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Introduction to GIS
Transcript of Introduction to GIS
”A computer system that can hold and use data describing places on the Earth’s surface” (Rhind)
”A system for capturing, storing, checking, integrating, manipulating, analysing and displaying data which are spatially referenced to the Earth” (UK Dept. Of Environment)
See: Heywood et al. (Ch. 1, Pg. 18)
Describing systems and architectures
Describing abilities, potential, uses
Themes and components
data manipulation, mapping, analysis
A comprehensive description
1. Quick and easy access to large volumes of data
2. The ability to:
Select detail by area or theme
Link or merge one dataset with another
Analyse spatial characteristics of data
Search for particular characteristics or features in an area
Update data quickly and cheaply
Model data and assess alternatives
3. Output capabilities tailored to meet particular needs
Heywood et al., 2006; pg. 18. (quoting UK Dept. of Environment).
Do these definitions introduce limitations or provide clarity?
Have/Will definitions change over time?
Why limit a GIS to terrestrial geography?
Discuss the definitions of a GIS and the implications of delimiting a GIS framework.
Come up with your own definition: base it on your reading or your own requirements.
Data have three modes or attributes (Heywood et al.)
- e.g. Acquisition time of a satellite image
- e.g. The type of object represented by point or vector data
- e.g. The location covered by the data
Attributes of Spatial Data
Heywood et al. Identify three types of spatial entity:
These are two dimensional
of something else. As such they are a simplification or generalisation.
Symbols or features are used to represent a reality. Collectively we call these spatial entities. Traditionally these would be encountered in maps (Heywood et al. P. 38). In a GIS spatial entities appear primarily as vector data (Heywood et al. Ch. 3).
Features and representations have a scale attribute
Scale affects the accuracy of the representation and the level of detail
If the data originate from a large spatial scale source they will be highly detailed
Data originating from a small-scale source would have a lower level of detail and be more generalised
Does a point have scale attributes? Without scale related generalisation a point or line feature would develop fractal edges as we zoomed in. Raster data, such as satellite imagery, have a spatial resolution attribute. Spatial resolution and scale are not the same. For a description of spatial resolution see Heywood et al. Pg. 62, or Mather, P. (2003) Ch. 2.2.1
Spatial data need to reference a (geo)spatial system. These can be:
A geographic co-ordinate system
e.g. latitude and longitude
A rectangular co-ordinate system
Such as map grid
A non-co-ordinate system
A system that uses a descriptor rather than co-ordinate system
Without ’georeferencing’ layers of data cannot be analysed together. Data need a spatial attribute to be used in a GIS.
The choice or use of spatial referencing system has implications for mapping and measuring. It is important that we know which system has been used (the system is described in the metadata- data that describes other data).
Measuring the area of an object, such as a lake or glacier, is not trivial.
In a cylindrical projection, such as Universal Transverse Mercator (UTM) areas are preserved, at the cost of spatial disortion near towards the poles (stretching). This leads to exaggerated distances between points. Latitude and longitude (a true geographic coordinate system) does not preserve area measurements as well as UTM meaning areas are distorted nearer the poles.
An Introduction to
Geographical Information Systems
Dept. of Physical Geography
an example of an arbitary grid
Satellites capture data that, in their basic form, are referenced to an arbitary grid (without geographic co-ordinates)
Each pixel (picture element) has a known spatial resolution (i.e. It extends over a known area)
Transformation to a projection or co-ordinate system is referred to as geo-referencing
Increasingly satellite data are the basis for mapping. It is therefore important we understand how to use satellite data.
Data are often delivered as an x,y grid with a known spatial resolution.
Metadata are often included describing the corner co-ordinates of the image, and/or the co-ordinates of the top-left pixel and the spatial resolution. These allow the user to transform the image to a map projection of geographic co-ordinate system.
A typical measurement of spatial resolution is the pixel size (also known as pixel spacing). A pixel is the picture element measured by the IFOV. In satellite data spatial resolution is used instead of scale to refer to spatial properties.
The scale of a map or other representation can be reduced by generalisation: in order to do so the spatial resolution of the raster data (e.g. Satellite image) would be reduced. Nevertheless, a large scale map, such as a 1:1000000, could still be produced from high resolution data (though it would be unnecessary to do so).
Satellite data have spatial, thematic and temporal attributes.
Discuss and list the attributes of satellite data.
Distance on the map compared to distance on the Earth
A respresentation of a reality
A geospatial representation of locations relative to a known point
GIS to Map
GIS are regularly used to produce map products from geospatial data.
Elements of a map
A GIS can be used to produce maps: hence we can take a satellite image dataset with a given resolution, and derive a map presenting features of the image at a given scale.
The map should include spatial referencing information such as a map grid, or co-ordinate graticules, a north arrow and scale bar!
Without such information the scale of spatial relationships is difficult to interpret.
A database structure able to access, store, manipulate and visualise (geo)-spatial data
Able to represent spatial, thematic and temporal attributes
Able to transform data between spatial referencing systems
Does this correspond with your list?
If not add to it while your memory is fresh.
Heywood et al., 2006. An introduction to Geographical Information Systems (3rd Edition) ISBN: 0-13-129317-6
Goodchild, Michael/NCGIA: What is GIS? http://www.ncgia.ucsb.edu/giscc/units/u002/
By the end of this presentation you should:
be able to describe some of the fundamental attributes of a GIS
be able to explain the attributes of spatial data
be able to identify different types of spatial entity
1:3 000 000 scale
1:1 000 000 scale
Sweden, 1:7 500 000 scale
Northern Sweden, 1:1 000 000 scale
Storglaciären, 1:17 500 scale
Plotting: Panopoly (NASA)
1:17 500 scale