1. Objects/ parts of a geologic map are held in a spatially referenced database.

Objects/ parts (map components) of a geologic map are held in a spatially referenced database, just like geochemical samples, land records, etc. Using appropriate software the objects can be displayed spatially (as a map). The database may hold information in a flat-file format or as related tables. Each map component or map object is represented by one record of data in the database.  Because the information is in a database, SQL-type queries may be performed in order to select various subsets of information or join related tables based on attributes (information/data) attached to the objects, as discussed below. Because the objects can be displayed on a digital map, subsets of the data (specific parts of the geologic map) can be selected on-screen using a variety of techniques based on visible spatial characteristics, or using SQL queries to select characteristics of the mapped objects which may not be visibly obvious or easy to select.

Features displayed on the legend of a standard geologic map are the sets of features that can be held in the database, with the addition of any other information you may have or obtain in the future. The exercise of defining the attributes (specific information about the map objects) is essentially one of parsing out the map legend.

2. Specific sets of attributes are attached to the objects in the database. 

An attribute is any kind of descriptive information that can be attached to an object or record in the database. The attributes being attached to geological map objects are equivalent to geochemical data for a single sample (held in a geochem database or spread sheet) like sample type, location, rock type and results for different elements.
As in many databases, the objects and their attached information can be viewed in Table or spreadsheet-like form where each object represents a record in the database and each column in the Table is a field holding a different type of attribute which may or may not be filled-in for each record (depending on the availability of that information for that particular object.) For example the geochronologic age may or may not be known, the specific rock type may or may not be known, even though it IS known that the map object is Mesozoic and igneous. 

Attributes fall into two categories: those that are always attached to certain types of objects and a second set which may be specific to individual objects or sets of objects. Using the example of a polygon or area of a geologic map that is bounded by contacts (in the general sense including faults), the unit may be referred to by a map code as Jg. All Jg objects on the map legend and in the database have the attributes of Mesozoic, Jurassic, Igneous, Plutonic, and granitic but some may have a date of 198ma, another may be 156ma and some may have no date at all. Some may be known to be metalliferous and others not. Some may be reduced, others oxidized or no information. Attributes of the first type (pertaining to all Jg, for example) lend themselves to being held in related tables if desired. 

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