Unit 1 - Understanding GIS
- Wendy Silva
Learning Objectives
Describe what a GIS is
Understand the two basic GIS data models
Describe the Geographic Approach to problem solving
Understand what you can do with GIS
Describe the two main types of coordinate systems
What is GIS?
Geographic Information System (GIS) is a system that can be used to capture, store, manipulate, organize, distribute, visualize and analyze data in a geographic context. If a thing or phenomenon can be mapped via an address, XY coordinate, intersection, etc., it can be made part of a GIS. Once the thing or phenomenon is mapped in a GIS, it can be displayed, queried, analyzed and made ready for presentation. Examples:
City Childcare locations each have an address. Therefore, these locations can be added to a map.
Coyote sightings in Toronto are a geographic phenomenon that can be mapped. Sightings can be collected by citizen engagement (submitted via 311, e-mail or a web form). This phenomena can be mapped using the location (address) at which the animals were sighted.
A GIS allows us to:
Organize this data into map layers stored in a database
Distribute this data via database connections or web services
Visualize this data by creating maps to draw the features using their differing characteristics
Analyze the data in their geographic context
A geographic information system (GIS) integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information. GIS allows us to view, understand, question, interpret, and visualize data in many ways that reveal relationships, patterns, and trends in the form of maps, globes, reports, and charts. A GIS helps you answer questions and solve problems by looking at your data in a way that is quickly understood and easily shared.
What is "Geographic Data"?
Geographic data is one of the most important components of a GIS. At its heart, GIS data is a digital map (spatial data) linked to a database (attribute information). Everything or phenomena (i.e. each childcare location) in the map is a feature that is linked to a record in a table. This integrated information is called geographic data.
For all intents and purposes, you are already part of a GIS. Your name is registered to your address. One mechanism that links your name to your address is your driver's information, which records your name, address, date of birth, and basic physical characteristics. With this geographic information, you can be mapped.
What is a "System"?
A working GIS integrates five key components involved in managing and processing geographic information.
Hardware
A GIS operates on a wide range of hardware types—from centralized computer servers to desktop computers (used in stand-alone or networked configurations) to handheld mobile devices like smartphones.
Software
GIS software provides the functions and tools needed to store, analyze, and display geographic information (including a database management system); tools for the input and manipulation of geographic data; and tools that support query, analysis, and visualization of that data.
Data
A GIS is useless without data to map and analyze. A GIS will manage and integrate geographic information with other data resources and can even incorporate a database management system (DBMS) to manage spatial data.
People
GIS technology is of limited value without the people who manage the system and develop plans for applying it to real-world problems. GIS users range from technical specialists who use it to design and maintain the system to those who use it to help them perform their everyday work.
Workflows
A successful GIS operates according to a well-designed plan and business rules, which are the models and operating practices (i.e., workflows) unique to each organization.
Turning geographic information into GIS data
Geographic information is defined as spatial information combined with attribute information. GIS data is the representation of geographic information as digital files in a GIS database. It is this digital data that can be mapped with a GIS.
Data is generally gathered by observing and recording information through a variety of devices, methods, and behaviors. As you record information available in the real world, you process, organize, and synthesize it as a collection of digital files in a GIS database. Mapping this data may provide you with new information and possibly new data.
The Geographic Approach
Geography is the science of our world. Coupled with GIS, geography is helping us to better understand the earth and apply geographic knowledge to a host of human activities. The outcome is the emergence of The Geographic Approach—a new way of thinking and problem solving that integrates geographic information into how we understand and manage our environment. The geographic approach involves integrating many factors on a map and interpreting their meaning in a holistic way by means of map overlay.
This approach allows us to create geographic knowledge by measuring the earth, organizing this data, and analyzing and modeling various processes and their relationships. The Geographic Approach also allows us to apply this knowledge to the way we design, plan, and change our world.
What can you do with GIS?
Using a methodology such as The Geographic Approach formalizes the analytic process with GIS, which allows a clearer understanding of the results and promotes a response that can be supported by the data. By applying The Geographic Approach to help us solve complex problems, we can make better decisions, conserve resources, and improve the way we work.
Creating a map with a GIS allows you to visualize spatial information. This often reveals relationships, patterns, and trends that provide you with the information you need to make more effective and useful decisions. The following are examples of the kinds of maps you can create with GIS.
Map locations - Mapping locations lets you find places that have the features you are looking for. It also enables you to see patterns in how features relate to one another geographically.
Map quantities - People often map quantities to find places that meet a particular criterion.
Map densities - People often map densities to show the number of features in an area unit to clearly see its distribution.
Proximity analysis: Find what's nearby - GIS can help you find out what is occurring within a set distance of a feature by mapping what is nearby.
Overlay analysis: Find what's inside - By mapping what is inside a specific area, you can monitor what is happening, identify which features are most likely to be affected, and take specific action, prioritizing your response.
Statistical analysis - Determine if there are patterns in your data to support your conclusions.
Map change - Mapping change in an area may allow you to anticipate future conditions, identify contributing factors, decide on a course of action, or evaluate the results of an action or policy.
Layers
A layer is a map or set of features that contain features of one type or theme, such as roads, population density, or coyote sightings. A map layer can be combined or overlaid with other layers that share the same geographic space in a GIS software tool to describe area of interest in different ways. Since these layers share the same space, we can perform spatial queries and analysis.
GIS Data Models
In the GIS world, there are two main data models used to represent real-world features: the vector data model and the raster data model.
Raster Data
A raster data model represents the surface of the earth as a grid of equally sized cells. In its simplest form, a raster consists of a matrix of cells (or pixels) organized into rows and columns (or a grid) where each cell contains a value representing information, such as temperature. The raster data model is often used to represent continuous phenomenon (i.e., without discrete boundaries) such as temperature, though it can be used to represent discrete phenomenon as well, such as landuse. Examples of raster data are: digital aerial photographs, imagery from satellites, digital pictures, or scanned maps.
An individual cell represents a portion of the earth, such as a square meter or a square mile. The level of detail is determined by the size of each cell. For example, a 1x1m cell size would provide a very fine level of detail whereas a 100mx100x cell size would provide coarser data.
Vector Data
The Vector data model is a coordinate-based data model that represents geographic features using sets of coordinate pairs (x,y) to represent locations on the earth. Using coordinate pairs we can define the three main types of vector geometry: points, lines, and polygons.
An individual entity that is constructed using coordinate pairs (i.e. an address point, a street centreline, or a ward boundary) is called a feature. Features represent real-world objects on the map, and they have specific locations on the earth's surface. A group of features of the same theme and geometry type is called a layer.
Each point feature is represented as a single coordinate pair, while line and polygon features are represented as ordered lists of vertices. Attributes are associated with each vector feature, as opposed to a raster data model, which associates attributes with grid cells.
Coordinate Systems
While a GIS represents reality, it is not reality. To be useful, a GIS map must accurately represent feature locations. To determine the location of features in the real world or on a map, you need a reference system—a standard framework for defining location. In a GIS, the reference systems used to determine feature locations are called coordinate systems. There are two main types of coordinate systems: Geographic Coordinate Systems and Projected Coordinate Systems.
Geographic Coordinate Systems
A geographic coordinate system (GCS) uses a three-dimensional spherical surface to define locations on the earth. A GCS is often incorrectly called a datum, but a datum is only one part of a GCS. A GCS includes an angular unit of measure, a prime meridian, and a datum.
A datum is a set of values used to define a specific geodetic system. A geodetic datum is a reference from which measurements are made. Datum is based on a reference ellipsoid, which is an approximation of the shape of the Earth. Ellipsoids are required because the earth is not a perfect sphere. A point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in grads). The following illustration shows the world as a globe with longitude and latitude values.
In the spherical system, horizontal lines, or east–west lines, are lines of equal latitude, or parallels. Vertical lines, or north–south lines, are lines of equal longitude, or meridians. These lines encompass the globe and form a gridded network called a graticule.
The line of latitude midway between the poles is called the equator. It defines the line of zero latitude. The line of zero longitude is called the prime meridian. For most geographic coordinate systems, the prime meridian is the longitude that passes through Greenwich, England. Other countries use longitude lines that pass through Bern, Bogota, and Paris as prime meridians. The origin of the graticule (0,0) is defined by where the equator and prime meridian intersect. The globe is then divided into four geographical quadrants that are based on compass bearings from the origin. North and south are above and below the equator, and west and east are to the left and right of the prime meridian.
Latitude and longitude values are traditionally measured either in decimal degrees or in degrees, minutes, and seconds (DMS). Latitude values are measured relative to the equator and range from -90° at the South Pole to +90° at the North Pole. Longitude values are measured relative to the prime meridian. They range from -180° when traveling west to 180° when traveling east. If the prime meridian is at Greenwich, then Australia, which is south of the equator and east of Greenwich, has positive longitude values and negative latitude values.
Projected Coordinate Systems
The transformation of the Earth's three-dimensional surface to create a flat map sheet is commonly referred to as a map projection. One easy way to understand how map projections alter spatial properties is to visualize shining a light through the earth onto a surface, called the projection surface. Imagine the earth's surface is clear with the graticule drawn on it. Wrap a piece of paper around the earth. A light at the center of the earth will cast the shadows of the graticule onto the piece of paper. You can now unwrap the paper and lay it flat. The shape of the graticule on the flat paper is different from that on the earth. The map projection has distorted the graticule.
A spheroid cannot be flattened to a plane any more easily than a piece of orange peel can be flattened - it will rip. Representing the earth's surface in two dimensions causes distortion in the shape, area, distance, or direction of the data.
A map projection uses mathematical formulas to relate spherical coordinates on the globe to flat, planar coordinates.
Different projections cause different types of distortions. Some projections are designed to minimize the distortion of one or two of the data's characteristics. A projection could maintain the area of a feature but alter its shape. In the graphic below, data near the poles is stretched.