Logical consistency February 24, 2006 Geog 458: Map Sources and Errors

Outlines What is logical consistency? Testing spatial consistency –Attribute consistency –Temporal consistency –Spatial consistency Documenting logical consistency in metadata

Logical 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 Year

Attribute 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 accordingly

Attribute 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 values

Temporal 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 time

Possible 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 intervals You can use this interval-logic for testing temporal consistency: e.g. session A should be during the conference

Spatial 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 layers

Topological 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 # polygons Read SDTS data quality (or reading in the course package)

Some encodings are not necessarily errors What ‘s encoded in the database? What it is indeed?Is it consistent?How to check errors? Dangling nodeDead endYes (no error)Check dangling node: too short one? How do I know if it’s a dangling node? Dangling nodeThere’s no such road segment No (error) Intersecting nodeRoad intersectionYes (no error)Check intersecting node: overlay with overpass layer if any Intersecting nodeOverpassNo (error)

Possible spatial relationships between entities Metric –Distance, direction Topological –Qualitative spatial relation between regions Disjoint, 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 boundaries (most of the time); hierarchical relationships should nest (cities inside counties inside states - except Bothell)... too many potential relationships to delineate them all

Detecting topological errors Functionalities available in GIS –Nodeerrors: check whether there are node errors (pseudo node, dangling node) –Labelerrors: check whether there are missing/multiple label points of polygon –Display node by types –Trace: check connectivity Your own method –Missing node: build/clean the data and compare the test data with the cleaned data…

Editing topological errors: automated vs manual Automated methods –Use “BUILD” and “CLEAN” command given a threshold; it can remove duplicate lines, sliver, undershoot, and overshoot given threshold in batch –But care should be exercised since topology building is dependent on threshold Interactive methods –Manual inspection with some visual aids

Editing topological errors: depending GISystem The Arc/INFO CLEAN and BUILD do topology building/cleaning. This is now hidden in the ArcToolbox, and invokes in hidden ways with less ability to control it. Since the coverage model is not as central anymore, An ArcMap shapefile can contain overlapping shapes. There is no easy way to get all the slivers and gaps edited away. SDE (Spatial Database Engine) does have advanced topological error checking even though it uses something equivalent to a shapefile (and quite different in storage details). SDE can find all shared segments and correct them.

Logical consistency and object- oriented database You can embed any rules (as much as you can come up with) in the database Spatial consistency by defining topological rules Temporal consistency by defining topological rules Thematic consistency by defining domain values It can also embed the relationships with other entities, in addition to internal consistency Quality of database can be ensured better as it checks the possible logical inconsistency internally based on the rules

Logical consistency test: general procedures Divide database into space, time, and attribute components (whenever necessary) Choose samples in space, time, and attribute Come up with logical rules to be followed for each component Check if the encoding in the database conforms to the rules Report on % violation of rules out of total cases tested for each component

Documenting logical consistency in the metadata State what elements are tested State the number of features that were checked State the number of inconsistencies encountered –Describe the methods used for detecting errors State a detailed specification of the nature of the problem and possible solutions –Mention whether it is corrected or not If possible, provide a graphical display of the error condition