Lecture 4: Data Source, Capture and Acquisition & Data Development

Data Source, Capture and Acquisition & Data Development

• GIS data is based on topographic features, which are part of the physical land structure. Topography involves the land's relief and the position of both natural and man-made features.
• Aerial photographs and satellite imagery can be incorporated into GIS systems to view data for the same region.
• Field and social surveys provide another rich source for powerful GIS applications
• Various data input methods are available, such as manual digitizing, scanning, direct entry (using a keyboard), transfer from databases and from GPS devices.

Presentation Outline

• Data sources for GIS
• Data capture and storage
• Primary and secondary data
• Data development
• Topology and topography
• Automated digitization

GIS Data Types

• GIS can handle several data types, including raster and vector data.
  • Raster data comes from scanned maps, aerial photographs, LiDAR imagery and satellite imagery.
  • Vector data includes contours, networks (roads, utilities), boundaries (post codes, electoral wards, geological boundaries) and instances (e.g., locations of crime events).
• Data may be analogue or digital and some data may require conversion to be usable in a GIS system.
• GIS data is available in hundreds of digital formats.

LiDAR (Light Detection and Ranging)

• LiDAR works similar to radar, but uses light waves rather than radio waves.
• Light is transmitted from a scanner to a target.
• The time it takes light to travel to a target and back is used to determine the distance to it.

Data Type

• Numerous sources of spatial information exist for GIS use.
• Data can be in various formats (analogue and digital).
• Some data is immediately usable in a GIS system.
• Other data needs conversion for GIS use during the acquisition process.
• Some data may not yet exist and needs collection.

Scanning of Maps

• Scanned maps are one source of GIS data.
• Scanned maps can be presented in formats that are compatible with GIS software, or they may require conversion.

GIS Data Capture And Storage

• Scanning
• Digitizing (Tablet and Onscreen)
• Remote Sensing
• Photography (Aerial)
• GPS
• Surveying
• Pen Computers

Significance of Scale in GIS

• Map scale is the relationship between the dimensions in a map and the real-world dimensions.
• A building that is 13m in the real world might appear as 13mm on a map with a scale of 1:1000

Scale of Capture

• All GIS packages allow zooming in and out on maps.
• All topographic data is captured at a specific scale.

Generalization

• Large-scale map data highlights specific details.
• Positional accuracy is high, causing the view to get cluttered, when zoomed out.
• Small-scale data shows fewer details when zoomed in.
• Features might need adjustments to accommodate the layout of the map symbols

GIS Data Storage (Polygon)

• GIS stores spatial data (map data) and attribute data (tabular data) separately.
• Data records for different regions and areas are stored separately.

Primary and Secondary Data

• GIS data can be classified as either primary or secondary.
• Primary data are newly collected for a particular purpose.
• Secondary data are existing data collected by others.
• Spatial data and attribute data can both be primary or secondary.

Collect Primary Spatial Data

• GPS, differential GPS, and total stations can collect primary spatial data, allowing for x, y, and z coordinate information.

Collect Primary Spatial Data Without Spatial Reference

• Spatial data can be collected without a formal spatial reference, but using known points in an image to conform to a spatial reference system.
• Digital photographs are an example.

Collect Primary Attribute Data

• Questionnaires, field observations, field surveys, and physical measurements are common methods for collecting primary attribute data.

Collect and Develop Your Own GIS Data

• Georeferencing, collection of spatially-referenced information (Using GPS or total stations), and collecting attribute information are key steps for developing GIS data.
• Combining this collected data and updating an existing GIS system is essential in maintaining GIS systems.

What about Secondary Data?

• Secondary data resources include analogue and digital data.
• Analogue data may need conversion before it can be used in a GIS system.
• Analogue data types include paper maps, aerial photographs, tabular data and photographs.
• Digital data types include digital maps, satellite imagery and digital photographs.

Data Development (Analogue to Digital Conversion)

• Four methods for analogue-to-digital conversion are scanning, keyboard entry, manual digitizing, and automated digitizing.

Keyboard Entry

• Keyboard entry involves entering data into computer files.
• It is used for attribute data.
• Two key issues are typographical errors and scale of entry.

Scanning

• A scanner creates a picture from a printed image.
• Map scanning might not capture attribute information.
• Storing the resulting raster data can use a lot of disk space.

Scanning (Automated Digitization)

• Scanning is an automated digitization technique.
• Scanning produces raster data which can be converted to vector data.
• Optical distortion is possible in the scanning process.

Manual Digitizing

• Manual digitizing involves several steps:
  • Spatial registration of the data source
  • Manual tracing of feature outlines
  • Error detection
  • Adding attribute data
  • Creating / verifying topology.

Digitizing Tablet

• A digitizing tablet is a board with a magnetic field, which enables the user to trace an image.
• The location of the cursor moves creates coordinate data

Onscreen Digitizing

• Onscreen digitizing is done using GIS software on a computer screen and a mouse.

Digitizing Process

• Digitizing is time-consuming.
• Each point or vertex needs to be digitized separately.
• Attribute information (e.g., type, shape) is attached to the features in the digitizing process.

Manual Digitizing (Points)

• Manual digitizing can be more efficient when using different modes (Point and Stream).

Manual Digitizing (Accuracy Issues)

• Physical factors affect the accuracy of manual digitizing, including shaky hands, and these must be accounted for.
• Digitizing errors can range from 0.8 mm to 0.054 mm depending on the factors.

Manual Digitizing (Shape / Topological Errors)

• Shape/topological errors like undershoot, overshoot, dangling nodes, pseudo nodes, missing label points and line crossings affect the quality of digitization work

Topology and Topography

• Topology describes logical relationships between objects in a GIS.
• Shape isn't emphasized in topology but important in topography.
• Topography describes precise location and shape of geographic objects.

Topological Building & Editing

• Important aspects in assessing objects are proximity, overlapping boundaries, connections between different features, and if one object completely fits within another.

Link-Node Topology

• Network analysis using link-node topology is one of the most important applications of GIS in real-world analysis.
• Examples of networks include roads, highways, water pipes, irrigation canals, and electrical power lines.
• GIS can be used in flood analysis, route planning and determining the distribution of resources

Topology

• Spatial relationships between geographic objects, are key to understanding how GIS systems analyze relationships between objects
• Geospatial data inherits topological relationships from the source data.

Automated Digitizing

• Automated digitizing utilizes algorithms to automatically trace lines.
• It is faster than manual approaches, but relies on the algorithm's quality.
• Details below the resolution of the data set may be lost in the automation process, requiring cleanup.

Vectorization

• Specialized computer systems convert raster data to vector data.
• Systems identify patterns in the image to generate lines (which might be linear features ).
• Vectorization is a faster method.
• However, it is typically less accurate than manual digitizing requiring additional cleanup steps.

Summary

• Several data sources exist for GIS, including warehouses, government departments, commercial providers, and satellite imagery.
• Primary data can be obtained through fieldwork.
• Secondary data from existing resources (warehouses, commercial providers) can be useful.
• GIS data can be enhanced by conversion of analogue data to digital and manual digitization, and automated processes.
• Accurate topological relationships between objects are essential, enabling GIS systems to understand object connections easily and accurately.

Test your knowledge on Geographic Information Systems (GIS) with this quiz. Explore concepts such as data types, sources, automated digitizing, and network analysis. Perfect for students and professionals looking to enhance their understanding of GIS.