Coordinates are most often pairs (x,y) or triplets (x,y,z, where z represents a value such as elevation).
The coordinate values depend on the geographic coordinate system the data is stored in. Coordinate systems are
discussed in more detail in Chapter 6, Preparing data for analysis.
ArcGIS stores vector data in feature classes and collections of topologically related feature classes. The attributes associated with the features are stored in data tables.
ArcGIS uses three different implementations of the vector model to represent feature data: coverages, shapefiles, and geodatabases.
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Raster models
In a raster model, the world is represented as a surface that is divided into a regular grid of cells.
The x,y coordinates of at least one corner of the raster are known, so it can be located in geographic space.
Raster models are useful for storing and analyzing data that is continuous across an area. Each cell contains a value that can represent membership in a class or category, a
measurement, or an interpreted value.
Raster data includes images and grids. Images, such as an aerial photograph, a satellite image, or a scanned map, are often used for generating GIS data.
Grids represent derived data and are often used for analysis and modeling. They can be created from sample points, such as for a surface of chemical concentrations in the soil, or based on classification of an image, such as for a land cover grid. Grids can also be created by converting vector data.
Grids can store continuous values, such as for an elevation surface.
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They can also store categories, such as for a grid of vegetation types.
Grids storing categorical information can store additional attributes about each category. For example, a grid of vegetation types might storefor each categorya numeric code, the name of the vegetation type, a habitat suitability rating for certain wildlife species, and a general type code. This is unlike feature data, where attributes are stored for each individual feature.
The smaller the cell size for the raster layer, the higher the resolution and the more detailed the map. However, because the cells form a regular grid over the whole surface, decreasing the cell size to store higher resolution data substantially increases the total volume of data that must be stored.
ArcGIS can recognize and use rasters from image files in many different file types and from grids stored in
workspaces. You can add raster datasets to a map just as you would features, and you can inspect and organize them with ArcCatalog.
TIN models
In a triangulated irregular network model, the world is represented as a network of linked triangles drawn between irregularly spaced points with x, y, and z values. TINs are an efficient way to store and analyze surfaces.
Heterogeneous surfaces that vary sharply in some areas and less in others can be modeled more accurately, in a given volume of data, with a triangulated surface than with a raster. That is because many points can be placed where the surface is highly variable, and fewer points can be placed where the surface is less variable. ArcGIS stores
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triangulated surfaces as TIN datasets. As with rasters, you can add TIN datasets to a map in ArcMap and manage them with ArcCatalog.
For more information about raster data and TINs, see Modeling Our World: The ESRI Guide to Geodatabase Design.
Tabular data
You can think of a GIS as a database that understands geometry. Like other databases, ArcGIS lets you link tables of data together. Just about any table of data can be joined to an existing feature class or raster dataset if they share an attribute. For example, you may have a shapefile of census tracts with a tract number field and a tabular file of
additional census data also containing a tract number field.
You can link the census data to the shapefiles attribute table and map the additional data.
Geolocating is another means of getting tabular data on a map. Perhaps the simplest example of geolocating is plotting points based on tables of geographic coordinates.
For example, you can plot the locations of soil samples based on latitudelongitude values obtained from a global positioning system (GPS) receiver. You can also plot points by geolocating tables of addresses on an existing street network. This is often called address geocoding.
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ArcGIS supports both file-based feature models and database management system (DBMS) feature models.
The two file-based models are coverages and shapefiles.
Coverages and shapefiles employ a georelational data model. They store the vector data for the features in binary files and use unique identifiers to link features to attributes stored in feature attribute tables in other files.
The DBMS feature model supported by ArcGIS is the geodatabase data model. In this model, features are stored as rows in a relational database table. The rows in the table contain both the coordinates and the attribute information for the features.
Coverages
Coverages are the traditional format for complex
geoprocessing, building high-quality geographic datasets, and sophisticated spatial analysis.
Coverages contain primary, composite, and secondary feature types. The primary features in coverages are label points, arcs, and polygons. The composite featuresroutes/
sections and regionsare built from these primary feature types.
Coverages may also contain secondary features: tics, links, and annotation. Tics and links do not represent geographic objects but are used to manage coverages. Annotation is used to provide text about geographic features on maps.