Page 51 - Geochemical Anomaly and Mineral Prospectivity Mapping in GIS

Spatial Data Models, Management and Operations 47

unique conditions map assigns a unique attribute (e.g., geo-object class or ID) to
polygons with unique combinations of attributes of the input maps. This type of intersect
operation is common in raster-based GIS software packages. The number of unique
conditions is usually less than but could be equal to the number of unique polygons. For
example, in Fig. 2-17 there are eight pairs of unique polygons and each pair has unique
combinations of attributes of the input maps. Thus, the attribute table associated with the
unique conditions map has four records less than the attribute table associated with the
unique polygons map. The reduced number of classes in a unique conditions map
compared to a unique polygons map, or the reduced number of records in the attribute
table associated with the former map compared to the latter map, is favourable in
speeding up modeling computations. This is a reason why the intersect operation that
results in a unique conditions map is usually preferred over the intersect operation that
results in a unique polygons map.
Of the different overlay operations, the intersect operation is widely used in the
analysis of at least two maps in order to obtain spatial information of interest. The
objective of overlay operations in the analysis of either a pair of maps or multiple maps
could be either (a) to combine maps according to certain conditions or rules or (b) to
examine spatial relationships between two different geological features. The conditions
or rules applied in combining a pair of maps represent a conceptual model of theoretical
relationships between factors of or controls on how and where certain geochemical
anomalies and mineral deposit-types occur (see Chapter 1).

SUMMARY
A GIS facilitates efficient capture, storage, organisation, management, query,
retrieval, transformation, analysis and integration of geoscience spatial data sets used in
mineral exploration. Such functionalities of a GIS, in turn, facilitate efficient modeling
of spatial geo-information such as geochemical anomalies and prospective areas. The
registration of spatial data to a common coordinate system, the representation of geo-
objects and their data attributes as either vector or raster data models, and the
organisation of spatial data attributes in relational databases all contribute to facility in
analysis and integration of various spatial data or geo-information as a series of data
layers. The way of analysing and integrating spatial data or geo-information in a GIS is
actually an extension of the traditional light table method employed by mineral
explorationists in the past. Spatial data operations on single or multiple data layers
provide efficient tools for analysis of inter-relationships between data, which are
important in modeling of geochemical anomalies and prospective areas. The various
functionalities of a GIS that are relevant in predictive modeling of geochemical
anomalies and prospective areas are demonstrated further in the succeeding chapters.