Intro to GIS
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What is a GIS?
What is a GIS?
A geographic information system (GIS) is a computer framework for linking geographic information (where things are), with descriptive information (what things are).
A GIS can be used to capture, store, manipulate, analyze, and present geographic data.
GIS tools help illuminate patterns and relationships based on geography.
'Everything is related to everything else, but near things are more related than distant things' - Tobler's first law of geography.
Applications of GIS
Applications of GIS
GIS covers a diverse set of tools and use cases. Everything from urban design, to infrastructure management, to environmental analysis, to digital humanities... GIS can be applied to almost any field.
Key concepts
Key concepts
GIS tools and workflows are based on the general principles of geography and utilize maps to help us analyze and visualize spatial patterns.
The key characteristics of a GIS are:
Geographic data are linked to descriptive information
Data in a GIS are referenced to a model of the earth
Data in a GIS are separated into thematic layers
GIS Data types
GIS Data types
GIS data comes in a variety of formats and data models, but the primary data types you will encounter are Raster and Vector.
Vector data
Vector data
Vector data is used represent real world features as Points, Lines, or Polygons. Vector features are usually discrete, meaning they have defined boundaries or exist only at specific locations. Each feature in a vector dataset will have an associated attribute entry in
Raster data
Raster data
Raster data represent the real world as pixels organized into a uniform grid or matrix (just like a digital image). Information in a raster is stored as values at the pixel location itself and can represent both continuous values (like elevation, precipitation, image colors) or discrete values where continuous data have been categorized into thematic classes (like soil classification or land use types).
Spatial reference systems
Spatial reference systems
A GIS spatial reference system (SRS) - aka coordinate reference system(CRS) - is a framework that defines the location of features on the Earth's surface.
A coordinate system in a GIS is composed of two main parts, the Datum and the Coordinate system itself.
Datum/Ellipsoid
Datum/Ellipsoid
A GIS datum in simple terms is a reference model for the shape and size of the earth. More accurately it sets the center point for various ellipsoidal models of the earth, onto which we map our coordinates.
There are a variety of different datums and they can be generally thought of as either global or local. Global datums distribute locational errors evenly around the model while local datums shift the ellipsoid to minimize locational error in specific regions, while exaggerating it in other parts of the model.
Coordinate systems
Coordinate systems
There are two main types of coordinate system in GIS, Geographic and Projected.
A geographic coordinate system uses latitude and longitude to locate points on a globe (degrees minutes and seconds). A projected coordinate system uses X and Y coordinates in linear units (feet or meters). While lat/long is fine for general use it doesn't work for most analysis tasks in a GIS, so we usually have to project our raw data into one of several different projected coordinate systems.
Geographic CRS
Projected CRS
Map distortion
Map distortion
All map projections distort the true shape of the earth. Only one characteristic of a globe can be fully preserved at a time (Area, Distance, Shape, or Direction).
Some projections are a Compromise between these characteristics, balancing each for a best fit but preserving none of them fully.
I Hate Coordinate Systems - a problem-based guide of common CRS issues, root causes, and solutions.
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