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.

1.3.1 Objectives

1.3.2 What is a geographic information system (GIS)?

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.

1.3.3 Geographic data

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.

graphic shows how pints on a map can be connected to rows of data
Source: Mayienda, R. 2003. Adapted from "Basic Training in Geographic Information Systems for Wildlife Conservation." Wildlife Conservation Society.

1.3.4 Spatial data models

Geographic data in GIS is stored using two data models:

1.3.5 Vector data model

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:

pictures of the three list items above
Source: Mayienda, 2003.

Points connect to make lines, lines connect to make polygons, and polygons are grouped to form regions.

shows a single point, a line made up of several points, and two polygons
Source: Mayienda, 2003.

1.3.6 File format for vector data

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.

combination image made from the ones above
Source: Mayienda, 2003.

In the example above

Most paper maps you are familiar with use the vector data model:

a color map
Gold Coast with Togoland. 1949. European Digital Archive of Soil Maps (EuDASM). Available at http://eusoils.jrc.ec.europa.eu/esdb_archive/eudasm/africa/maps/afr_ghgc.htm.

1.3.7 Raster 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.

shows a high altitude map, a portion zoomed in and a lined drawing of a grid.
Source: Mayienda, 2003.

Below is an example of how the same data can be represented using the vector and raster data models.

shows two color-block maps. the vector has smooth lines and teh raster is cubic. S
ource: Mayienda, R. 2014. Unpublished Map

The raster model is best for representing

shows a full color and two-color map
Source: Mayienda, 2003.

1.3.8 Vector versus Raster

In practice, the data model that you use depends on software and data.

1.3.9 Map projections and coordinate systems

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).

1.3.9.1 Geographic (spheroid) coordinate reference systems

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.

graphical representation of data in preceeding paragraph.
Source: Mayienda, 2003.

1.3.9.2 Projected (planar) coordinate reference systems

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.

see caption
Source: Wikimedia Commons, 2007. The longitude and latitude zones in the Universal Transverse Mercator system. https://commons.wikimedia.org/wiki/File:Utm-zones.jpg.

1.3.10 Basic steps of mapping data in a GIS

We map data to see geographic patterns and relationships between data features. As a general rule, mapping follows four basic steps:

  1. Frame your question of interest. In other words, what do you want to know about your data?
  2. Gather available data to answer your question.
    • Are existing data available to answer the question?
    • Can you triangulate multiple data sources?
    • Do you need to collect data?
    • Do you need to digitize data?
  3. Determine whether the question can be answered using a geographical approach.
    • Would a chart, graph, or table be a better format to communicate your findings?
    • Are you interested in displaying values or do you need a map to reveal patterns or relationships in the data?
  4. Select a method or combination of methods to help answer your question of interest.
ESRI. 2013. GIS Dictionary: GIS. ESRI 2013. Available online at http://support.esri.com/en/knowledgebase/GISDictionary/search/GIS, accessed on September 1, 2015.

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