2 What is geographical query? Queries and reasoning Measurements
Geographical Query3 What is geographical query? Geographical query is the most basic of analysis operations. To answer simple questions posed by the user. No change occur in the database. No new data are produced. The operations vary from simple to complex, well-defined queries. Query and reasoning Measurements: simple numerical values that describe aspects of geographical data.
Geographical Query5 Selective map display Displays the locations or all objects using points and arcs, with or without background. Attributes and entity types can be displayed by varying colours line patterns and point symbols. Relational query Different systems use different ways of formulating queries. Structured Query Language (SQL) is an attempt to provide a “standard” way in querying data base. The user can select object interested and produce maps. Selective map display
Geographical Query21 Measurements Distance and length Area Shape
Geographical Query22 Distance and length (x 1, y 1 ) (x 2, y 2 ) Computation of line length The poly line representation of a smooth curve tends to be shorter. The length of a path as travelled on Earth’s surface may be longer than its horizontal projection.
Geographical Query23 Shape of an area Measure the shape of zones in terms of their proximity to a perfect circular shape For each zone in X measure the area of the zone (C) measure the perimeter of the zone (P) Circle U = 1 Any other shape 0 < U < 1
Geographical Query24 Continents and islands Determine the shape of region by analysing the number of enclosed islands and number of zones which make up the region itself. If the number of zones is greater than the number of enclosed islands, then the region is closer to the shape of “islands”. Otherwise, when the number of enclosed islands is greater than the number of zones, then the region is closer to the shape of “continents”. Continents Islands
Geographical Query25 The Euler number An area is closer to either a continent or island shape can be determined by it Euler number. The Euler number is calculated by subtract the number of islands which make up the region itself from the number of enclosed islands by that region. If the Euler number is greater than 0, the region is closer to a “continent”, while the Euler number is less than 0 then the region is closer to “islands”. For each region in X count a count b U = a - b where a = number of enclosed islands b = number of islands which make up the region itself A B For A: a = 0; b = 5; U = -5 For B: a = 3; b = 1; U = 2
Geographical Query26 Summary Geographical query is the most frequently used operation in GIS. Geographical query includes spatial, attribute and spatial relationship queries. The purpose of geographical query is to retrieve geographical information from GIS based on user-defined criteria. Spatial measurements can be made in GIS including distance, area and shapes.