Geographic Information Systems
Spatial data types
Field Vs. Object
(Geographic) objects populate the study area, and are usually well distinguishable, discrete, bounded entities. The space between them is potentially empty.
A (geographic) field is a geographic phenomenon for which, for every point in the study area, a value can be determined.
Field View Vs. Object View
Examples:
Object View: Trees, Houses, Streets.
Field View: Elevation, Temperature, Rain Intensity.
General rule-of-thumb is that natural geographic phenomena are more often fields, and man-made phenomena are more often objects.
Geographic Objects
For example roads are objects, they are
characterized by:
• location (where does it begin and end)
• shape (how many lanes does it have)
• size (how far can one travel on it)
• orientation (in which direction can one travel on it)
Computer representations of geographic information
In GIS, fields are usually implemented with a
tessellation/raster approach, and objects with a
(topological) vector approach.
Regular tessellations
A tessellation (or tiling) is a partition of space into
mutually exclusive cells that together make up the
complete study space.
The simplest example is a rectangular raster of unit
squares, represented in a computer in the 2D case
as an array of n × m elements
Raster Example
Regular tessellations
• Square, regular tessellations are known under
various names in different GIS packages: raster
or raster map.
• The size of the area that a raster cell represents
is called the raster’s resolution.
Point representations
• Points are defined as single coordinate pairs (x, y)
when we work in 2D or coordinate triplets (x, y, z) when
we work in 3D.
• Points are used to represent objects that are best
described as shape- and sizeless, single-locality
features.
Line representations
• Line data are used to represent one-dimensional
objects such as roads, railroads, canals, rivers and
power lines.
• The two end nodes and zero or more internal nodes
define a line.
• Another word for internal node is vertex (plural:
vertices);
Line representations
• Another phrase for line that is used in some GISs is
polyline, arc or edge.
• A node or vertex is like a point (as discussed above)
but it only serves to define the line
Start Point End Point
VertexVertex Vertex
Area representations
• Employed when area objects are stored using a vector
approach
Spatial Data Models
Raster
exhaustive regular or irregular partitioning of space
associated with the field view
location-based
Vector
points, lines, polygons
associated with the object view
object-based
Spatial Data Models
ESRI Shapefile
Designed by ESRI for ArcView
Implementation of the vector model
An individual layer stores a single type of geometry (i.e. point, line, polygon)
ESRI Shapefile
Four primary files in a shapefile: .shp, .shx, .dbf and.sbn
All files must share the same prefix for one shapefile,
e.g. road.shp, road.shx, and road.dbf
.shp : stores the feature geometry (binary)
.shx : index for .shp file
.dbf : attribute data stored in dBASE format
.sbn: for indexing
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