Map as Database, continued from previous page
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
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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