Data source for Google earth Satellite imagery at least 15 meters of resolution Aerial photography the highest resolution, at 15 cm (6 inches)

Digital terrain model NASA's Shuttle Radar Topography Mission

3D buildings for many major cities in the US and Japan

GIS road, boundaries

Geographic coordinates (latitude/longitude) on the World Geodetic System of 1984 (WGS84) datum

Data: Usually less than 3 years old

• Google Earth approximate the sphere of the planet's surface with flat tiles. The further away your viewpoint is from the surface, the lower the resolution of these tiles can be.

• Download high-resolution data when the viewer is actively zooming towards it.

• Images for places you have already looked at are stored locally on your hard drive

How it works?

Primary & Secondary Sources

Primary geographic data sources are captured specifically for use in GIS by direct measurement.

Secondary sources are those reused from earlier studies or obtained from other systems.

Classification of geographic data for data collection purposes

Raster Vector

Primary Satellite remote sensing Aerial photographs

GPS measurementsSurvey measurements

Secondary Scanned maps or photographsDigital elevation models from topographic map contours

Topographic maps

GIS Data: Primary & Secondary Sources Primary data sources

– Created “in house”• Through your own or your team’s field data

collection• By transforming data from sources not yet

available digitally• For use by the same organization

– High level quality control– Often customized for specific

project/application– Costly

Secondary Data

Format conversion often required Datasets may be difficult to find

– Result: Data reproduced costly redundancy Data costs & sensitivity may limit access Need to be aware of vendor’s quality control

procedures to be able to judge data quality What type of information included about

data?– Scale, resolution, field names & descriptions,

codes & meaning– Need enough info to be able to make decisions

about whether data use is appropriate

Errors & Editing

Types of errors:– Entity error: Error in position

– Attribute error: Incorrect attribution

From Fundamentals of Geographic Information Systems, Demers (2005)

Entity Errors

Undershoot Overshoot

Entity errors Weird polygons: Polygons with missing

nodes– Cause: Point digitized in wrong place or wrong

order– Prevention: Map preparation or digitizing method– Fix: Moving lines to correct locations

http://www-users.aston.ac.uk/~elgyj/data_structures.htm

Error in position:

Incorrect Attributes

Missing attributes

(e.g., -9999)

Incorrect attribute values

difficult to detect, both in raster and in vector

Sources for error:– Hardware (digitizing, scanning)– People

• Digitizing• Attribute inputs • Uncertainty in data transformation and analysis

Data & Editing

Before beginning a spatial analysis or mapping project check data

Error detection and correction may include:– Compare data with input document– Check for missing spatial objects– Check attributes of spatial objects

Vector Editing

– Entities • Missing objects, incorrectly placed objects,

incomplete links

– Attributes• Typos, incorrect codes, attributes incorrectly

associated with a field

– Entities & attributes together• Codes associated with incorrect entities

After making changes:

– Rebuild topology• Changes modified some spatial relationships topology

needs to be updated

– Save!!

Incorrect attributes– Prevention: Keep track of attributes as typing in– Fix: Correct them! Save!

Raster Data

– Attribute values for grid cells – Associated tables

Editing– Concerned with verification correct

positions of grid cells• Examine column & row position, attribute code

Metadata

Data about data– Content, quality, condition

Component of the GIS data input process ArcCatalog

Why?– Organizations want to maintain their investment– To share information about available data

• Data catalogs & clearinghouses

– To aid data transfer & appropriate use

Introduction to Maps

DefinitionA graphic depiction on a flat medium of all or part of a geographic realm in which real world features have been replaced with symbols in their correct spatial location at a reduced scale.

Map: to transform information from one form to another --- Mathematics

Earth surface Paper map

--- Geography

Types of Map

Topographic Map:

A map showing the outlines of selected natural and man-made features of the Earth. It also shows roads and other prominent features. Topography = shape of the surface

Thematic Map:

A map showing the spatial patterns of a geographic theme, such as land use, climate, population etc.

Characteristics of Maps

• Abstracted – requires careful interpretation• Show only static situation --- one slice in time• Often highly artistic Map making is as much an art as it is a science.• Usually out of date• Questions easy to answer by maps ○ how to get there from here? ○ what is at a certain location?• Questions difficult to answer by maps ○ what is the area of this lake? ○ what can I see from here? ○ what this point on the topo map is on the thematic map

Maps and GIS

Disadvantage of Paper Maps Consistent, accurate retrieval of data from maps is difficult Only limited amount of info can be show due to constraints of paper

Advantage of Digital Maps Remote access High density of info. Powerful map analysis (impossible with paper maps) Accurate and rapid measurement of area Spatial analysis GIS offers significant advantages of map display over paper map Zoom to change scales freely Allows 3-D perspective views

Map elements• Most common:

– map/spatial data

– title

– legend

– scale

– north arrow

– inset(s)

– textual information (incl. spatial metadata)

– border, neatlines

– coordinate grid

Legend

Scale

Credits

North ArrowPlace nameInset

Ground

Figure

Neat lineBorder

Title

Map Elements

Major Map Elements Necessary components of a typical map

– Title

– Legend: Interpretive key to symbols • Symbols: Used to describe features

– Scale bar

– North arrow

Scale Issues

Map Scale:

The ratio between map distances and Earth Distances. Something from the Earth… (a geographic entity or phenomenon) … is scaled to be mapped at a much smaller size.

Note: We must always be aware of the implications of scale when using geographic data

General Classification of Scale

Large scale

– 1:400 to 1:50,000 Intermediate scale

– 1:50,000 to 1:250,000 Small scale

– 1:250,000 and beyond

The scale used for most U.S. topographic mapping is 1:24,000

1:24,000 1:100,000

Representing Scale on Maps

representative fraction “1:50,000”

Verbal “1 inch = 50 miles” Graphic scale

10 miles

Questions: How many miles does 5 cm map distance represent in the real world on a map of 1:320,000? How many kilometers does 5 inches represent in the real world on a map of 1:100,000?

What is the real world distance for 10 cm on a map with a scale of 1:500,000?

(A) 50 km.

(B) 500 km.

(C) 100 km.

(D) Not enough information to answer the question.