GIS Data - Features & Layers

Concepts:
   features
   feature types
   layers

Any map is composed of one or more layers.  A layer is composed of one or more features.  These are the key building blocks of GIS data.

Features
Most 'things' in this world have a location.  That location can be plotted on a graph (a map) with x and y coordinates.  This is the simplest level of GIS data, locating something on a map.  These 'things' are known as features.  A feature is composed of two parts - its geography and its attributes.  The geography (also known as the geometry) is the location of the 'thing', typically represented as a line, point, or polygon.  The attributes are facts that we know about the 'thing'.  Some examples are shown below.

              
In these examples, we can see the location of certain ozone monitors (point locations based on their X and Y coordinates), and certain towns (the polygon representing the area of the town, as a series of coordinates which encloses an area).  We also can use the 'Identify' tool  in ArcMap to see the feature's attributes (what we know about the feature).

Feature Types
Typically spatial features can be categorized into one of three types:
   - point - the feature is represented by a single x,y coordinate.  Example: wells, monitoring stations.
   - line - the feature is represented by a series of x,y coordinates that are connected.  Example:  roads, streams.
   - polygon - the feature is represented by a series of x,y coordinates that enclose an area.  Example:  towns.


This example shows the three different types of features - ozone monitoring stations as points, the 3-mile coastal limit as lines, and the towns as polygons.

Layers
Features can be combined together into a layer.  A layer is a single entity in GIS that can be turned on and off in a map or analysis, and typically is a collection of features with a common purpose (such as all the roads, or all the streams in Maine).  Not just any group of features can be combined into a layer; they must meet these two standards:
   - All the features in a layer must be the same feature type (point, line, or polygon)
   - All the features in a layer must have the same attribute fields (you must know the same things about each one - though null values are acceptable).

In the example shown above, the ozone monitors are in a single layer because they are all points, and they have the same attribute fields (the name, number, status, etc.).  A McDonald's restaurant could be plotted as a point as well, but it is unlikely to be included in the same layer as the ozone monitors because it would have different attribute fields, and it is not likely to be related to the ozone monitors.

Turning layers on and off
Features that are organized as layers can be turned on and off together.  In this example, all of the ozone monitors are a single layer, and all of the towns are another, separate layer.  You can turn a layer on or off by clicking the check-box in the TOC.

Adding new layers to a map

The 'Add Data' tool on the 'Standard' toolbar is used to add new data layers to the map.  When this tool is clicked, a window will come up allowing you to browse for GIS data.

From here, you will need to browse to the folder storing the GIS data, select the data layer(s), and click 'Add'.

You will learn more about where to find data in the next section.

Removing layers from a map

To remove a layer, right-click on the layer name in the Table of Contents, and choose 'Remove' from the context menu.


Note that this does not actually delete the GIS data layer, it only removes it from your map.

Layer names
Layer names are typically the name of the data in the layer.  However, you can change this to whatever you want by single-clicking in the Table of Contents to select the layer name, and then single-clicking again to type in a new name.  Note that this does not change the actual name of the data layer, it only changes its name in your map.