raster and vector 2 major gis data models data models: represent real world in digital form raster:...

Raster and Vector

2 Major GIS Data Models

Data Models: Represent real world in Digital Form

Raster: - divides entire study area into a regular grid of cell in a regular sequence

- Each cell = single value

- Space filling: every location in study area corresponds to a cell. Shows what occurs everywhere

- one set of cells & associated values = layer

Vector: - uses discrete line segments or points to identify locations

- Objects such streams or boundaries are formed by connecting line segments

- Objects do not necessarily fill space. Shows where everything occurs.

- one set of points, lines, or polygons = layer

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Creating a Raster:

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B.

C.

Features in the “real world”

Grid is placed over ‘real world” features

The smaller the grid, the more accurate the map & the larger the data storage or file size

Each block is a cell

Each cell is assigned a value

Automated methods used to enter data

Satellite image such LANDSAT contains7.4 million cells

Raster data can be compressed

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Classes of Raster Data Values

Multiple layers with cell values can be overlayed

Can add or subtract layers

Resampling may be needed in order to allow cellsfrom different layers coincide.

Remote sensing or satellite photos interfaces bestwith raster since images can be transferred to cellvalues using automated methods

Conversion between data structures: Vector to Raster Raster to Vector

Topology:

Mathematical method used to define spatial relationshipsbetween points, lines, and polygons

Topology Use in GIS

x, y coordinate or locationAdjacencyContainmentConnectivity

Topology is required to determine spatial relationships in a GIS

With or without Topology:

Computer software can NOT see features ashumans do:

Humans see implicitly

In GIS, features must be defined explicitly

Topology uses points, lines, & polygons (0D, 1D, & 2D)to explicitly define spatial relationships

Lower dimensional objects build higher dimensional objects: points-> lines-> polygons

Topology Importance:

1) Spatial Analysis- allows GIS to perform spatial queries.ie. Point in polygon, buffering , network analysis, polygon overlay

2) Processing Speed- is faster since relationships are defined

3) Data Storage- less data is required to be stored; thus allows smaller filesie. Polygon A & B share same polyline, thus need to store only once

Map Design and Components

Attributes of GOOD Map Design:

Clarity

Balance

Figure-ground

Hierarchical Organization

Color

Pattern

Lettering

Clarity: Point, line, & polygon symbols as well as text must clear & readableLines- clean, sharp, uniformShapes- symbols & shading are distinguishableText- legible

Balance: proper arrangement of component parts of map (title,legend, map body)

3 to 5 ratio pleasing to eye

Figure-ground: figure- portion of map that is being portrayed ground- formless area or background

Hierarchical Organization: To show levels of relative importance or comparisons on the map. Can be shown with point, line, or polygon objects

Color: by choosing one hue (one color range) can look at map to distinguish different objects or interpret hierarchy

Patterns: by choosing patterns that are increasing dense patterns, you can look at map and interpret hierarchy (without reading the legend)

Lettering: Text position and orientation rules to follow (Remember these are guidelines; sometimes rules must be broken)

Point objects: same font size & positioned horizontally above , below, left, or right

Line Objects: text should run parallel to the feature (ie. road or river) if text must follow vertical feature, it is easier to read going

up on left (of line) or down on right.

Polygon Objects: Text should be within boundary. Text should be horizontal if at all possible. If text must be tilted, curve the text.

Do not split text by other objects

Good LetteringBad Lettering

Components of a Map:

Title: Subject matter,date of data, & by what geography

Legend: explain symbols,color, patterns used.For thematic legends:Legend title = UNITS used,Ascending or descendingorder

North arrow (optional)

Scalebar (optional)

Source: Who, what, when

Prepared by: who, include date prepared

Map body