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Basic GIS Concepts

1. What is a GIS?

2. What is SPATIAL Analysis?3. What is a Theme?4. Points, Lines and Polygons5. Overlaying Data6. Conducting the Query

(Set/subset basics and, or, not etc)

What is a GIS?

A GIS or Geographic Information System is a system designed for storing,analyzing, and displaying spatial data. Today, GIS manipulations are primarilyaccomplished through the use of computers, software and computer users.*

What is SPATIAL analysis?Spatial analysis is simply analysis that involves spatial data and gives youinformation that is spatial in nature. For example, if you were querying your USdata layer for high human population density, the areas of high density would belocated primarily on the West and East Coasts of the United States. That answeris spatial in nature.

What is a Theme?Each of our towns has many features such as roads, schools, shopping centers,rivers, lakes, forests, farms, to name just a few. A GIS stores these features asseparate data layers.

In Arcview, each data layer is called a theme.

Points, Lines and Polygons:Themes (data layers) are stored in the computer as 3 different types: points, linesor polygons.

Point theme: cities, wells, schools, airportsLine theme: roads, streamsPolygon theme: vegetation, land ownership

The significance of this is fairly obvious in these examples, but is important formore complicated analysis. You can compute the length of a line theme but not

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the area. You can compute the area or perimeter of a polygon theme. For thecomputer the theme type determines how it stores the data internally.

Overlaying Data:Each of the themes in a GIS can be overlayed upon one another. Think of an

overhead transparency projection system where each transparency shows adifferent data type. One shows roads, another streams, and yet another land usetypes. You can put all of these together, or look at just two data types.*

Also, you could do a simple analysis by creating a 1 mile buffer around the roadstheme. By defining a one mile strip around the roads theme, one could find outwhat types of land uses exist within that one mile strippotentially very usefulinformation for planners and developers.

A GIS such as Arcview GIS works in a similar way as described above onlyoperations and analysis are all done through the use of a computer and digital

data.

Conducting the Query:A query is a question that you form into an equation to ask a question of theparticular theme you are interested in. In Arcview you use the query builder tohelp you form your query. For example, in the theme land cover, you might wantto see where all the Aspen vegetation is in your county. For the query: Whereare all the Aspen forests? You would build the query:

([vegetation type] = Aspen)

Thus you have defined a subset of data from the original set. Note thatparentheses must be around your query. Also note that names such as Aspenmust be quoted. This is only true for names; numbers do not need to be quoted.

Suppose you wanted to query to ask the question: Where is the Aspen andDouglas Fir vegetation in my county? You would need to use an operator toconnect two statements such as and, or, not. Your query in Arcview would looklike:

([vegetation type] = Aspen) and ([vegetation type] = Douglas Fir)

* GIS in K-12 Education, April 1995, Arcview White Paper Series, Environmental SystemsResearch Institute, Inc. Redlands, CA.