Asking Questions about Spatial Data

Queries

Analysis technique/method used to extract certain records from a map or table.

Aspatial queries

All parcels with value greater than $100,000

Spatial queries

All parcels that lie completely within the flood plain

Selecting features of interest

For example, selecting aspen stands from a forest vegetation layer. 

Exploring patterns

For example, are aspen stands randomly scattered or clustered? Do they occur in partictular portions of the forest? What are the distributions of stand densities?

Isolating for more analysis

For example, are there any mature stands with large trees and open crowns? where ate they?

Exploring spatial relationships

For example, what fraction of stands are intersected by roads? What types of trees are adjacent to aspen stands?

Queries involving surfaces

Over what range of elevations do aspen occur? Do aspen occur above 1500m elevation?

Interactive query

User visually identifies the desired features in a map or records in the table

• Ex. States west of the Mississippi River

Attribute query

An expression is used to find records with values meeting a specified condition

• Counties with more than 100,000 people

Spatial query

A spatial relationship between two layers is evaluated

• Cities within 50 miles of a major earthquake

Operators

Act on objects to produce a result.

Arithmetic operators

Act on numbers (+−×÷^)

Ex. 3 + 5

Logical operators

Test conditions and return true or false ( = > < ≥ ≤)

Ex. STATE= NJ, GPA >3.0

Boolean operators

Test pairs of conditions and return ture or false (AND OR XOR NOT)

Ex. MAJOR = Geography AND GPA ≥ 3.5

Spatial operators

Test spatial relationships between features ( intersect contains proximity)

Ex. Cities within 50 miles of a volcano

Attribure queries

Use expressions applied to a table to find records that meet one or more conditions. 

Structured query langyage (SQL)

Expressions are executed using…

Multiple conditions must be tested

Boolean operators are used when…

AND

Returns true if both conditions are true

OR

Returns true if either or both conditions are true

XOR

Returns true if one condition is true and the other is false (doesn’t matter which one is the true one)

NOT

Returns true if the first condition is true and the second condition is false.

Venn diagrams

Help visualize how boolean operators work.

Order of precedence

Multiple conditions are evaluated from left to right unless parentheses are used to change the order. 

Intersect

Tests whether features touch

Contains/within

Tests whether a feature is inside another

Proximity

Tests whether features are within a specified distance of another

Contains operator

Permits the feature to lie on the edge of (share a boundary with) the containing feature.

Completely contains operator

Does not permit a shared boundary. 

Target layer

The one from which the features will be selected.

Source layer

The one that the target features are compared to.

Select Layer by Location tool

A way to conduct a spatial query.

Logical consistency

How well a data set reflects real-world relationships.

Buffers

A type of spatial query that delineate the area (or boundary) of the region within a specified distance of a set of features. 

Dissolve buffers

Used to generate a single, clean buffer region.

Clip and Erase

Extraction functions:

Clip

Works like a cookie cutter to extract features that lie inside the boundary of another data set.

Erase

Keep features outside of the boundary and remove the ones within the boundary. 

Extraction functions

May change the lengths or areas of features that cross the boundary.

Interactive selection

User visually picks the desired features form a map.

Selection type

A parameter that determines what happens to an existing selection if another query is performed on the same layer.