Using Data In GIS

A GIS stores information about the world as a collection of themed layers that can be used together. A layer can be anything that contains similar features such as customers, buildings, streets, lakes, or postal codes.

This data contains either an explicit geographic reference, such as a latitude and longitude coordinate, or an implicit reference such as an address, postal code, census tract name, forest stand identifier, or road name.

To work, a GIS requires explicit references. A GIS can create these explicit references from implicit references by an automated process called geocoding—tying an implicit reference like an address to a specific point on the earth.

Map Projections and Coordinate Systems

All the data layers you use must match up correctly for you to draw them on top of each other or combine them to see relationships. This means they must be in the same map projection and coordinate system.

Several issues are involved in choosing a map projection and coordinate system, including where on the globe the area you are mapping is located, how large the area is, and whether you need precise measurements of distance or areal extent.

Map Projections

This elevation map of North America uses the Lambert Azimuthal Equal Area projection.

A map projection translates the locations on the globe onto the flat surface of your map. All map projections distort the shapes of the features being displayed to some degree, as well as measurements of area, distance, and direction. (This is why on some maps of the earth, Greenland appears to be almost as big as North America.)

If you are mapping a relatively small area such as a town or county, this distortion is usually negligible. The larger the area mapped, the more this distortion will affect you.

• Download a projections table from ESRI for a comprehensive comparison of different projection systems.
• Visit the University of Texas Map Projection Overview to learn more about map projections.

This world map is shown in Gall cylindrical projection and the polar areas are displayed in Postel azimuthal projection. Physical Map of the World courtesy of T-Mapy and SHOCart, Czech Republic.

Coordinate Systems

A coordinate system specifies the units used to locate features in two-dimensional space and the origin point of those units. Latitude and longitude is a coordinate system (often called the "geographic" coordinate system).

If you are using an established GIS database, the data should already be in the same coordinate system and projection. If you are collecting data from various sources, you will probably need to rectify different systems.

• Download a table of map projections from ESRI for a comprehensive comparison of projections and their uses.
• Visit the University of Texas Coordinate Systems Overview to learn more about coordinate systems.

Using Geographic Attributes

Each geographic feature has one or more attributes that identify what the feature is, describe it, or represent some magnitude associated with the feature. The type of analysis you do depends partly on the type of attributes you are working with.

Types of attribute values:

• Categories
• Ranks
• Counts
• Amounts
• Ratios

Categories

Categories are groups of similar things. They help you organize and make sense of your data. All features with the same value for a category are alike in some way and different from features with other values for that category. For example, you can categorize roads by whether they are freeways, highways, or local roads, crimes by whether they are burglaries, thefts, assaults, and so forth.

Category values can be represented using numeric code or text. Text values are often abbreviated to save space in the table.

Ranks

Ranks put features in order from high to low. Ranks are used when direct measures are difficult or if the quantity represents a combination of factors. For example, it is hard to quantify the scenic value of a stream. You may be able to state, though, that the section that passes through a mountain gorge has a higher scenic value than the section near a dairy farm.

Since the ranks are relative, you only know where a feature falls in the order—you do not know how much higher or lower a value is than another value. For example, you may know a feature with a rank of 3 is higher than one ranked 2 and lower than a 4, but you do not know how much higher or lower.

This map ranks the vulnerability of groundwater in Indiana to pesticide contamination. Green is low risk, pale yellow is slight risk, orange is medium risk, and red is high risk. Map courtesy of Purdue University, West Lafayette, Indiana.

You can assign ranks based on another feature attribute, usually a type or category. For example, you would assign all soils of a certain type the same suitability for growing a particular crop.

Counts and Amounts

Counts and amounts show you total numbers. A count is the actual number of features on the map. An amount can be any measurable quantity associated with a feature such as the number of employees at a business. Using a count or amount lets you see the actual value of each feature as well as its magnitude compared to other features.

Ratios

Ratios show you the relationship between two quantities and are created by dividing one quantity by another, for each feature. For example, dividing the number of people in each tract by the number of households gives you the average number of people per household. Using ratios evens out differences between large and small areas or areas having many features and those having few, so the map more accurately shows the distribution of features.

This Households With Children ratio map shows steep declines in the proportion of households with school-age children in the inner core of the Portland Public School District. Learn more and see the full map.

Two special ratios are proportions and densities.

Proportions show you what part of a total each value is. For example, dividing the number of 18- to 30-year-olds in each tract by the total population of each tract gives you the proportion of people aged 18 to 30 in each tract. Proportions are often presented as percentages (the proportion multiplied by 100).

Densities show the distribution of features or values per unit area. For example, by dividing the population of a county by its land area in square miles, you get a value for people per square mile.

Continuous and Noncontinuous Values

Categories and ranks are not continuous values—there are a set number of values in the data layer, and more than one feature may have the same value.

Counts, amounts, and ratios are continuous values—each feature potentially has a unique value anywhere in the range, between the highest and lowest values. That is important to realize, because knowing how the values are distributed between the highest and lowest values will help you decide how to group them for presentation, so you can see the patterns.

Metadata

Metadata is frequently described as "data about data." Metadata is additional information (besides the spatial and tabular data) that is required to make the data useful. It is information you need to know in order to use the data. Metadata represents a set of characteristics about the data that are normally not contained within the data itself. Metadata could include

• An inventory of existing data
• Definitions of the names and data items
• A keyword list of names and definitions
• An index of the inventory and the keyword list for access
• A record of the steps performed on the data including how it was collected
• Documentation of the data structures and data models used
• A recording of the steps used on the data for analysis

Spatial metadata is important because it not only describes what the data is, but it can reduce the size of spatial data sets.

Spatial data also supports software-based and organizationwide standards. The benefit of having software-based data standards is that the program is easier to use, and users can readily move data between systems and platforms. By creating metadata, you are creating a standard in naming, defining, cataloging, and operating standards for all departments. This in turn is a vital foundation for understanding, collaborating, and sharing resources with others.

Spatial metadata is important because it supports easier spatial data access and management. Metadata provides a guide to the casual and novice user's question, "How do I know what to ask for?" Metadata can provide information on what is available in an area of interest, where the information is, how current it is, what format it is in, and what use constraints apply. For spatial data professionals, metadata provides feature- and attribute item-level metadata management. This way, updates are easily accommodated and integrated into daily use of the data. Metadata is not an end in itself; it is a tool that will greatly improve your work with spatial data and increase your overall GIS benefits.

Cartography

Mapping is an essential function of GIS. A map can present data in a fashion that other types of presentation media cannot.

Maps from a GIS are created from data in the GIS database. This means that any changes in the GIS database will be automatically reflected in the next printing of a map, allowing changes to a map to be made with minimal effort and cost.

GIS gives you the layout and drawing tools that help you make clear, compelling presentations in print or digital format.