ESRI Data Models

The geospatial data model evolved:

• as a metaphor to old analog methods
• along with database technology
• in respsponse to faster cheaper computer hardware

CAD

CAD/CAM came around before pong was a video game. It was a way of capturing manual drafting techniques in a digital format. A CAD file is a digital representation of a design or existing object.

CAD charachteristics:

• Primarily design and drawing tool
• Developed to automate drafting functions
• Features represented by graphic symbols Graphics driven
• Often stored in “local” coordinate system
• Layers often not well organized
• Attribute info not commonly stored or stored as annotation

ArcIfo Coverage

The coverage evolved out of relational database technology with spatial functions being user defined (or ESRI defined) extensions of the relational database model.

Georelational data model represents geographic features as interrelated Spatial and attribute data. In 1981, ESRI introduced ArcGIS and a georelational data model called “Coverage” as the data format for ArcInfo. The key features of the Coverage are:

• Spatial data stored as indexed binary files which are optimized for access and display of geographic data. Attributes are stored in Info tables with each row linked to a single related spatial feature by a common identifier.
• Topological relationships between vector features can be stored and updated. A line is defined as a sequence of points or nodes. By inference lines with common nodes are connected. Fields for which polygon is to the right and left of a line can define polygon boundaries.

Disadvantages to the ArcInfo Coverage:

1. The ArcInfo coverage represents features as having generic behaviors so that lines representing roads and streams have the same behaviors and unique behaviors such as flow direction and flow rates for rivers could not be added.
2. Topological features (polygons, regions and routes) are not ready to use until topology is explicitly built and must be rebuilt after editing using the “clean” and “build functions”
3. Coverage was limited to single user editing due to need to synchronize topological graph with feature geometries. Large data sets had to be broken into smaller grids or tiles which required which required extra work and time.

System generated and maintained fields - For points and polygons the (PAT and PAT) tables have Area, Perimeter, coverName#, and coverName-ID

Shape files

The shape file was created to meet a need for faster display and querying of geospatial data by users who neither needed to nor wanted to work with and maintain topological graphs. Faster cheaper hardware, especially color displays and compact cheap data storage along with GUI interface softeware began opening up GIS data to a larger audience. Shape files were introduced along with ArcView, an extended viewer with analysis and cartographic capabilities.

A shapefile is a set of at least three files (shp, shx, dbf) that store and index spatial feature geometry as a set of vector coordinates and attributes in a file with dBASE format, with optional files that can store coordinate system information, metadata, and other information. Single features are noncontiguous and can overlap.

Disadvantages of shapefiles:

• Data redundancy in shared segments between adjacent polygons
• Shapefiles do not store topology, model only generic features, do not store relationships with features
• have limited ability to enforce topological data integrity

System generated and maintained fields - FID, Shape, ID (or OID)

Geodatabase

A geodatabase is an object relational container for storing spatial data along with attribute data and the relationships that exist among them.

System generated and maintained fields - OBJECTID*, SHAPE*, SHAPE _Length, SHAPE _Area

More on the Geodatabase in other posts