GIS Standards and operations

•          Need for standards and policies within the GIS communities has been addressed by

ü  OGC (Open Geospatial Consortium)

ü  FGDC (U.S Federal Geospatial Data Committee)

ü  ISO (International Organization for Standardization)

OGC defined the core data types and methods that should supported by GIS systems also defined GML (Geographic Markup Language) which is an XML encoding tailored for geographical information

Methods for testing Spatial Database

ü  Equal - Is the geometry spatially equal to another geometry?

ü  Disjoint - Do the geometry share a point?

ü  Intersect - Do the geometries intersect?

ü  Touch – Do the geometries spatially touch?

ü  Cross – Do the geometry spatially cross?

ü  Within – Is the geometry spatially within another?

ü  Contain – Does one geometry completely contain another?

ü  Overlap – Do the geometry overlap (must be same dimension)?

ü  Rotate – are the geometries spatially related?

Methods for Analyzing Spatial Geometries

ü  Distance – returns the shortest distance between any two points in two geometries

ü  Buffer – returns a geometry that represents all points whose sistance is from given geometry is less than or equal to distance

ü  ConvexHull – returns a geometry that represents the convex hull of the geometry

ü  Intersection – returns a geometry that represents the set intersection of the geometry with another geometry

ü  Union – returns a geometry that represents the point of union of geometry with another

ü  Difference – returns geometry that represents the point set difference of geometry with other

ü  SymDifference - returns geometry that represents the point set symmetric difference of geometry with other

Assertions

ü  Standards define assertions for different geometries

ü  Example:

o   A polygon is defined as a planar surfaces, described by one exterior boundary and may not have several interior boundaries which defines a hole in polygon

For the above geometry the assertions are

o   Polygons are topologically closed

o   The boundary consist of set of linear rings that define its exterior and interior boundaries

o   No two rings in the boundary cross

Feature

ü  Defined as, an object with spatial location and geometric attributes

o   Features are stored as rows in tables and each geometric attribute is a foreign key that reference a geometry table or a view

o   Relationships are defined as foreign key references between feature tables

Spatial reference system

ü  Also known as Datum in geometry which stands for size, shape and origin of an ellipsoid that represents earth

ü  Each feature should stored in a row and must have a column associated with its spatial reference system

ü  Spatial reference system must identify the coordinates of the system

Spatial reference system table

ü  Stores spatial reference system in database

ü  Example: creation of states table

CREATE TABLE STATES

( Sname                       VARCHAR(50)           NOT NULL

  State_shape    POLYGON                  NOT NULL

  Contry                       VARCHAR(60),

PRIMARY KEY(Sname),

FOREIGN KEY(Contry) REFERENCES CONTRIES (Cname),

);

Example 1:

ü  Retrieve states with an area greater than 50000

SELECT Sname

FROM STATES

WHERE (AREA(State_shape)>50000);

ü  Area is a method defined in OGC standard

Example 2:

A different statement that will retrieve all the states that share a boundary with Texas, The touches method returns 1 when the geometry spatially touch

SELECT S1.Sname

FROM STATES S1, STATES S2

WHERE ((TOUCHES (S1.State_shape,S2.State_shape)==1) AND (S2.State_name=‘Texas’))