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Digital spatial databases that can be used in computer software packages
for visualization and mapping store spatial data and share some of the
same features as paper maps. Paper maps store many kinds of spatial data
in a single sheet, and are used as-is without further processing. Digital
spatial database layers, however, generally store only one kind of information
or information on only one kind of object. To create a map, the user of
the digital spatial databases must use a computer software system to integrate
the data layers based on location. If data layers exist in different scales
at different projections, the spatial data must be processed further before
it can be used.
Scale -- For raster data, the size of a pixel
in terms of its area on the earth's surface affects the size of an object
that can be discerned in a digital image, thus determining the spatial
resolution of the data. A common ground dimension for remote sensing
data in the U.S. is 30m x 30m, the pixel size of Landsat Thematic Mapper
data. The total region covered by a raster database is defined by the number
of rows and columns of pixels in the database.
For vector data, resolution also refers to the smallest feature that
can be discerned. The minimum length of a line object, the minimum separation
required to display objects as separate and distinct, and the minimum
mapping unit size are affected by the scale of the database. The total
region covered by a vector database is defined by a bounding polygon within
which all of the objects in the database can be placed.
Projection -- Primary sources of positional
data usually report locations in latitude/longitude, that is, the spatial
data are not projected. For positional information digitized from an existing
map, the data are tied to the projection system used to make the original
map.
Symbols -- The entity represented in a
digital map may be a point, line, or area. Every entity has a number of
different attributes. Some attributes are spatial, describing the locations
of the entities. Some attributes are graphical (the symbols used by the
system to represent the point, line, or area in the visual display). Some
attributes are textual/numeric. Others are temporal.
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Spatial |
100 Main Street |
| Graphical |
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| Textual Numeric |
Jones' House 2000 sq. Feet |
| Temporal |
Year Built: 1950 |
Unlike paper maps that store attribute information in a given symbolic
form, digital spatial databases store objects and attributes. Users of
spatial data select graphical symbols from an available set in the particular
software package used to view and map the data and can change map classification.
Spatial Data Quality -- Five
dimensions of spatial data quality have been identified (lineage, positional
accuracy, attribute accuracy, completeness, and logical consistency). Librarians
need to understand how to describe spatial databases (metadata) they provide
access to so that they can help end users determine whether a particular
digital spatial database can meet the needs of the user and what types
of processing will be required to get the database to that point.
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