7 Spatial data acquisition and management

Training on Natural Resource Management through GIS/RS1

There are five essential elements that a GIS must

contain:

• Data Acquisition• Pre-processing• Data Management• Manipulation and Analysis• Product Generation

GIS Functional Elements


Data Acquisition


Methods of Data Capture

Field surveysLarge scale topographic data acquired through Field surveys.

PhotogrammetryAerial photographs/ stereo images used for capturing topographicFeatures.

Satellite dataEarth Resources Satellites a major source of data for GIS applications.Sensors on satellites measure radiation from earth’s surface which are later put to proper spatial relationship.

GPS dataGPS provides accurate and flexible positioning for navigation, surveying and GIS data capture.


• Digitizing or scanning analogue maps

Manual digitizing or scanning to convert analog maps into digital format.

• Socioeconomic statistical files

National censuses major source of socioeconomic datawhich can be linked with spatial data at different levels ofAggregation.

• Geophysical and environmental data files

Global organizations working for GIS database on various geophysical and environmental themes, e.g, Digital chart of the world (DCW) and FAO soil maps.



•Data Capture from existing maps

Maps provide spatial and non-spatial information of geographic phenomena Attention needs to be given to map properties like scale, resolution, accuracy, precision and time of map productionwhile using them as a data source for GIS.

•Manual digitizing

Digitizing is the process where features on a map or image are converted into digital format for use by a GIS.Map to be digitized is affixed to a digitizing table. A pointing device used to trace the features on the map Digitizing table electronically encodes the position of the cursor with a precision of a fraction of a millimeter.



On-screen digitizing is a combination of scanning and manual screen digitizing is a combination of scanning and manual digitizing.

The main steps in on-screen digitizing typically include:

Scanning the map – a user can scan the map at a high resolution.

Registering the map – using the transformation methods user can enter control points on screen and transform the enter control points on screen and transform the scanned image to real world coordinates.

Digitizing the map – the user can zoom to specific areas on screen and trace points, lines, or polygons on the map. Because the maps are already in the correct geographic coordinate system anything digitized on top of the map will also be in the correct coordinate system.

On-Screen Digitizing


Data quality measures allow judgment to be made about spatial data’s worthiness for sharing and data integration.

Components of spatial data quality:

• Lineage • Positional accuracy• Attribute accuracy• Completeness• Logical consistency Timeliness

Data Quality and Assessment


Components of spatial data quality

• Lineage refers to the history of the data set such as source ofdata, methods of acquisition, date, scale and structure ofsource material, accuracy and precision of instrumentationand assumptions made.

• Positional accuracy is defined as the nearness of location ofa real world entity to its true position.

• Attribute accuracy is defined as the closeness of attributevalues to their true value.



Components of spatial data quality

• Completeness concerns with the degree to which the dataexhausts the universe of possible items.

• Logical consistency deals with the logical rules of structureand attribute rules for spatial data

• Timeliness of data deals with time when the data werecollected and published. Timeliness is a very important factorwhen analyses are based on data from different sources.



• Accuracy - absolute accuracy

• Precision - relative accuracy

• Accuracy refers to the location of the map feature

• Precision refers to the measurement of that location

• Precision and accuracy together define the reliability of thedatabase coordinates.

Accuracy and Precision


Direct and indirect spatial data acquisition

Direct spatial data acquisition Indirect spatial data acquisition


Point and stream mode digitising


• On-screen more comfortable for the operator• On-screen generally more accurate ( zooming

facilities )• On-screen faster (semi-automatic, digitising and

editing at the same time)• On-screen up-dating procedure ( geometrically

corrected satellite imagery and scanned aerial photo’s can be overlaid with the old vector data )

• Digitising tablet –easier to do larger maps

On – Screen VS Tablet Digitising


Captured image is a raster. Records intensity of illumination Scanned image needs to be converted to a vector Vectorization process that attempts to distill points, lines and areas from scanned image consisting of pixels removal of all pixels that make the line wider than one pixel

Scanning


Original paper map


• Problems in Data Input and Editing

There are two main problems. – Manual operations

• It is difficult to automate data input and editing because of un-• removable noise and incomplete original maps, which result in a • large amount of manual work with resultant inefficiencies in time • and cost.

– Unreliability of input data• As the input involves many kinds of errors, mistakes and • misregistration because of the manual input, further effort should be • applied to obtain data of high quality and reliability.


Spatial Data Management

• Partitioning– Adaptive Tiling– Fixed Tiling

• Spatial Indexes• Metadata

Main Components– Identify the data– Identify the custodians and access conditions for the

data– Projection, status, content, lineage, quality

7 Spatial data acquisition and management

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