Week 7: Investigating Earthquake Activity
Getting to Know Spatial Querying in GIS
In week 5, you were introduced to two kinds of queries that can be carried out on features in a GIS: 1) database queries and 2) spatial queries. You then spent time creating and carrying out database queries. This week's focus is on spatial queries. The spatial information about any given feature drawn on a map involves geometric characteristics such as its length, area, perimeter, and shape.
What Does it Mean to Ask a Spatial Question?
Spatial queries, or questions, come in several forms. All spatial queries ask spatial, or geometric, questions of a dataset. The simplest type of spatial query is a question about one object. These simple questions might include asking about the object's shape, size, direction, orientation, perimeter, or length. For example, how long is the Ohio River? Or which mountain ranges are oriented North and South?
A spatial relationship takes place between 2 or more features and involves direction, distance, or topology. If you recall the example in the Introduction to Mapping Tabular Data in a GIS section, the breakthrough that Dr. Snow had in solving the mystery of the cholera outbreak was that he mapped the data and looked for the spatial relationship between the cases of cholera and the contaminated well water.
There are three broad categories of spatial questions:
Direction questions ask about the orientation of features on a map. They include questions such as:
What cities are west of Phoenix? Which rivers are north of 40 degrees latitude? Which direction do hummingbirds migrate? Which direction do tornadoes or hurricanes travel?
Distance questions ask about features within a given distance of other features. Distance questions can also be thought of as proximity, or how close is one object to another. They include questions such as:
How far is it from Boston to New York? How far apart are all the earthquakes with magnitude of 7 or greater? How far are the US thermal springs from faults? How many cities are within X distance of an earthquake? How many earthquakes are withing 100 miles of plate boundaries?
Topology questions ask about how the geometric features, i.e. points, lines and polygons are spatially related. They include questions such as:
Do they share boundaries? Do they cross? Is one contained within another? Do they overlap? Are they clustered around a central feature? Are they one side or the other.
One type of topology query is a containment question. Containment questions involve a polygon that contains (or excludes) other datasets, including points, lines and/or polygons.
Some examples of containment questions are listed below.
- How many rivers are contained within in the United States?
- What types of soils do Tamarix or other plants prefer to grow in?
- Which US counties have populations of Africanized bees?
- How many states have borders that are along the Ohio River?
- How may Earthquakes occurred in China in 2009?