Spatial Data is a kind of data that reflects the real world which has become too complex for the direct and immediate understanding of data consumers. Spatial Data are used to create models of reality and designed to have some similarity with selected aspects of the real world including status and nature of the reality.
A Spatial Database is therefore a collection of spatially referenced data which are acting as model of the real world in that it sets and approximates selected phenomena. These phenomena are then converted in digital form and may representations of past, present or future time.
Data Layers are actually a set of spatial objects, coverage or themes. A Data Layer may represent a single entity from the Spatial Database or a group of related entity types which are conceptually the same.
For example a layer representing a real life landscape may have only stream segments or may have streams, coastlines, lakes and swamps.
The entity set to be included in a Data Layer actually depends on the system as well as the database model although in some cases, the database may have already been built by combining all the entities into a single Data Layer.
The basic elements in a Spatial Database are actually the same as a regular database. It also has an entity but this entity refers to any "a phenomenon of interest in reality that is not further subdivided into phenomena of the same kind" such as a "city" entity which could be further broken down into component parts or "forest" entity which could be further subdivided into smaller forests.
An Entity also has an attribute as in the case of regular databases.
An Attribute refers to any characteristic of an entity selected for representation.
An Object is "a digital representation of all or part of an entity".
In a Spatial Database, the very distinct difference between an Entity an Object is that an Entity is the element in reality while the object is the element as it is represented in the physical database. Both of these elements are modeled in a Geographic Information System (GIS) database.
To better illustrate a Data Layer in a Spatial Database, let us take an example of a Spatial Database dealing with geographical data of a particular place. The Spatial Database may contain a Data Layer which consists of specific types of geographical data such as soil map, cadastral map or any image.
In the implementation of object-relational DBMS data storage, each layer may be associated with a set of Data Tables to store both the spatial and non-spatial components. There will be a different table which will be associated with each Data Layer for storing each of the different geometric elements such line, point, polygon and raster.
Accessing the Spatial Data within the DBMS may be possible through a generic Application Programming Interface (API). The API can encapsulate any internal differences among database systems. The API can also map spatial data types into the specific DBMS implementations with the use of spatial indexing or their in-built optimization facilities
It is common nowadays to have Data Warehouse that have database systems which can integrate spatial data types in object-relational data base management systems.
New advancement will make this set up even more popular in the future with the development of GIS technology. Companies can generate reports not just in traditional tables but with graphical maps reflecting data about the company as well.