Lecture # 8GIS Data and Input

GIS data

Obtaining data is an important part of any GIS projectYou need to know

What types of data you can use with GISHow to evaluate itWhere to find itAnd how to create it yourself

Data Sources

Two types of data sourcesPrimary data

Data measured directly by surveys, field data collection, remote sensing

Secondary dataData obtained from existing maps, tables or other data sources

Primary data

We cannot usually observe the spatial distribution of a variable throughout the study areaTherefore we need to sample:

Take measurements of a subset of the features in the area that best captures the actual spatial variation

Sampling

The sampling density determines the resolution of the dataSamples taken at 1 km intervals will miss variation smaller than 1 kmStandard approaches to sampling:

RandomSystematicStratified

Random samples

Every location is equally likely to be chosen

Systematic samples

Sample points are spaced at regular intervals

Stratified samples

Requires knowledge about distinct, spatially defined sub-populationsMore sample points are chosen in areas where higher variability is expected

Stratified samples

Secondary data

More and more ready-made digital GIS data sets become availableGovernment agencies: census geographyTopographic surveysPrivate companies

Secondary data

Meta-data: “data about the data”Procedures used to collect or compile the dataData lineageAccuracy and measurement standardsCoding schemes

Required for both spatial and attribute data

Secondary data

Meta-data often absentThis leads to

MisinterpretationMisuseFalse perception of accuracy

Data sources

Framework dataSocioeconomic dataEnvironmental data

Framework data

Reference data to provide context for other dataRoads, rivers, elevation contoursTopographic survey

Topographic Map

Framework data

Digital Chart of the World (DCW)Largest scale consistent digital data set for the whole world (1:1 million)Designed for air navigation -> not necessarily appropriate for other uses

Socioeconomic data

Data about humans, human activities, and the space and/or structures to conduct human activities

Demographic dataMigrationHousingTransportationEconomic activity

305

306304

303

302

154 156

157

160

155

158159

Socioeconomic data

Referenced by

- Administrative units

- Settlements / villages

- Individual houses or facilities

Data input

Data input involves digital encoding of both geographic and attribute data

Data input

For attribute data:SpreadsheetsDatabase management systems

For geographic data:Coordinate entryDigitizingScanning

Data input

Conversion of hardcopy to digital maps is the most time-consuming task in GIS

Up to 80% of project costsEstimated to be a US $10 billion annual marketLabor intensive, tedious and error-proneDatabase development sometimes becomes an end in itself

GIS DATA INPUT SYSTEMTo do transfers of data (entities and attributes) from design to computer.Allows input in either vector or raster form of geographic entities.It is desirable to have a Cartesian grid and a projection system.Capability to link graphic elements and attribute editing

Seven rules of data inputKnow the userKnow the use (the objective or purpose)Avoid "exotic" data sourcesUse the best, most accurate, data necessary for your taskRemember the law of diminishing returns when considering accuracy levelsInput multiple coverages from the same map sheetEach coverage should be as specific as possible

Data Preparation

Input Devices

Coordinate conversion

Latitude is y-coordinate, longitude is x-coordinate Often need to convert from degrees, minutes, seconds (DMS) to degrees decimal (DD)

DD = D + M * (1/60) + S * (1/3600)

Global Positioning Systems

Determine current position based on signals sent by a number of satellitesGPS readings are in digital form -can be read directly into the GIS

Global positioning systemGPS uses satellite signals to obtain exact latitude/longitude coordinate

Handheld GPS are now available for about US $300.

Manual digitizing

Digitizing tables25 x 25cm to 200 x 150cmCost 300$ to 5000$Most common formof coordinate datainput

Digitizing steps

Fix map or aerial photograph on digitizing tableSelect control points (tics): easily identifiable points with known real-world coordinates (e.g., road intersections, graticule intersections, mountain peaks)

Digitizing steps

Trace features to be digitized with pointing device (cursor)Point mode: click at positions where direction changesStream mode: digitizer automatically records position at regular intervals or when cursor moved a fixed distance

Digitizing steps

Point mode: requires judgment by the userStream mode: creates lots of redundant coordinates

y

x

Digitizing table

Grid of wires in the table creates a magnetic field which is detected by thecursorx/y coordinates indigitizing units arefed directly into GISHigh precision in coordinate recording

Digitizing errors

Undershoots

Dangles

Spurious polygons

Digitizing errors

Any digitized map requires considerable post-processingCheck for missing featuresConnect linesRemove spurious polygonsSome of these steps can be automated

Scanning

Scanner output is a raster data setUsually needs to be converted into a vector representation

Manually (on-screen digitizing)Automated (raster-vector conversion) line-tracing - e.g., MapScan

Often requires considerable editing

ScanningAutomated vectorization: operator sets “global parameters” and system converts entire mapInteractive line following: operator points at specific line and system follows and converts the line

Heads-Up Digitizing II:

Raster-scanned image on computer screenOperator follows lines on-screen in vector mode

Scanning

Pre-processing can reduce editing requiredE.g., trace important features manually first (re-drafting)Scan clearer, simpler map

Scanning

Direct use of scanned imagesE.g., scanned air-photosDigital topographic maps in raster format

Digitizing / scanning

Source map is registered in a real world coordinate system with a projection and associated parameters; usually recorded in meters or feetDigitized coordinates are recorded in digitizing units (e.g., cm or inches from the table’s origin)

++

+

++

Settlements

Admin.Units

Reference Grid

Rivers

Longitude

Transformation

For spatial data integration, coordinates need to be transformed into real world units