EAT 2333 GEOGRAPHIC INFORMATION SYSTEM GIS CO 3

EAT 233/3 GEOGRAPHIC INFORMATION SYSTEM (GIS) CO 3: Ability to produce detail mapping using geographic information systems (GIS) BY : AYU WAZIRA AZHARI

Data Input, Editing & Maintenance

Data Input • Data input is the most costly and time consuming process in GIS. • Data input include: v. Data capture v. Editing and error correction v. Geocoding

Methods of Data Input i. Keyboard entry ii. Manual digitizing iii. Automatic digitizing (including scanning)

Figure 1: Digitizing table and PC workstation

Figure 2: On-screen digitizing

Figure 3: Types of scanner

Figure 4: Data input process

Data Editing • Editing is the process of making changes to a geographical database by adding new objects or changing existing objects. • This process is required to: �� correct data input errors �� making geographical data more usable �� update geographical information.

Errors in Data Input • Paper maps are unstable so that the control points must be re-entered if the map is moved. • The map may be stretched or shrunk in the interim of digitising making digitised points slightly off in their location. • Errors on the map will be carried into GIS database. • The level of error in GIS is directly related to the error level of the source maps.

Cartographic Errors • Maps are meant to display information and do not always accurately record locational information. • e. g. when a railway, stream and road all go through a narrow mountain pass, the pass may actually be depicted wider than its actual size. Original location Depicted wider Final map

Edge Matching Errors • Discrepancies across map boundaries can cause discrepancies in the total GIS data base. • e. g. Roads or streams that do not meet exactly when twomap sheets are placed next to each other.

Digitising errors • User error causes overshoots, undershoots and spikes at intersection of lines. overshoot undershoot spike

Error Detection and Editing Common errors in spatial data: • polygon not closed • sliver • gap, etc. Polygon not closed Sliver Gap

Examples of errors in spatial databases: Dead end Leaking polygon Invalid polygon Slivers Edge shift Attribute errors

Error Correction Procedures • Setting a fuzzy tolerance • Connecting nodes • Re-building topology

Data Pre-Processing • Purpose: convert data sets for GIS database (data integration) • Geographical data sets need to be converted to match: �� data type �� geo-referencing system �� data structure • Products: coordinated set of thematic data layers

Format Conversion • Data structure conversions: 1. Raster to raster: conversion between data formats 2. Raster to vector or vector to raster • Data medium conversions

Figure 5: Raster to raster conversion

Vector to raster conversion Vector superimposed on raster Original data set Resulting raster Skeletonizing Data after one thinning pass Figure 6: Vector to raster conversion Final skeleton

Data Reduction and Generalisation • Data reduction �� Simplify data records �� Change scale • Data generalisation �� Less precise representation �� Vector: less vertices and polygons �� Raster: resampling to larger pixel size

Figure 7: Map generalisation

MMU = 9 ha MMU = 1 ha *MMU = Minimum Mapping Unit Figure 8: Small area elimination

Merging and Establishing Topology • Clear minor errors and build spatial relationship • Building from elemental points • Creating topological structure • Linking spatial and aspatial data sets

Edge Matching • Joining map sheets • Automated or manual procedures Original two map sheets Two sheets brought together showing discontinuities Derived single sheet with edges adjusted

Rectification and Registration • Exact numerical approaches to rectification �� Transformation and map projection • Approximation approaches to rectification �� Rubber sheeting • Rotation, translation and scaling

Figure 11: Rubber sheeting