Using location to get information

Location, location, location. That's the mantra of all real estate agents. The information stored in a GIS is linked to feature locations.

Visually, it's easy to compare the locations of some features on a map and draw conclusions. You can see that one freeway intersects another or that a lake is located within a particular county.

No man is an island, and, in a GIS, no feature is an orphan. All features can be related to other features based on their locations. Relationships based on location are called spatial relationships.

There are four basic types of spatial relationships: distance, containment, intersection, and adjacency. It's important to understand these spatial relationships because they provide a basis for making decisions and solving many problems. Getting answers to questions that are based on spatial relationships is the reason people use a GIS.

Because a GIS stores feature coordinate locations, it can quickly find and select features that meet some spatial criteria.

 

Distance

Points within a given distance of the red point are selected.

 

Containment

Points contained by the red polygon are selected.

 

Intersection

Lines that intersect the red line are selected.

 

Adjacency

Polygons adjacent to the red polygons are selected.

 

Examples of questions that can be answered using feature spatial relationships are:

·   How many houses are less than 1 mile from the airport?

·   Which parcels are contained by the contamination plume?

·   Which bridges intersect the fault line?

·   Which land uses are adjacent to the proposed subdivision?

A GIS answers questions based on spatial relationships by selecting features on the map. The map allows people to visualize information at a glance and detect patterns that aren't obvious in a written report or on a graph.

 

  • Distance

Distance

Which countries contain a lake completely within their borders?

Which cities are located within 25 kilometers of a river?

 

 

  • Intersection

Distance

Which countries have a river that intersects their border?

 

  • Adjacency

Distance

Which countries share a border with Russia?

 

  • Containment

Distance

Which countries contain a lake completely within their borders?