GIS

GIS

1. Geo-referencing & Map Co-Ordinate Systems

   • Geo-references
     • -How locations are determined Common types:  -place names -postal codes -linear references -co-ordinates
     • A geo-reference must be: 1-Unique (links to exactly one location) 2- Understood (so different users understand the meaning of a geo-reference) 3- Consistent (same projection is used for all map layers) 4- Persistent (through time, so today's geo-references are still meaningful tomorrow)
   • Ellipsoid
     • A reference surface for horizontal measurements 
   • Geoid
     • A reference surface for vertical measurements-ellipsoids don't take into account for the Earth's complex topography - A equipotential surface based on gravitational irregularities
   • Datum
     • 'a system for anchoring an ellipsoid to known locations on the Earths surface
   • Map Projections
     • Mathematical formula to transform the curved surface of the Earth onto a flat map

2. Vector

   • Object
     • Implementation of the object model of spatial data  Object: - discrete spatial features - each object has precisely defined spatial boundaries, buildings, roads, lakes -represented in the DB through points, lines & polygons - these are generally things we can see on the landscape 
   • Analysis
     • Queries
       • Queries do not change existing data sets or create new data by combining existing data sets
     • Measurements
       • Distance  Length or PerimeterArea ShapeRatio of perimeter to the square root of area Volume
     • Proximity-based Transformations
       • Buffering Overlays
       • The point in polygon algorithm - a point inside a polygon has a odd number of crossings - a point outside a polygon has an even number of crossings
       • Overly issues -Spurious polygons (sliver polys)    -Complexity of output map layers   -Dissolving adjacent polygons (spatial aggregation) to solve these issues

3. Raster

   • Regular tessellation of cells -Represents fields by assigning attribute values to cells 
   • Raster model is generally much better for modeling continuous features than vector and performing analyses that involve spread, flow, or diffusion processes
   • It is relatively easier to understand as a method of representing spatial features and processes and is usually computationally faster and simpler than vector
   • Field
     • -have properties that vary continuosly over space eg elevation, temperature, noise
   • Encoding Methods
     • Presence/Absence
       • This involves recording what you want into the cell. If a feature of interest is found within a cell, code the cell to reflect the presence of that feature and ignore all other features
     • Centroid of a Cell
       • Presence of an entity is recorded only if a portion of it occurs directly at the central point of each cell Appropriate for continuous data such as elevation, temperature
     • Dominant Type
       • Encodes the presence of an entity if it occupies more than 50% of the cell
     • Percent Occurrence
       • Identify what feature to record in raster layer (e.g. urban land) If several types of land use occur in a cell, then value of each cell reflects the percentage of the cell that the feature of interest (e.g. urban land) occupies
   • Compression Methods
     • Run Length Coding
       • scan a row, noting each unique cell value and the # of occurrences for each cell value
     • Chain & Block Coding
       • similar to Run Length Encoding but scans rows & columns to define 2D regions with same cell values
     • Quadtree Data Structure
       • recursive subdivision of raster cells into quadrants with the same cell value - uses variable cell resolution to reduce data storage requirements
   • Sources
     • Remote sensing Raster scanning Vector to raster conversion 
   • Analysis
     • Local
       • Is the most basic of the associative operations in Raster GIS. These functions produce a new grid where the value of each cell is determined as some arithmetic operation among the values of same-location cells on any number of input layers Boolean statements Reclass: aggregating values to produce new classes 
       • Overlay -Create new data by combining map layers of different features based on  -Combines 2 layers (fields) to produce a 3rd field (layer) f x f ---> f have to have the same # of cells and location 
       • Reclass - cell by cell operations  -neighbouring or distant cells have no effect on the output grid values -local operations include:  -reclassification (assign a new value to each unique value on the input layer) f--->f scalar -overlay 
     • Zonal
       • This means of association produces a new grid of zones that summarize the values of a raster grid over the areas covered by the zones in a zone grid.  Operation on groups of cells 
     • Focal
       • This class of associative functions evaluates a new grid by summarizing statistics in the neighbourhood around each cell. This may be on the same or a different layer
       • Surface Analysis Functions: associations of cells in a raster of elevations, a DEM, can be analysed to calculate slope & aspect
       • Filtering -moving window that is passed across the entire raster Slope and aspect derivation uses a method similar to filtering
       • Viewsheds: a surface that represents the field of view across a study area from a given location, calculated from a DEM 
     • Map Algebra
       • input raster layer(s) ---> [operation] ---> output raster layer 3 categories: local, focal, zonal 
     • More analysis
       • Inverse Distance Weighting- assumption that space is correlated -values nearby are not independent, and this correlation may be used to support the prediction of values at nearby sites Weighted Distance Buffers -can reflect differences in 'cost of distance' using a 'friction'surface -Resultant buffer will be wider in areas of low friction and narrower where cells represent higher friction values If moving straight across, it is a cost of 1*the cost of the cell itself. Or if moving diagonally, it is 1.414* the cost of the cell itself. 
       • A surface is a continuous field of values over an infinite number of points eg elevation, temp, concentration of a chemical 
       • Independence and Spatial Correlation- consider dependence between nearby points in space, a sample that occurs near in space is likely to be more similar than one that is further away (aka auto-correlation)
       • Cost Surfaces & Least Cost Paths
         • Cost Surface represents the total 'cost'of travelling overland from one or more starting points 1 - Generate a Friction Surface --absolute barriers- you cannot travel through that cell --relative barriers- you can pass through cell, but incur travel costs 2- Create an accumulated cost surface from the point(s) of origin to the destination cell(s) 3- Create a least cost path across the accumulated surface from origin to destination 

4. Spatial Data

   • Data is the foundation of a GIS It determines: -what theme - what types of analysis -where -when - quality of your analysis
   • Collection
     • Someone else
       • Buy it from vendors- government or commercial  Get it from free websites Data sharing agreements
     • Yourself
       • -Primary- collection of raw data from direct measurement in the field, eg GPS, Surveying, Remote Sensing -Secondary - data derivation from other sources -Hardcopies (scanning or digitizing) -Digital copies (data model or format conversion)
   • Metadata
     • Data about data Describes the identity, extent, quality, spatial and temporal schema, origin, condition and many other characteristics
     • It can come in many forms: file headers, XML documents, data dictionaries & definitions, time date and authorship
     • It adds value to an already valuable resource
   • SDIs
     • Spatial Data Infrastructure - a relevant base collection of technologies, policies & institutional arrangements that facilitate the avaliability of and access to spatial data. An SDI provides a basis for spatial data discovery, evalutaion, and application for users and providers. 
   • Uncertainty
     • Has 3 components: Error: the known uncertainty due to systematic and human limitations (chief uncertainty) Vagueness:uncertainty associated with a spatial or attribute & concept eg boundary or density  Randomness:  the component of of uncertainty that despite minimised error it cannot be modelled eg Human error or GPS signal affected by atomsphere
     • Error: accuracy( how close to the recorded data values are to the 'true'value precsion: interpreted in terms of how 'exact' data measurement & storage area aka as we repeatdly collect data for a point, how precise we are 
     • Sources of Positional Error - map projection, datum & their parameters...as soon as you use an ellipsoid/geoid there will always be some kind of error - primary measurement error - error determining location of features - instrument or human error...humans operate the instruments & they all have a range of skill levels -secondary data acquisition errors- digitizer/scanner resolution - media, quality of source maps & photos (the older it is, the more time has elapsed to distort that media) - operator, registration/interpretation error...friday afternoons -improper representation or conceptualisation of objects or phenomena- scale effects, resolutions, data model effects 
     • Screen or “heads-up” digitizing - digitizing features on computer screen based on digital orthophotos, it is relatively easy, but not as accurate as table digitizing
     • Attribute Error: 
     • Modifiable Areal Unit Problem

5. Topology

   • Branch of maths that describes the geometric relationships among objects that remain constant when the objects are subjected to certain types of geometric transformations
   • Planar Enforcement
     • A set of rules used to define a consistent method of building point, line and polygon features from spaghetti-digitized data. For example, planar enforcement includes rules that polygons of differing soil types cannot overlap, and that lines must be split at intersections.

6. Management

   • GIS Professionals
     • Must be educated Must follow ethical behaviour Should continue with their education In order to become a GISP, they are submitted for review
   • Ethics
     • Unethical behaviour involves: -lying with maps -using GIS to plan/support war -using GIS as a surveillance tool -plagarism 
   • Privacy /Legal Issues
     •  The need for ready availability of information for the social and economic benefit of the community; vs. The responsibility to ensure that the rights of individuals and groups are not infringed by abuse of that information.  e.g. In e-commerce, consumers’ private and  personal information is subject to collection,  storage, manipulation and analysis using:  Cookies, Personal digital certificates, Customer tracking systems,  Mass customization and personalized marketing, Dynamic  content servers, DBMS and data mining
     • Copyright (protects the expression of an idea) Trade Secrets(confidential info-providing a competitive advantage) Patents (protects the idea itself) Trademarks- word or symbol used to distinguish the wares or services of an organization from other
   • Project Management
     • Investigation  Analysis Documentation  Critical Appraisal & Review Presentation 
     • RFPs You have to convince the client that you are the best for the job.  Tips: -Take notes of what they are actually requesting Elements such as ToC and clear structure   Diagrams + figures preferable to lots of text   Do not give away too much   Contingencies: inflate the price by 10-15%

7. Hydro-logical Applications

   • Hydro-logical Geomorphometry
     • Geomorphometry is the quantitative science of land surface analysis Inputs: DEMs Methods: Image Analysis, Statistical, Mathematical, Algorithmic Outputs:  -surface parameters & objects
   • Surface
     • The land surface could be theoretically be described by a function of x & y (N,E)
     • Flow direction  -Direction in which water will flow -The direction of steepest descent  -Similar to aspect
   • Pits & Catchments
     • Local minima in the surface -Raster: cells with no lower neighbours
     • A catchment is the region contributing flow to a single outlet point Usually depicted as a polygon or contiguous region of a nomial raster values A basin is the catchment of a pit The lowest point on the perimeter of a basin is the pour point, and so is the outlet. A basins pour point is the point where pits will overflow
   • Automated Watershed Analysis
     • Take a DEM ---> Fill sinks---> Determine flow direction---> Determine flow accumulation---> Determine watersheds

8. 3D

   • 'the natural choice, as its how we see the world"
   • Isarithmic Map- depicts continuous smooth phenomena such as precipitation or elevation  2.5D  ArcGIS ArcScene- is a 3D vizualization app that allows you to view your GIS data in 3D Building & Texturing- you can use various apps to do 3D modelling like GoogleSketchUp, ESRI CityEngine

9. Time

   • It is unidirectional Yet it has no obvious physical form or properties, so it is difficult to conceptualise Can be absolute (6/06/2014, 1:50pm) or relative (In lecture time slot 5 aka 12pm  Time really hasn't received much attention in GIS until recently, this may be due to GIS databases being designed to handle static not dynamic situations 
   • Time data is stored as separate events in a GIS. There are 3 approaches to event representation  Datestamps: time recorded when an attribute changes, this means that the history of a single object can be selected & ordered.  Timeslices: separate layers used to represent passing time, this is the most common way to represent time in a GIS  Timeline: timeslices into single dataset DBMS enabling temporal queries aka Spacetime composites
   • Diagramming Examples
     • OpenSim Demo Static Time Mapping- Cartograms- map of London underground, you can click on any staion and it morphs the map into travel time, rather than distance (can literally turn time into space)  Time Geography Constucts - lifeline represents where we go in the day, a station is a location, domain represents a land area and a prism is the potential for movement. 

10. Distributed GIS

    • User, software, hardware and data can all be in different places, accessed via a network. Data in the cloud, client and server interaction with the desktop and mobile GIS
    • Network & Internet
      • Moving/moved away from desktop GIS delivery, to it being delivered from a remote server. This means it can handle concurrent access 
    • 3 tier architecture
      • Clients are situated at the top Middleware is in the middle  Server is at the bottom 
      • File-based doesn't handle large amounts of info well The DBMS Oracle cant directly talk to ArcGIS, so it has to use the middleware ArcSDE
    • Municipalities
      • Local government authorities- it is the most persuasive force in the industry - in charge of land management, zoning, property evaluation, road &  asset management, emergency response
    • Web Mapping Options
      • There is a limited number of options & the commercial  industry is dominated by ArcGIS, though a few use AutoDesk-MapGuide 
      • There are also plenty of non-commercial opensource tools
    • GI Services
      • 'replaces a local GIS function with one provided remotely by a server'
    • Mobile
      • Devices have become more compact & mobile in recent years
      • Issues -Egocentric Context - the location of the user is important -Limitations of the display due to its small size' -Certain environments unsuitable for certain media -Battery life is limited -Wifi/3G issues  -Security 
    • LBS
      • Location Based Services - information that it spatially fixed and can be delivered to network devices... “A location-based service (LBS) is an information service provided by a device that knows where it is and is capable of modifying the information it provides based in that knowledge"

11. Neogeography

    • Neogeography (literally "new geography") is the use of geographical techniques and tools for personal and community activities or by a non-expert group of users