data input and editing

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GIS Data Acquisition and Editing

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Data Input and Editing in GIS

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  • GIS Data Acquisition and Editing

  • IntroductionData Acquisition is the process of getting data into the computerSpatial data can be obtained from different sources, in different format and can be input in GIS in different methodsThe whole process of data editing and encoding is called the data streamAnalogue & Digital distinction.

  • Data Collection WorkflowPlanning includes establishing user requirements, garnering resources, and developing a project plan. Preparation involves obtaining data, redrafting poor-quality map sources, editing scanned map images, removing noise, and setting up appropriate GIS hardware and software systems to accept data. Digitizing and transfer are the stages where the majority of the effort will be expended. Editing and improvement covers many techniques designed to validate data, as well as correct errors and improve quality. Evaluation is the process of identifying project successes and failures.

  • Stages in Data Collection Projects

  • Important Guidelines for Data CaptureScale or resolution: How much detail do you need for your study?Measurement level: Do you need ordinal data, or are categories enough?Accuracy: How well can your measurement tool capture your data?Sampling method: Do you collect all the data in all the places you need?Timeliness: Do you work with time-sensitive data that change quickly and need to be collected right away?Data type: Are the data the appropriate data for you application, both in subject matter and in format? (Do you need field data or satellite data, for example, or do you need soil data rather than temperature data?)Data classification system: Do you use the same data classes as other layers in your database (for example, land-use classes from 1955 versus 2005)?Completeness: Have you collected all the data that you need to answer your question?

  • Types of Data AcquisitionPrimarySensory Data: those data most commonly associated with distant sensing devices, such as the Global Positioning System (GPS), Total Station and various forms of imagery, including both aerial photography and digital satellite dataStatistical Data: include field data and census data, both of which usually rely on some form of direct contact by a person to collectSecondary Data collected for other specific purposes can be converted for use in GIS e.g. Keyboards, Scanning and Digitizing

  • Primary Data Captureis data captured specifically for GIS useRaster - remote sensing (Primary Capture)usually involves actual sensor collectione.g. SPOT or IKONOS satellites and aerial photographyPassive and active sensorsResolution is key considerationSpatialSpectralTemporal

  • Primary Data Capture in Raster DataDisadvantages are: Resolution is often too coarse (especially with Satellite Mounted Sensors); Most Optical Sensors are restricted by cloud cover(Except Thermal and Radar sensors).

  • Vector Primary Data CaptureSurveyingLocations of objects determines by angle and distance measurements from known locationsUses expensive field equipment and crewsMost accurate method for large scale, small areasGPSCollection of satellites used to fix locations on Earths surfaceDifferential GPS used to improve accuracy

  • Total Station

  • Primary Data Capture: Field DataAssembling field data can involve conductingHouse-to-house surveyscollecting traffic data along roads recording the air temperature and other atmospheric datagathering soils, vegetation, insects, or any number of other environmental samples.

  • Primary Data Capture: Field Data SamplingIts physically impossible to collect temperature/elevation data everywhereIn each case, one is forced to collect data from a sample of the totalFor GIS, sampling of geographic space is required

  • Primary Data Capture: Field Data Sampling MethodsClustered: Sampling focuses on distinct areas that have a lot of features from which one can sample.Systematic: Use a specific, often regular, pattern to sample. For example, one sample at every meter along a line.Random: The sampling has no pattern at all

  • Primary Data Capture: Field Data Sampling Methods (Cont..)Stratificationdivide data into groups, or strata

  • Primary Data Capture: Field Data Sampling Methods (Cont..)Stratification (Example) To stratify your sample of who watches certain television programs in your city, you could divide the city into sub portions, or neighbourhoods. Then, you pick a certain number (for example, 25 people) in each neighbourhood to sample randomly, systematically (for example, every fifth house), or clustered (such as where housing density is highest).

  • Secondary Geographic Data Capture (SGDC)Data collected for other specific purposes can be converted for use in GISRaster conversionScanning of maps, aerial photographs, documents, etcImportant scanning parameters are spatial and spectral (bit depth) resolution

  • SGDC: Vector Secondary Data CaptureCollection of vector objects from maps, photographs, plans, etc.DigitizingManual (table) Heads-up and vectorizationPhotogrammetry the science and technology of making measurements from photographs, etc.COGO Coordinate Geometry

  • SGDC: 1- Keyboard EntryKeycoding, is the entry of data into a file at a computer terminal.This technique is used for attribute data that are only available on paper.Its may be appropriate for tabular data, or for small numbers of coordinates pairs read from a paper map source or pocket GPS.Text scanners and OCR software can be used to read data automatically.

  • SGDC: 2- Manual DigitizingThe most common method of encoding spatial features from paper maps.its also used for map encoding where topology is required and for digitizing features of interest from hard-copy aerial photographs.Manual digitizing requires a digitizing table that is linked to a computer workstation.Two modes of digitizing: Point & Stream modes

  • Digitizing

  • SGDC: Digitizing Cont.Manual digitizing of paper maps is one of the main sources of positional error in GIS.The accuracy of encoding depends on factors like : scale & resolution of the source map, the quality of the equipment and software being used.Errors can be introduced by incorrect registration of the map document or hand-wobble

  • SGDC: Digitizing Cont. Manual digitizing can also be used to digitize low volume of data on demand from scanned and geocorrected digital map images.

    Many GIS packages provides facilities for onscreen digitizing using raster backdrop images as a guide

  • SGDC: 2 (a)- Heads-Up DigitizingHead-Up Screen Digitizationto create vectors from raster layers is to digitize vector objects manually straight off a computer screen using a mouse or digitizing cursor and a GIS Software.

  • 3- Automatic DigitizingManual digitizing is a time consuming tedious process.When a large number of complex maps need to be digitized then a more expensive alternative is used: automatic digitizing Two methods: ScanningAutomatic line following

  • Scanning

  • ScanningMost commonly used method and appropriate when raster data are required.It is a piece of hardware for converting an analogue source document into a digital raster format.3 types: Flat bed scannersRotating drum scannersLarge-format feed scanners

  • Scanning (Flat Bed)

  • Scanning (Rotating Drum)

  • Scanning (Large-format feed scanners)KartoScan FB VLS

  • Practical problems: scanningThe possibility of optical distortion when using the flat bed scanners.The automatic scanning of unwanted information.The selections of appropriate scanning tolerance to ensure important data are encoded and background data are ignored.The format of files produced and the input of data to GIS software.The amount of editing required producing data suitable for analysis.

  • Automatic line followerAppropriate where digital versions of clear, distinctive lines on a map are required.It mimics manual digitizing and uses a laser and light sensitive device to follow the lines on the map.It is a vector device and produces output as (x,y) co-ordinates.Some difficulties faced when digitizing dashed or contour lines.

  • 4- Electronic Data TransferIts appropriate when the data is already available in digital formIn most time there is a need to transform of convert the data to an appropriate format compatible with the GIS software.Most GIS software will allow data conversionObtaining data from other sources requires users to address a range of important questions.

  • Cont.Spatial data may be collected in digital form and transferred from devices such as GPS, total stations, and data loggers.Data may be purchased from a supplier or obtained from an agency.Remotely sensed data are normally provided in electronic form.

  • Data EditingAfter data encoding, data may include some errors derived from the original source data, or errors introduced during the encoding processIts better to intercept errors before they contaminate the GIS DB.Data editing or cleaning can be done through four processes

  • 1- Detecting & Correcting errorsErrors in input data may derive from three main sources:Errors in the source dataErrors introduced during encoding (inputting)Errors propagated during data transfer & conversionErrors in attribute data are easy to spot & may be identified using manual comparison with the original dataErrors in spatial data are often more difficult to identify and correct.

  • Figure 5.11 Examples of spatial error in vector data

  • Figure 5.12 Examples of original data problems and the corrected data after processingSource: Laser-Scan. Copyright 2005 LS 2003 Ltd. All rights reserved

  • Cont.Most GIS packages will provide a suite for editing tools.Corrections can be done on-screen or automatically.Errors are also present in raster data.

  • 2- Re-projection, transformation & GeneralizationOnce spatial and attribute data have been encoded and edited, it may be necessary to process the data geometrically in to provide a common framework of reference.The projection systemDifferent sources (co-ordinate system)Different origins Different unit of measurementsDifferent orientationScale & Resolution

  • Figure 5.15 Topological mismatch between data in different projectionsSource: Courtesy of Peter H. Dana

  • 3- Edge matching & rubber sheetingWhen a map extends across two or three more map sheets differences or mismatches between adjacent map sheets may need to be resolved.The process involves three basic steps First mismatches at sheet boundaries must be resolvedSecond topology must be rebuilt as new lines & polygons have been created from the segments that lie across map sheetsThird, redundant map sheet boundary lines are deleted or dissolved

  • Figure 5.17 Edge matching

  • Cont.Rubber sheeting involves stretching the map in various directions as if it were drawn on a rubber sheet.Objects on the map that are accuratly placed are tacked down and kept stillOthers that are in the wrong location pr have the wrong shape are stretched to fit with the control points

  • Figure 5.18 Rubber sheeting

  • Geocoding address dataIts the process of converting an address into a point locationThe address itself, a postcode or another non-geographic descriptor is used to determine the geographical co-ordinates of a locationGeocoding can be affected by the quality of data

  • 4- Updating &maintaining spatial DBsThe world is a very dynamic place and things change.Using old and out-of-date map information would cost time and moneyKeeping Dbs up-to-date avoids problems and is a key aspect of ongoing data editing and maintenance.

  • Towards an integrated DBEach thematic layer in the DB must be encoded, corrected and transformed to create a GIS ready for analysis.

    he latest flatbed scanner has an interpolated resolution of 5400 ppi and almost 12,000 ppi for a drum scanner. 300300 dots per inch