地理信息系统chap.ppt

WHAT IS A GIS?
is a computer system for capturing, storing, querying, analyzing, and displaying geographically refeust be based on the same coordinate system.
Projection
is the process that can transform the Earth’s spherical surface
to a plane surface and bridge the two spatial reference systems.
A basic understanding of projection and coordinate systems is therefore crucial to users of spatial data.
Spatial Data
Data model
defines how spatial features are represented in a GIS.
Vector data model
uses points and their x-, y-coordinates to construct
spatial features of points, lines, and areas.
Raster data model
uses a grid and grid cells to represent
the spatial variation of a feature.
Spatial Data
vector data are ideal for representing discrete features;
raster data are better suited for representing continuous features.
Difference of the two data models
The raster data model uses a simple data structure with rows and columns and fixed cell locations.
The vector data model may be georelational or object-based, may or may not involve topology, and may include simple or composite features.
Spatial Data
Georelational data model
uses a split system to store spatial data and attribute data
Object-based data model
stores spatial data and attribute data in a single system.
Recent trends suggest that GIS vendors have adopted the object-based data model in their software development. For example, the geodatabase data model, introduced by ESRI, Inc. in ArcGIS, is object-based.
Spatial Data
expresses explicitly the spatial relationships between features.
Topology
Topological or topology-based data are useful for detecting and correcting digitizing errors in geographic data sets and are necessary for some GIS analyses.
Such as two lines meeting perfectly at a point and directed line having an explicit left and right side.
Spatial Data
can not expresses the spatial relationships between featu