ArcGIS Basics[转载]

Feature and Feature Classes.

Feature classes are homogeneous collections of common features, each having the same spatial representation, such as points, lines, or polygons, and a common set of attribute columns, for example, a line feature class for representing road centerlines. The four most commonly used feature classes in the geodatabaseare points, lines, polygons, and annotation (the geodatabase name for map text).

In the illustration below, these are used to represent four datasets for the same area: (1) manhole cover locations as points, (2) sewer lines, (3) parcel polygons, and (4) street name annotation.

FeatClassPointLinePoly.gif

In this diagram, you might also have noted the potential requirement to model some advanced feature properties. For example, the sewer lines and manhole locations make up a storm sewer network, a system with which you can model runoff and flows. Also, note how adjacent parcels share common boundaries. Most parcel users want to maintain the integrity of shared feature boundaries in their datasets using a topology.

 Feature class storage in the geodatabase

 In the geodatabase, each feature class is managed in a single table. A Shape column in each row is used to hold the geometry or shape of each feature.

 In the feature class table:

 Each feature class is a table.

  • Individual features are held as rows.
  • Feature attributes are recorded in columns.
  • The Shape column holds each feature's geometry (point, line, polygon, and so forth), or a reference to the geometry.
  • The Object ID column holds the unique identifier for each feature.

 In ArcSDE geodatabases, relational databases hold feature classes as tables in the DBMS. ArcSDE is supported on five RDBMSs: Oracle, DB2, Informix, SQL Server and PostgreSQL.

SQL scripts for GDB_FeatureClasses:

 1 CREATE TABLE [sde].[GDB_FEATURECLASSES]
 2 (
 3 [ObjectClassID] [int] NOT NULL,
 4 [FeatureType] [int] NOT NULL,
 5 [GeometryType] [int] NOT NULL,
 6 [ShapeField] [nvarchar] (32) COLLATE Latin1_General_CI_AS NOT NULL,
 7 [GeomNetworkID] [int] NULL,
 8 [GraphID] [int] NULL
 9 ) ON [PRIMARY]
10 
11 GO
12 CREATE TRIGGER GDB_FC_TR ON sde.GDB_FeatureClasses FOR INSERT, UPDATE, DELETE AS BEGIN UPDATE sde.GDB_tables_last_modified SET last_modified_count = last_modified_count + 1 WHERE table_name = 'GDB_FEATURECLASSES' IF @@ROWCOUNT = 0 BEGIN INSERT INTO sde.GDB_tables_last_modified VALUES ('GDB_FEATURECLASSES', 1) END END 
13 GO
14 ALTER TABLE [sde].[GDB_FEATURECLASSES] ADD CONSTRAINT [GDB_FC_UC] UNIQUE NONCLUSTERED ([ObjectClassID]) ON [PRIMARY]
15 GO

Spatial Indexes:

The geodatabase uses a system of grids to create spatial index. When performing such tasks as panning, zooming, or selecting features in ArcMap, the spatial index is used to quickly locate features. That is, the geodatabase uses the spatial index to increase the efficiency of spatial searches on your data.

Spatial reference:

The spatial reference describes where features are located in the real world. You define a spatial reference when creating a geodatabase feature dataset or stand-alone feature class. The spatial reference includes a coordinate system for x-, y- and z- values as well as tolerance and resolution values for x-, y-, z- and m-values. The geodatabase uses the spatial reference to accurately display a features location and carry out geoprocessing functions.

Configuration Keywords:

Configuration keywords specify how data is stored in the geodatabase. Configuration keywords represent a setting or group of settings that tell the geodatabase where or in what format to store data contents in each dataset. 
The geodatabase uses configuration keywords to optimize the storage parameters of your data.

posted on 2012-03-30 15:19  Joshua Leung  阅读(247)  评论(0)  编辑  收藏  举报

导航