Mapping Concepts

A GIS is composed of data layers that represent the different spatial properties of an area. Example data layers that would be important for agricultural geophysics would include data layers of topography, soil type, crop coverage, hydrologic features, land cover type, crop yield, and, of course, the geophysical measurements. New spatial data layers can be derived from existing data layers. This relationship can be defined as

When using geospatial data models, the spatial properties of each data layer need to be considered. This functionality can be expressed as the following:

F (P1, P2, ..., Pn) x,y,z = f (P1)x,y,z + f (P2)x,y,z + ... + f (Pn)x,y,z

where F ( ) is the model, and P1 ... + (Pn) are the parameters of the model. Unlike traditional methods, GIS data are organized based on geographic relationships among the data attributes. Areas that share the same property are grouped into a data layer. The attribute data in GIS layers inherit the spatial properties of the data. Data modeling is the focus when manipulating layers in a GIS. In general, the GIS model can be expressed as the following:

F (L1, L2, ., Ln) x,y,z = f (L1)x,y,z + f (L2)x,y,z + ... + f (Ln)x,y,z (10.3)

where F ( ) is the model, and L1 ... + (Ln) are the data layers in the GIS.

Geographic data, such as the topography of an agricultural field, crop yields, soil type, and geophysical measurements, are composed of two types of data: the spatial components (x,y) and the nonspatial attribute data (a). The spatial component stores the position of a feature, and the attribute data store the property or condition of that particular feature. Geospatial data can be categorized into four fundamental types—point, line, polygon, and surface—and are stored as either a vector or a raster format in a GIS (Figure 10.2).

Vector and raster data structures are the two major types of spatial data structures used in a GIS. A vector data structure is an object-based approach to represent the real world and includes points, lines, and polygons as spatial objects. A raster data structure is a field-based approach to represent geographic phenomena that are continuous over a large area.

A vector data format uses a series of points and mathematical functions to describe the shapes and boundaries of features. For example, a line is composed of two or more points. The polygon is represented by a series of points that closes at the original starting point. Currently, two types of vector data models are commonly used in a GIS. These include traditional vector data using Cartesian coordinates (the spaghetti model) and a topological vector model. The primary difference between these two types of vector data models is that the topological vector model adds the topolog-

Data Characteristics Space-feature locations Attribute-feature attributes, qualities and characteristics of geographic places Relationships between Features Time-additional spatial dimension

Data Characteristics Space-feature locations Attribute-feature attributes, qualities and characteristics of geographic places Relationships between Features Time-additional spatial dimension

GIS Data Elements and Characteristics

GIS Data Elements and Characteristics

Point

Polygon

Point sis

Lines

Image

Polygon

Surface

Surface

Image

Grid

Grid

Data Types

Vector

-Based on mathematical function -Point, line, polygon, and surface Raster

-Data present on a fixed grid structure (matrix) -Image, grid

GIS Data Layers

FIGURE 10.2 The four fundamental data types—point, line, polygon, and surface—in a geographic information system (GIS) that can be represented by a vector or raster data structure.

ical relationships—adjacency, containment, connectivity—to the basic elements of the vector data. These topological descriptions provide information in addition to the spatial relationships between features. Some common vector data formats include GBF/DIME, TIGER, DLG, AutoCAD DXF, IGDS DGN file, ArcInfo coverage, ArcInfo E00, shapefile, and CGM.

A raster data format is used to store data in a grid cell array. The size of a grid cell is fixed throughout the entire data set with each cell equally spaced. A regular grid is used and can be the shape of a square, rectangle, triangle, or hexagon. Because there is a fixed grid size, the spatial resolution of the raster data layer will be equivalent to the size of the grid cell.

The use of raster and vector data has strengths and weaknesses. The decision to implement a raster or vector data format relies on the convenience of implementation, the type of GIS operations to be performed, the desired scale and accuracy, and the format of the original data sets. One should be familiar with these data types and select an appropriate format. Table 10.1 provides a summary of the advantages and disadvantages of raster and vector data.

Nonspatial attribute data in a GIS can be a set of tables or individual data records from a database. These attribute data provide a description of features. Two methods are used to link spatial and attribute data: attribute relationships and spatial relationships.

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