map scale, resolution and data models. components of a gis map maps can be displayed at various...

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)

Copyright © 2006 by Maribeth H. Price

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


Talking about map scale

• A large denominator gives a small fraction a small scale map. It shows a large area.

• A small denominator gives a larger fraction a large scale map. It shows a small area.

1--------

50,000,000

1--------

500,000

1--------5,000

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 ScaleMap 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. Original scale

1:10 million

Original scale1:100,000

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


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.


2-9

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 Page units


2-10

Display scale

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

Helps avoid clutter.

1:260,425

1:325,582


Reference scale

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

When set, symbols change size after zoom

Reference scale not set Reference scale set

Reference scale


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

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


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


Impact of resolution

• Storage space increases by the square of the resolution

• Portraying large areas at high precision is problematic

90m resolution

10m resolution

Vector vs. Raster

Point s

L ineA r ea

Vect or Rast er

Point s

L ineA r ea

( x ,y )

X

Y

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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