Basic Entity-Relationship Modelling for GIS Database

Basic Entity-Relationship Modelling for GIS Database Various tools to support the conceptual database modelling activity have been developed. One widely accepted and used technique is the entity-relationship modelling technique developed by Chen (Chen, 1976). The E-R technique is a graphical method of representing entities of a GIS database, relationships between the entities, and all attributes of either entities or relationships which must be captured in the database. Figure 1 E-R Diagram symbols for GIS database modelling (Calkins, no date) Each component has a graphic symbol and there exists a set of rules for building an ER model of a database using the three basic symbols Rules for identifying E-R diagram components Rule 1: A noun in English corresponds to an entity type on an E-R diagram Rule 2: An adjective in English corresponds to an attribute of an entity in an E-R diagram Rule 3: A transitive verb in English corresponds to a relationship type in an E-R diagram. Example Provincial government will use the database to identify soil, rainfall and farms within a local municipality. The database shall keep for soil and rainfall values on type and annual averages respectively. Local municipalities have English and African names. Each farm will be identified in the databases using farm_id as in the land records. A farmer owns at least one farm. Address and dominant agriculture activity are kept as part of farmer records. The provincial government will use the database to assist farmers to identify from their farm closest town by name. Information of each town’s population will be made available to help farmers understand their market. Identifying entities Rule 1: A noun in English corresponds to an entity type on an E-R diagram. IMPORTANT: You need to consider the following when identifying the entities of GIS database.   A thing of significance about which information needs to be known or held.  Each entity instance has specific values for the entities attributes.  Each entity must have multiple occurrences or instances. Each instance of must be uniquely identifiable from other instances of the same entity. PROVINCIAL GOVERNMENT will use the DATABASE to identify SOIL, RAINFALL and FARMS within a LOCAL MUNICIPALITY. The database shall keep for soil and rainfall values on type and annual averages respectively. Local municipalities have English and African names. Each farm will be identified in the databases using farm_id as in the land records. A FARMER owns at least one farm. Address and dominant agriculture activity are kept as part of farmer records. The provincial government will use the database to assist farmers to identify from their farm closest TOWN by name. Information of each town’s population will be made available to help farmers understand their market. Identified English nouns which correspond to GIS database entities are highlighted in bold. The Provincial Government and database are English nouns by they will not be considered as database entities. Although, they are nouns they could not qualify because both do not represent information content that needs to be held in the database. The Provincial government is the owner of the GIS database therefore it will not be stored in this database. The database represents the component for storing requirement data therefore it cannot be stored inside itself. Identifying attributes Rule 2: An adjective in English corresponds to an attribute of an entity in an E-R diagram. PROVINCIAL GOVERNMENT will use the DATABASE to identify SOIL, RAINFALL and FARMS within a LOCAL MUNICIPALITY. The database shall keep for soil and rainfall values on TYPE and ANNUAL AVERAGES respectively. Local municipalities have ENGLISH and AFRICAN NAMES. Each farm will be identified in the databases using FARM_ID as in the land records. A FARMER owns at least one farm. ADDRESS and DOMINANT AGRICULTURE ACTIVITY are kept as part of farmer records. The provincial government will use the database to assist farmers to identify from their farm closest TOWN by NAME. Information of each town’s POPULATION will be made available to help farmers understand their market. Entity Soil Rainfall Farm Local Municipality Farmer Town Attributes type annual average farm_id  English name  African name  Address  Dominant agriculture activity  Name  Population Table 1 Identified entities and their attributes The attributes of the GIS database entities are highlighted in blue above. These were identified by searching for adjectives of nouns listed as the database entities. Identifying relationship between entities Figure 2 More Symbols for spatial relationships (Calkins, no date) Rule 3: A transitive verb in English corresponds to a relationship type in an E-R diagram. PROVINCIAL GOVERNMENT will use the DATABASE to identify SOIL, RAINFALL and FARMS within a LOCAL MUNICIPALITY. The database shall keep for soil and rainfall values on TYPE and ANNUAL AVERAGES respectively. Local municipalities have ENGLISH and AFRICAN NAMES. Each farm will be identified in the databases using FARM_ID as in the land records. A FARMER owns at least one farm. ADDRESS and DOMINANT AGRICULTURE ACTIVITY are kept as part of farmer records. The provincial government will use the database to assist farmers to identify from their farm closest TOWN by NAME. Information of each town’s POPULATION will be made available to help farmers understand their market. Transitive verbs have a noun phrase as object. They have a pattern is N + V + N (noun + verb + noun). The word “within” in the following statement is a transitive verb because it connects the two nouns: “FARMS within a LOCAL MUNICIPALITY”. Other transitive verbs are highlighted in green. In addition, entities relate in various ways. Figure 2 presents typical words for identifying relationship types and their E-R Diagram symbols. A complete E-R Diagram is presented below. Designing a database with the following, specific user needs:   The GIS database will be used to create maps that guide new students to locate academic buildings.  It is important to record the names of each academic building and the number of floors.  around the campus.  The system shall allow identification of parking lots close to each academic building.  The system shall use GIS overlay operations to identify parking lots for all sport facilities. The system shall use GIS overlay operations to identify roads, which connect parking lots  Each road shall be identified using the name approved by the SRC.  Data on parking lots should allow one to determine if it is small, medium or large. Further for each sporting facility is important to know the main sporting activity. Based on the user needs above identify the E-R Diagram components.