User:Aude/Intro to GIS

=Intro to GIS= Geographic Information Systems (GIS) help manage and facilitate analysis of spatial data.

What is a GIS?
GIS includes a number of components and capabilities:
 * Allows spatial data to be collected and data input into the system
 * Spatial data storage and retrieval
 * Permits data maintenance and updates
 * Allows manipulation and analysis of data
 * Displays data, in form of maps and other visualization

Some software packages only do some of these things, such as mapping only. Those would not be considered a GIS.

The mapping capabilities of GIS are best known, but other things GIS can do include measuring the distance between two locations, the area of a feature (such as a lake), identify features that overlap, etc.

Development of GIS
The first GIS developed in Canada, during the 1960s, to help in carrying out a nation-wide land use inventory. Manual methods for gathering data and making maps were extremely laborious, with immense amount of time involved to manually do this land inventory for such a large, vast country. At this point, computers were beginning to have the capability to store data. A GIS would allow Canadian natural resources and land use to be represented in a numerical, digital format, which would also allow analysis to be carried out and information to be synthesized in a way useful for decision-making. The Canadian Geographic Information System was an idea conceived by Roger Tomlinson. In the late 1960s, the United States Census Bureau developed DIME, a geographic information system to handle spatial data.

Commercial GIS companies and products began to develop in the 1970s, with some of the first including ESRI and Intergraph. GIS evolved from several types of systems, including engineering mapping, CAD, and photogrammetry (aerial photography) systems, property or parcel information systems, general thematic and statistical mapping systems, base map systems (e.g. DIME), image processing systems (e.g. for processing Landsat and other satellite imagery), and Geographic Names Information Systems that stored basic attribute information about places.

Types of data
GIS handle two basic types of together:
 * Location - where is the feature or phenomena
 * Attributes - characteristics about a place

GIS has the ability link or join the two together.

Location
Spatial data represents the location of features (and their attributes - "characteristics"), usually measured as a location (x, y, z) within a coordinate system. The location may be measured as latitude, longitude, and elevation, or any number of other geographic coordinate systems.

Measuring location
Numerous tools exist to allow us to measure location with a high decree of accuracy, such as Global Positioning System (GPS) or more traditional surveying methods.

Accuracy
The degree of accuracy in mapping systems and GIS does vary, depending on requirements. GIS data used for engineering purposes, such as construction of infrastructure, requires extremely high precision and accuracy. The consequences of error in engineering are high. But, the high level of detail and accuracy comes with a cost, namely very large data storage requirements, along with extensive time and effort to compile the GIS data. In other applications, such as census taking, level of accuracy required is less. In order to save on costs and reduce data storage requirements, spatial data is often generalized, with less precision.

Representing geographic phenomena
There are two basic ways that geographic phenomena can be represented in GIS:

Vector
Vector representation of geographic features is done using points, lines, and/or polygons.

Raster
With raster representation, geographic space is divided into an grid with cells of uniform size, evenly spaced across the grid. The grid is georeferenced to a coordinate system. Each cell has a numerical value that represents the value of an attribute at that particular location. Examples of raster datasets include satellite imagery and digital elevation models (DEM).

Topology
Topology is often used in GIS to represent relative location between geographic features.

Data input
Methods for inputting geographic data include:
 * Global positioning systems (GPS)
 * Manual digitizing of paper maps, with a graphics tablet and usually a puck which helps with accuracy.
 * Scanning maps, often with a large-format drum scanner, and then processing images on the computer (e.g. referencing, on-screen digitizing, automatic raster to vector conversion).
 * Import a text file, spreadsheet, or other file with a list of x,y coordinates.
 * Import a text file, spreadsheet, or other file with a list of addresses, and geocode them.

Cleaning data
GIS data that is digitized into the system often needs to be cleaned and processed. This includes edgematching, to make sure features, such as roads, match from map sheet to adjacent map sheets. The process of digitizing (tracing) geographic features from a map may also result in various errors and pseudo-features, such as "donut" polygons and dangling arcs.

Integrating spatial datasets
Integrating multiple spatial datasets requires each to use the same coordinate system, same units of measure, etc.

Applications of GIS

 * Boundary surveys and disputes - e.g. Where is Ellis Island?
 * Census
 * Environmental impact assessments
 * Infrastructure planning, design, construction, and maintenance
 * Land use zoning
 * Natural resources management
 * Property tax assessments
 * Transportation planning
 * Urban planning

Global issues
GIS has been useful in scientific research and management of global problems, such as desertification and climate change. It has the ability integrate data and information across borders.