Map Scale, Resolution and Data Models

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Map Scale, Resolution and Data

Models

Components of a GIS Map

Maps can be displayed at various scales

Scale - the relationship between the size of features on a map and the size of the corresponding places in the real world

Scale is most commonly expressed as a ratio

1:100,000 or 1/100,000

Verbal

One inch represents 2,000 feet (1:24,000)

One cm represents 20 km (1:2,000,000)

Map scale

Ratio of distance on the map to distance on the ground

Dimensionless: cm or inches or mm…

1 cm on map = 100,000 cm on ground

Copyright © 2006 by Maribeth H.

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Talking about map scale

A large denominator gives a small fraction

 a small scale map.

It shows a large area.

1

--------

50,000,000

1

--------

500,000 • A small denominator gives a larger fraction

 a large scale map.

It shows a small area.

1

--------

5,000

Copyright © 2006 by Maribeth H.

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Large scale vs. Small scale

Large scale maps represent smaller areas and thereby show greater detail

Typically large scale maps > 1:24,000

Small scale maps represent larger areas with less spatial resolution

Typically small scale maps < 1:500,000

Typical scales

Metric

1:10,000 or 1:25,000

1:50,000

1:100,000

English System:

1:9,600 (one inch represents 800 feet)

1:24,000 (one inch represents 2000 feet)

1:62,500 (one inch represents (slightly less than one mile)

Map Scale

Map Scale

Digital vs original scale

Once in GIS data may be displayed at any scale, BUT

Original scale of the map does impact the precision and accuracy of the data.

You should not display or analyze data at scales very different from the original source data.

Copyright © 2006 by Maribeth H.

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Original scale

1:100,000

Original scale

1:10 million

Units

The cursor position on the screen may be read in several different types of units.

Map units are determined by the x-y coordinates stored in the data file.

Usually they are in degrees, feet, or meters

Display units can be set by the user, so that the coordinates may be viewed in any desired unit, such as miles.

Page units show the location on the map page layout, usually in inches or cm.

Copyright © 2006 by Maribeth H.

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Scale bar shows current scale as you zoom in/out

Set map units and display units in layer properties

Position bar shows current location of cursor

Display units

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2-9

Page units

Display scale

Visible range set less than or greater than a specific scale.

Helps avoid clutter.

Copyright © 2006 by Maribeth H. Price

1:325,582

1:260,425

2-10

Reference scale

Reference scale sets the size of features relative to a specific scale.

When set, symbols change size after zoom

Reference scale

Reference scale not set

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Reference scale set

Don’t confuse these scale terms

The map scale is the ratio of the map units to the ground units, e.g. 1:24,000.

It changes when you zoom in or out.

The display scale range controls whether a layer is visible only at certain scales.

The reference scale determines whether symbols/text also change size when the map scale changes

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GIS represents the real world

Data Model – the objects in a spatial database plus the relationships among them

(Bolstad 2002)

Each spatial feature in our data model is represented by two distinct types of data:

Coordinate or Geometric

Attribute data

REPRESENTATION AND DATA STRUCTURES

Coordinates and Attributes

Bolstad 2002

Data Models

Vector Data Model

Conceptualizes the real world using three different types of elements:

Points

Lines

Polygons

Bolstad 2002

Polygons

Bolstad 2002

Data Models

Raster Data Model

A conceptualization that uses a grid of cells to represent an area of interest.

Usually used to represent variables that change continually over an area

i.e.: elevation, slope, depth, mean temperature, rainfall, etc.

Raster contains 1 value indicating a single attribute— i.e.: road type

Only numeric attributes may be stored

Raster Data Model

Bolstad 2002

Raster Data Model

Bolstad 2002

Continuous data

Raster is the best way to store continuously changing values such as elevation

Analysis faster and more flexible than vectors for many applications

Some analysis only possible using rasters

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Impact of resolution

90m resolution

10m resolution

Storage space increases by the square of the resolution

Portraying large areas at high precision is problematic

Copyright © 2006 by Maribeth H.

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Vector vs. Raster

Y Points

Vector

( x,y )

Area

Line

X

Raster

Points

Line

Area

Bolstad 2002

Which model do I use?

Raster Model Advantages

Represent frequent changes in space well

Other raster layers overlay easily due to grid nature

Raster data is “simpler” for a computer to handle

Best method for digital images

Which model do I use?

Vector Model Advantages

Less memory intensive

Excellent for representing linear features

Best map representation for discrete objects

Allows for more accurate analysis on adjacency and connectivity

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