Center for Modeling & Simulation

A Map is the most effective shorthand to show locations of objects with attributes, which can be physical or cultural in nature

It is an abstraction process by which real-world objects are measured, documented, and stored on a medium which generally is a paper

It is therefore simplified and reduced in size

It is a graphical / dynamic representation of geographic data

Spatial data stored in the maps can be described by three concepts Entity Attribute relationship

Entity: it is a distinct spatial object of interest e.g. river, road etc

Attribute: a description of some aspects of the entity e.g. name , length, width, volume of flow etc

Relationship: spatial association among the entities etc river draining into the lake or sea

Share knowledge about world in many ways

Identify a location and its attributes

Locate where you are

With the aid of GPS you can also monitor speed of travel and direction

Identify distributions, relationships, and trends which are not otherwise discernible

Integrate data from diverse sources into a common geographic reference

Combine and overlay data to solve spatial problems

Can find best path between one point and the other

Model future events

If both describe the spatial data by concept of entity, attribute and relationship

then

Why is GIS better then maps?

Maps GIS

Unstructured representation entity generally represented by symbols as attributes

Structured data according to entity type along with the table of attributes (database)

Relationships on the maps is depicted visually

They are stored either explicitly as topology

General purpose/Reference Maps:

Not designed for any specific application

Focuses on locations

Show variety of physical and cultural features

Good base-maps for determining distance, areas, directions and cordinates

In GIS used for locational information of the spatial data

E.g. toposheets

Special purpose/Thematic Maps

Designed to depict a particular type of feature or measurement only

Produced at different scales using the reference maps

In GIS depict geographical phenomenon and processes

E.g. maps of population distribution, landforms, rainfall, rivers etc.

Geographic features can be natural or manmade

Three ways of presenting the geographic features on the map are

Discrete features Image or sample grid surface

Have distinct shapes represented by points, lines and polygons

Points: features too small to represented by lines or areas

Lines: features too narrow to be represented by area

Polygons: closed figures representing the shape and location of the homogeneous features

They are raster formats

It is a two-dimensional matrix of cells

Represent qualities such as colour, spectral reflectance, rainfall

Some aspects of the earth’s surface can be drawn as features like ridges, peaks, streams etc

Surface display portrays the shape of the earth, like elevation, slope etc

Similarly population density can also define the surface

Features on the maps are present within the database table and can be accessed through links to other databases

There can be numerous attributes to a feature

The most common are

Descriptive string: giving name, condition, type or category

Coded Value: Giving type of feature (numeric or abbreviated string)

Discrete numeric value: resents something that can be counted like, lanes on roads

Real numeric value: represents continuous data that can be measured or calculated like distance, area or flow

Primary key: A key to access attributes in the external database

Depicting type attributes: Coded values are used to draw symbols which depict the type of object

Illustrating measured attributes: numeric values can be drawn on the map by varying size of the symbols

Classified attributes: coded or numeric values can be resented by colours, which represent features which share a common value

Descriptive attributes: Descriptive strings can be drawn, next to, along or inside the features

Maps visually reveal

Which features connect to others Which features are adjacent to others Which features are contained within an area Which features intersect Which features are near others Difference in elevation of features Relative position among features

Maps in GIS also support spatial queries that list and select

◦ Title◦ Direction◦ Scale◦ Legend◦ Features

http://geodepot.statcan.ca/Diss/Reference/Tutorial/RM_tut2_e.cfm

Mapmakers usually orient their maps to show north at the top.

• How distance on the map relates to distance on the ground.

• Any scale can be used for a map, but a few common scales have been settled on for use by most organizations, like 1:24,000 by USGS

1:63,360 - 1 inch equals 1 mile 1:50,000 1:250,000 1:1,000,000

• Scale is expressed in 3 ways.

1.In words (One cm = One km)

2.Representative fraction (RF as 1:63,360, 1:25000)TRICK - REMOVE LAST 2 ZEROS TO GET ACTUAL SIZE ON GROUND IN METRES

3.Linear scale – Bar scale

Smaller the number on the bottom of the map scale, the more detailed the map will be and is called as large scale maps.

Larger number on the bottom of the map shows

fewer details and is called as small scale map

A 1:10,000 map will show objects ten times as large as a 1:100,000 map but will only show 1/10th the land area on the same sized piece of paper.

Large scale maps have a scale of 1:50,000 or greater (1:24,000, 1:10,000, ...).

Maps with scales from 1:50,000 to 1:250,000 are considered intermediate.

Small scale maps are those with scales smaller than 1:250,000.

Here are 3 views

The legend is the key to unlocking the

secrets of the map.

Typical RF

Large Scale Medium Scale Small Scale

Depicts small features

Small feature disappear

Symbolise features eg. Areas represented by points

Characters Show geometric shapes

Generalise geometric shapes

Show macro features like climate zones

Good compromise between map details and extent of map coverage

Things displayed on the map: towns, mountains, rivers, etc.

Which features depends on map’s purpose.