities•Process: a series of changes with some unifying principle•Event: countable occurrence located in point in space–Can be point-based (at some point in time) –Can be line-based (at some interval in time)Possible qualitative relations between two intervalsYou can use this interval-logic for testing temporal consistency: e.g. session A should be during the conferenceSpatial consistency•Mostly refers to the conformance to topological rules•Can arise at various stages of data handling–Digitizing/ updating error–Error propagation through processing–Miscoding of topological relationships•Topological rules can be checked on topological vector data model (link-node model); can be checked within a single layer or between layersTopological rules within a layer•Missing node makes a correct topological description impossible (e.g. road intersection will make it connected to other links)•Pseudo nodes (nodes where only two edges meet) can lengthen computation time•Undershoots and overshoots (edges that end in only one node) may be fictitious line due to digitizing errors•Duplicate lines: comes from manual digitizing, when two data sets are to be merged; can create slivers•Label points (reference points or centroids): used to link polygon to attributes to place labels–Missing label points: can cause the inconsistency between geometric data description and topological data description–Multiple label points: # polygons =/= actual # polygonsRead SDTS data quality (or reading in the course package)•Some encodings are not necessarily errorsWhat ‘s encoded in the database?What it is indeed? Is it consistent?How to check errors?Dangling node Dead end Yes (no error) Check dangling node: too short one? How do I know if it’s a dangling node?Dangling node There’s no such road segmentNo (error)Intersecting node Road intersection Yes (no error) Check intersecting node: overlay with overpass layer if anyIntersecting node Overpass No (error)Possible spatial relationships between entities •Metric–Distance, direction•Topological –Qualitative spatial relation between regionsDisjoint, meet, within, covered by, cover, contain, equal, overlap You can compare this relational info. encoded in the database to the actual relational info. (e.g. King County should be within Washington State)Topological rules between layers•For example, •the street center lines should fall inside the pavement area; •rivers should be inside their floodplain; •zoning boundaries should follow certain parcel


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