This curriculum assumes a basic understanding of GIS. In this section, we will review basic GIS concepts and terminology. Users who require an in-depth introduction to GIS concepts would benefit from taking the MEASURE Evaluation course, Geographic Approaches to Global Health.
A geographic information system (GIS) is software designed to capture, manage, analyze, and display all forms of geographically referenced information.1 It is essentially a database linked to a map.
Geographic data refers to features that have a spatial component. These features are commonly referred to as spatial data layers. A spatial data layer is composed of two components: spatial data, or the location of a feature (i.e., latitude and longitude); and attribute data, or information about the feature.
Geographic data in GIS is stored using two data models:
The basic element of the vector data model is a point. It is used to represent discrete objects such as locations of towns, rivers, roads, or region boundaries. Within the vector model, there are three data types:
Points connect to make lines, lines connect to make polygons, and polygons are grouped to form regions.
The most common file format for storing vector data is an ESRI shape file. Other file types exist that are software-dependent. A shape file is comprised of a minimum of three files with the following extensions: .shp; .shx; .dbf. However, it can also include files with other extensions, such as .sbx and .prj.
In the example above
Most paper maps you are familiar with use the vector data model:
In the raster data model, geographic features are represented on a grid of square cells, all the same size. The basic element is a grid cell.
Below is an example of how the same data can be represented using the vector and raster data models.
The raster model is best for representing
In practice, the data model that you use depends on software and data.
A coordinate system is a reference system used to represent the locations of geographic features, imagery, and observations such as GPS locations within a common geographic framework.
There are two common types of coordinate systems used in GIS:
Geographic reference systems use degrees as units of measurement, while projected reference systems units are either in metric or statute. So far we have worked with data in geographic coordinate systems. Coordinate reference systems are used to accurately identify locations on the Earth's surface. There are two types of reference systems: geographic (spheroid) and projected (planar).
Geographic coordinate reference systems are spherical systems referenced by their latitude and longitude values, where the units of measurement are degrees. Decimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many geographic information systems. DDs are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180°, respectively. As shown in the map below, the coordinates for Buenos Aires are in decimal degrees; those for Dar es Salaam are in degrees minutes, while those for Lhassa are in DMS.
Projected coordinate systems are planar reference systems based on a map projection. Map projections represent the three-dimensional Earth as a two-dimensional Cartesian coordinate plane. One commonly used system is the Universal Transverse Mercator (UTM). Other systems include Albers equal area and Robinson. The UTM Coordinate Reference System CRS divides the earth between 84°N and 80°S into 60 zones, each of which covers 6 degrees of longitude. The map below shows an example of the world using the UTM CRS.
We map data to see geographic patterns and relationships between data features. As a general rule, mapping follows four basic steps: