Selected GIS and Epidemiology Terminology and Concepts

When exploring or using the LI GIS, it may be helpful to become familiar with the following terms and concepts:

Attributes: Characteristics of a geographic feature described by numbers, characters, and images, typically stored in a tabular format and linked to the feature by an identifier. A column in a database table. Attribute data are usually alphanumeric (consisting of letters and numbers).

Base Map: Map on which information would be placed for purposes of comparison. It may be used to construct other maps by the addition of information. Base maps usually contain mapped data that seldom changes and is used repeatedly.

Case: A person who meets the definition of who is to be identified as having the particular disease under investigation. The definition of a case will depend on whom one is trying to describe.

Census Data: The U.S. Census Bureau conducts a census of the U.S. population every ten years and publishes detailed descriptions of subgroups within the overall population for a variety of purposes. The U.S. Census Bureau organizes this data according to the following hierarchy of designated census areas.

Census Tract – Subarea of a county or city containing an average of approximately 4,000 inhabitants that have statistically comparable population characteristics, economic status, and living conditions.
Block Group – Subset of a Census Tract containing a population of approximately 800 inhabitants.
Census Block – Subset of a Block Group, the smallest geographic area for which census data are collected.

Centroid: The point that may be considered the center of a one- or two-dimensional figure.

Cluster: Aggregation of relatively uncommon events or diseases in space and/or time in amounts that are believed or perceived to be greater than could be expected by chance. Learn more about cancer clusters.

Data: General term for things known about real world entities; results of observations or measurements of such features from which conclusions can be inferred. (Note: The singular of data is datum.) Data has three potential components:

Attributes (see description above);
Geographic information describing the position of the datum in space relative to other data; and
Temporal information describing the instant or period of time for which the datum is valid.

Database: An organized set of data or collection of files that can be used for a specified purpose.

Dataset: A named collection of logically related data records arranged in a prescribed manner.

Demographic: Data characteristics of human populations, such as size, growth, density, distribution, and vital statistics.

Environment: All that which is external to the individual human. Can be divided into physical, biological, social, cultural, etc., any or all of which can influence the health status of the population.

Exposure Assessment: The determination of the proximity or actual degree of contact of a defined population with a putative (supposed) health hazard. Exposure is sometimes contrasted with dose, the amount that enters or interacts with the organism.

Features: Regions, lines, or points, usually but not always contiguous in nature.

Geocoding: Assignment of latitude/longitude coordinates to points, lines, or features.

Geographic Information System (GIS): A GIS is a computer system designed to allow users to collect, manage, and analyze large volumes of spatially referenced and associated attribute data. The major components of a GIS are a user interface system, database management capabilities, database creation and/or data entry capacity, spatial data manipulation and analysis packages, and display/product generation functions.

Incidence: The number of persons who fall ill with a certain disease during a defined time period.

Incidence Rate: The rate at which new events occur in a population. The numerator is the number of new events that occur in a defined period; the denominator is the population at risk of experiencing the event during this period. In a changing population, the denominator is the average size of the population at mid-period.

Land Cover: The materials that cover a study site, such as vegetation, bare soil, rock, sand, and water.

Legend: The part of the drawn map explaining the meaning of the symbols used to code the depicted geographical elements.

Line: A level of spatial measurement referring to a one-dimensional defined object having a length and direction and connecting at least two points. Examples are roads, railroads, telecommunication lines, streams, etc.

Map: Graphic representation of a part of the whole of the Earth’s surface using signs and symbols or photographic imagery at an established scale.

Map Layer: Refers to the various “overlays” of data, each of which normally deals with one thematic topic, such as breast cancer relative incidence, hazardous waste sites, or pesticide detections in the water supply. The overlapping layers of data are registered to each other by the common coordinate system of the database. Layers can be viewed individually or in combination with other layers.

Metadata: Data about data. This often includes the data’s source, accuracy, data type, projection, date of origination and other general descriptions.

Modeling: Applying structured rules and procedures to one or more spatial database overlays to conduct spatial and/or network analysis to derive new information to aid in problem solving and planning. It also refers to a process of simulation, prediction, and description that involves changing the parameters and generating new results or prediction of outcomes to what-if scenarios.

Overlay: The ability to superimpose graphically one map on another and display the result on a computer screen or on a plot.

Point: A single location in space. Many different natural and man-made features are modeled as points in a spatial database, including trees, hydrants, poles, buildings, etc.

Polygon: A two-dimensional figure with three or more sides that represents an area on a map. Many different natural and man-made features are typically represented by polygons in a spatial database, including soil types, water bodies, lot boundaries, etc.

Query: A logical search specification finding spatial features with linked records that contain matching geographic and/or nongraphic attributes. Once selected, additional operations can be performed, such as drawing them, listing their attributes, or summarizing attribute values.

Relational Database: A method of structuring data in the form of sets of records so that relationships between different entities and attributes can be used for data access and transformation.

Risk: The probability that an event will occur, e.g., that an individual will become ill or die within a stated period of time or age.

Scale: The ratio between map distance and real distance, where the map distance is usually represented as 1. For example, a map scale of 1:24000 means that one unit of measure on the map equals 24,000 of the same unit on the Earth. (1 inch would equal 24,000 inches, or 2,000 feet.)

Spatial Analysis: Study of geographic features and the relationships between them.

Spatial Data: A geographic dataset which is a collection of data that are individually or collectively attached to geographic locations, such as points, lines, or polygons.

Theme: The overall topic of a map in which the spatial variation of a single phenomenon is illustrated, such as breast cancer incidence, hazardous waste, or pesticide detection in water supply wells.

TIGER: Acronym for Topologically Integrated Geographic Encoding and Referencing database standard developed by the U.S. Bureau of the Census to support the 1990 census. Introduces political boundaries, feature names, and ZIP Code boundaries.

Topography: The features of the actual surface of the Earth, considered collectively according to their form, such as grassland, cultivated, desert, forest, swamp, etc. A single feature, such as one mountain or one valley, is called a topographic feature.

Topology: Relationships between spatial features including such things as continuity, nearness, inside versus outside, etc.