Two GIS Concepts You Need to Know

Lots of folks spend years learning about Geographic Information Systems technology. But since we promised to keep this brief, we're going to boil it down to the two basic GIS concepts that you need to know before you can effectively use Geocortex web-based maps.

GIS Concept #1: Features have attributes associated with them

Imagine a tree. How would you keep track of and communicate information about this tree to other people who need to know all about it? You might use a database to keep track of what species it is, how old it is, how tall it is, how healthy it is, and any other attributes (characteristics) that are important. This tree is one record in a database. We call each category (i.e. tree height) a field.

Figure 1. Sample Record in a Database

ID: Type: Age: Height:
12 Cedar 110 67'

Now imagine a grove of trees for which you need to keep track of attributes. Because we are now dealing with more than one tree, it becomes relevant where each tree is so we know what information relates to which tree.

Figure 2. Database with Multiple Records

ID: Type: Age: Height:
12 Cedar 110 67'
13 Pine 135 80'
14 Spruce 120 72'
15 Cedar 120 70'
16 Spruce 105 65'
17 Pine 115 75'

We map the location of each tree and identify which attributes belong to which tree. This is the foundation of GIS. A GIS tells us where something is and what it is. Computers are synonymous with GIS, and using a computer we can have hundreds of fields (different attributes) for millions of records (trees).

Figure 3. Mapping Locations

You will be able to examine the attributes of various features as you use a Geocortex web-based map.

GIS Concept # 2: Information is separated into layers

We can also have other layers of information in our GIS. Our information on trees would constitute one layer of information. We could also have a layer with rivers and a layer with soil types (Figure 4). Any information can be represented as a layer.

A map represents the landscape in an artificial way. Vector layers represent features in one of several ways:

Points
A point is good for representing information in which it is necessary to show where a feature is, but its physical shape is not important (i.e. trees in the old growth tree layer).
Lines
A line is suitable to represent many real world features (i.e. the rivers in the river layer).
Polygon
Don't be intimidated by the name. It is really just a solid multi-sided shape. When you see a polygon, remember that everything inside the boundary has the attributes associated with the record (i.e. soil types in the soils layer).

You might hear people talk about coverages , themes , or shapefiles . All these terms are other names for layers of information.

Figure 4. Vector Layers

Sometimes a layer of information can be a raster layer, which is a grid that contains information. Rows and rows of pixels make a grid. If you see a photo from above on a web-map, it's a raster layer. In fact, this is how all digital camera Images are stored. Raster layers don't have attributes associated with them like vector layers, though they all contain pixel values. That single value might be the color of a roof tile, or it might be a measure of hurricane force for a location, or just about anything else.

Figure 5. A Raster Layer (left) contains pixel values (right)

With individual layers we can conduct analysis between layers or, for example, only display layers that are of interest to us. As you work with Geocortex web-based maps, you will be able to turn different layers on and off as need be.

Guess what? You're done with the theory! With that under your belt, you're ready to make use of this knowledge.

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