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

Data Source, Capture and Acquisition & Data Development Dr Lawal Billa Introduction to Geographical Information Systems [email protected] School of Geography, University of Nottingham.MC Presentation outline Data sources for GIS Data capture and storage...

Data Source, Capture and Acquisition & Data Development Dr Lawal Billa Introduction to Geographical Information Systems [email protected] School of Geography, University of Nottingham.MC Presentation outline Data sources for GIS Data capture and storage Primary and secondary data Data development Topology and topography Automated digitization  GIS DATA SOURCE GIS data is based on topographic features – that make up the physical structure of the land surface. -Topography includes the relief of an area and the position of both natural and man-made features Aerial photographs and satellite imagery can be incorporated into a GIS and viewed along with other data for the same area, as long as the ground extent of the image can be identified Field and social surveys The most powerful GIS applications use data taken from a range of different sources.  Data Input Remote sensing Maps Direct Entry Database GPS Keyboard  What sorts of data can a GIS handle? Scanned Maps 3D surfaces Contours Networks OS Map Sheets RASTER (roads, services, Drainage etc…) Aerial VECTOR Photographs Boundaries (postcodes, election wards, walls, geological boundaries LiDAR Imagery etc…) Instances (e.g. locations of crimes) Satellite Imagery Plus many others… Introduction to Geographic Information Systems Dr Nick Mount School of Geography, University of Nottingham [email protected]  Data type Potentially hundreds of sources of spatial information for use in a GIS Hundreds of formats – some analogue and some digital Some data will be in a format ready for use within a GIS Others will need converting for use as part of the acquisition process Some data won’t exist and will require collection So, lets look at some examples of data types you may wish to acquire… Dr Nick Mount [email protected] Introduction to Geographic Information Systems School of Geography, University of Nottingham Scanned Maps  LiDAR(Light Detection and Ranging) 1. Works like RADAR, except it uses light waves rather than radio waves 2. Light transmitted from a scanner to target 3. The time it takes for light to travel to target and back provides a range Introduction to Geographic Information Systems School of Geography, University of Nottingham  Boundaries Introduction to Geographic Information Systems School of Geography, University of Nottingham Dr Nick Mount [email protected]  Ordinance Survey Map Sheets Department of Survey and Mapping Malaysia (JUPEM) Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Aerial Photographs Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Satellite Data Wide range of satellite types including passive and active sensors Satellites generally transmit and/or receive electromagnetic radiation at certain ‘spectra’ to form an image Different image ‘bands’ correspond to different wavelengths of radiation Malaysian Remote Sensing Agency (Formerly MACRES) Dr Nick Mount [email protected] Introduction to Geographic Information Systems School of Geography, University of Nottingham Network data Introduction to Geographic Information Systems School of Geography, University of Contours Introduction to Geographic Information Systems School of Geography, University of Nottingham  Instances Introduction to Geographic Information Systems Dr Nick Mount School of Geography, University of Nottingham [email protected]  Where do I get this GIS data from? 1. Collect primary data yourself (e.g. fieldwork) Or… use a pre-existing, secondary data set: 2. Data warehouses (e.g. Digimap) 3. Commercial data provides (e.g. Ordnance Survey, Department of Survey in Malaysia) 4. Government (e.g. Census, statistic, economics) 5. Etc… Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Where to get GIS data in Malaysia?  Department of Survey and Mapping Malaysia (JUPEM : Jabatan Ukur dan Pemetaan Malaysia). GIS data; Topographical maps: Aerial photographs; GPS location etc  Meteorological Service of Malaysia : Rainfall , Temperature, Air quality, wind speed, air pressure, humidity etc.  Department of Environment Malaysia : Air quality, Air pollution, Landuse , Land cover, Forests etc,  Department of Irrigation and Drainage Malaysia (DID) : Rainfall, discharge, water level, evaporation, floods , flood levels. Drainage and River networks, Temperature, Humidity etc,  Malaysia Remote Sensing Agency (MRSA): Satellite images (LandSat, Modis, AVHRR, Spot, Ikonos etc) Introduction to Geographic Information Systems School of Geography, University of Nottingham  GIS data from Jupem Name : GIS digital data  Data is in DXF (Data Exchange File ) format, requires conversion to compatible GIS software format (shape, Coverage etc). Data also available in shape files (slightly more expensive)  Data represent different topographical sheet at scale 1: 50000, 1:25000,. Also available 1:10000 and 1:5000 (but not for sale to public). You must understand the unique codes of the Topo sheet to select your desire location and data.  Data contains thousands of uniquely-identified features which are updated periodically  Made up of 1 DXF file: data should then be sub separated into layer 1. Line feature layer: ( boundary, contours, roads , rivers etc) 2. Polygon feature layer (building foot print, lakes , rivers etc). 3. Point feature layer ( bus stop, trig stations, bench marks etc) 4. Annotation layer: (text, names etc) Layers can be separated into sub-layers Introduction to Geographic Information Systems School of Geography, University of Nottingham GIS DATA CAPTURE AND STORAGE Scanning Remote sensing Digitizing Photogrammetry - Tablet - Onscreen - Aerial photographs Vectorization GPS Surveying Pen computers  Significance of Scale in GIS A map scale is the relationship between the dimensions on the paper to the real distance on the ground A building is 13m in the real world but on paper/map it is 13mm. thus scale is 1:1000. In GIS and computerized mapping things are more complicated. A description of scale can lose its meaning on different computer monitor sizes. The image above may appear 13 mm long on some computer screens but not others  Scale of capture:- All GIS packages enable you to zoom in and out on the map data as much as you like All topographic data have a scale of capture – that is the source data was captured at a particular scale (paper map or an aerial photo). Importance of the source scale of data:- 1. Data from a particular scale should only be viewed within a certain range of magnification for it to make sense visually. 2. Combining two or more datasets together is only appropriate if they have an equivalent scale of capture.  large-scale map data shows details. The Generalization: positional accuracy of features shown on this type of map is very high but there is so much detail that if you zoom out the view becomes very cluttered The Maps that we recognize have been deliberately simplified. This large-scale data (left image) when viewed at a small scale (zoomed out), appears cluttered. The small-scale data (shown on the right) viewed at a large scale (zoomed in), appears very sparse Sometimes it may be necessary to alter a feature's true survey position slightly to make space for the map symbols. The thick red lines of a road are shown much wider on the map than the actual road is on the ground. This science of small-scale map production is known as generalization. Raster Vector GIS Data Storage:- (Polygon) Spatial data 2 3 (ARC functions or graphical map) 4 5 FID Area Postcode 1 Chera 56000 Attribute data 2 KL 57000 (INFO or TABLES) 3 A-4 Semenyih 22061 43500 4 Serd 43400 5 Kaj 43000  PRIMARY AND SECONDARY DATA  GIS data can be considered primary and secondary  Primary data are (New) data collected by the user for a specific purpose  Secondary data are (Existing) data collected by others that may provide useful information for a given purpose  Spatial and attribute data may be primary or secondary  It is OK to have primary attribute data linked to secondary spatial data or vice versa Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Collect primary spatial data You can use GPS, differential GPS and total stations to collect primar x,y,z coordinate information relative to a spatial reference system Dr Nick Mount [email protected] Introduction to Geographic Information Systems School of Geography, University of Nottingham Collect primary spatial data without spatial reference  There are instances in which spatial data are collected without a formal, spatial reference but by locating known positions on an image it can be made to conform to a spatial reference system  A digital photograph is an example: Introduction to Geographic Information Systems Dr Nick Mount School of Geography, University of Nottingham [email protected]  Collect primary attribute data  Questionnaires  Field Observations  Field Surveys  Physical measurements  Etc… Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham Collect and develop your own GIS data Update GIS with data Link both Geo-reference spatial and attribute information Collect Collect Collect attribute information spatially-referenced information without spatial information reference (GPS /total station) Introduction to Geographic Information Systems Dr Nick Mount School of Geography, University of Nottingham [email protected]  What about secondary data? Analogue Secondary Data Digital Aerial Photos Digital Maps Paper maps Satellite Imagery Tabular Data Digital Photos Existing GIS data Analogue data will need conversion to a digital format for use in a G Dr Nick Mount [email protected] Introduction to Geographic Information Systems School of Geography, University of Nottingham  DATA DEVELOPMENT Analogue to digital conversion Four main methods for analogue to digital conversion: 1. Scanning 2. Keyboard Entry 3. Manual digitizing 4. Automated digitizing Dr Nick Mount [email protected] Introduction to Geographic Information Systems School of Geography, University of Nottingham  Keyboard Entry  Entry of data into a file at the computer terminal  Used for attribute data that are only available on paper Two major issues: 1. Typographical errors (difficult to eradicate) 2. Scale of entry (may be solved through text scanning) Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Scanning:- The scanner will take any printed image and take a picture of it By capturing the image in digital form it can be stored on the computer and displayed on screen Map scanning does not provide for the capture of attribute information for features Scanning a map to raster data takes up a lot of disk space, so is not always the most efficient method It is however a very good method for storing and preserving the cartographic style of the map.  Scanning Also an automated digitisation technique Outputs raster data only (which may then be converted to vector) Optical distortion a possibility Scanning resolution an important consideration Produces a raster that will then need to be registered/geo-referenced to provide a spatial reference across the raster Dr Nick Mount [email protected] Introduction to Geographic Information Systems School of Geography, University of Nottingham  Manual Digitizing  Involves 4 main steps: 1. Spatial registration of the data source 2. Tracing the outlines of features manually 3. Error detection 4. Adding attribute data 5. Encoding /enforcing topology Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Digitizing:- Digitizing tablet A digitizing tablet is a board with a magnetic field embedded in the flat surface, as the cursor is moved around the map, its location is identified A map is laid flat on the tablet and an electronic cursor is passed over the features of the map. coordinate points of different shapes are transferred in digital form to create a vector layer  Onscreen digitizing Uses a tool of a GIS software to map out feature on a map Digitizing is done on the screen of a computer using a mouse  Digitizing process Digitizing is very time consuming because every single point or vertex must be captured individually Digitizing attaches attribute information to features. Eg. its type and shape  Manual Digitising: How many points? Point mode Stream mode Can be solved by using stream mode digitising in which computer algorithms optimise the digitisation by placing points at set intervals Digitiser slows around complex lines so more points are added at these locations Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham Manual Digitising: Accuracy issues Shaky hands produce differences in location of digitised points and actual locations Studies show positional accuracy between ±0.8 mm and ±0.054 mm For a 1:50,000 scale map, this is equivalent to error between ± 40 m and 2.7 m Greater error is expected on winding lines Occasional/first time digitizers will produce most error Dr Nick Mount [email protected] Introduction to Geographic Information Systems School of Geography, University of Nottingham Manual Digitising: Shape/Topological Errors Introduction to Geographic Information Systems Dr Nick Mount School of Geography, University of Nottingham [email protected]  TOPOLOGY AND TOPOGRAPHY Topology describes the logical relationships between the position of those objects  In a topological map the precise shape of the objects is not important  It shows a shape called Hyde Park and a shape called Serpentine lake  What is important is that the Serpentine Lake object/feature is entirely contained inside the Hyde Park object  Topography Topography describes the precise physical location and shape of geographical objects A topographic map, shows an accurate depiction of the shape of the park A precise alignment of the shape of the objects within it (a lake). Topological building & editing Are the objects next to each other? Do they overlap? Do they form a connection? Does one lie completely within another?  Link-node topology One of the most important examples of GIS in real world application is the ability to model networks ( Roads, highways, water pipes, irrigation canal , electric power lines etc.) A GIS can analyze the potential flow around these networks, a useful ability in flood analysis or route finding, water distribution etc. Network analysis can only be possible if data has correct topology Network model  Topology Spatial relationship is key to understanding how the computer analyses the relationships between objects Geospatial data will have topology inherited from the source material. When you digitize a map, the topology which is implicit in the visual interpretation of the map, should be built into the data. A lot of care is required to maintain this relationships. Unless the data is topologically correct the computer will not pick up the relationships  AUTOMATED DIGITIZING  Vector-based data uses automatic line tracing algorithms  Much quicker than manual approach  Wholly dependent on the quality of the algorithm  Features less than the resolution of the data set will be lost  Algorithm has no concept of the context of the information being digitised  Usually requires a great deal of cleaning Introduction to Geographic Information Systems Dr Nick Mount School of Geography, University of Nottingham [email protected]  Vectorization:- Specialized computer systems are able to convert raster data to vector data by recognizing patterns in the image. The system guesses/identifies the sequence of colored pixels that form a line across the image and are showing a linear feature of some kind. The system knows the extent of the real position of the image, it thus convert these shapes into vector information. This vectorization from raster data can be a fast method of GIS data capture because it is automated However, it is usually less accurate than manual digitizing and require thorough cleaning  SUMMARY There many potential sources of data for GIS including ware houses, government department, commercial GIS and satellite image providers Primary data can be collected for GIS through field work Secondary data can be acquired for GIS from warehouses and commercial clearing houses GIS Data can be developed by manual digitization, automated digitizing and data conversion from analogue to digital The correct topological and logical relationships between the position of objects, ensures that the computer will understand these relationships during analysis Introduction to Geographic Information Systems School of Geography, University of Nottingham