9. Analyzing Spatial
Relationships
Select features nearby other features
Select features within other features
Select features that intersect other features
9. Analyzing Spatial
Relationships
ArcView GIS lets you
answer various types of queries. The ArcView view document shows the location
of features, thus allowing you to answer the question “Where is it?”. Next, the
Identify tool 
answers the
question “What is it?” for each feature, and working both view and table
document together answers the question also. Furthermore, you can select
features by both location (using the Select Feature tool 
) and attributes (using the Query Builder button 
). Sometimes we need to know the spatial relation between
different themes – how they are spatially related?: Near? Within? Pass
through?. For example, Where is the nearest hospital from your place? Is this
store in Erie county or Niagara county? (to figure out different property tax
applied to two counties.) Which route to take to go from A to B? What is the
demographic characteristic of neighbors near casino? and so on.
To do this, we have to
figure out the spatial relationship between map features. This kind of
operation is sometimes called spatial overlay. Since most of spatial entities
are represented as point, line and polygon, the task of spatial overlay is to
determine the qualitative (also quantitative (e.g., distance)) relation between
features. For example,
Point A is near point B.
(e.g., ECMC is near UB south campus)
Point A is near line C1.
(e.g., UB is near Interstate 90)
Point A intersects line
C2. (e.g., Her car breaks down on the road)
Point A is within
polygon D. (e.g., Park Hall is within UB campus)
Line A is near point B1.
(e.g., Mississippi river is near my dorm)
Line A intersects point
B2. (e.g., Amtrak line runs across our archaeology site)
Line A intersects
polygon C. (e.g., Erie canal runs across New York State)
Polygon A contains point
B. (e.g., UB campus contains Park Hall)
Polygon A intersects
line C1. (e.g., An unstable fault passes through Seattle )
Polygon A contains line
C2. (e.g., Orlando contains international drive)
Polygon A intersect
polygon D1. (e.g., American life style dominates the world)
Polygon A contains
polygon D2. (e.g., UB campus contains the lake La Salle)
From the example above,
feature A is called target theme, and the object is called selector theme. The
selection is made in the target theme based on the spatial relationship
between target and selector theme. If the selector theme is selected,
the operation will be performed over the selected features. Otherwise, the
operation will be performed over all features of the selector theme. The
available spatial relation type available in ArcView are expressed as the
following phrases:
Are Completely Within -
selects features in the target themes if they fall completely within one or
more of the selector theme's features.
Completely Contain -
selects features in the target themes that completely contain one or more of
the selector theme's features
Have their Center In -
selects features in the target themes if their center falls inside the selector
theme's features.
Contain the Center Of -
selects features in the target themes that contain the center of one or more of
the selector theme's features
Intersect - selects
features in the target themes that intersect the features in the target.
Intersection implies that at least one point is common to both the selector and
the target or one of them is completely within the other. If the selector and
target are the same, Intersect will select adjacent features.
Are Within Distance Of -
selects features in the target themes that are within a specified distance of
the selector theme's features. You can specify the type of distance units in
the View Properties dialog box.
Select
features nearby other features
To
illustrate this, suppose you work for the emergency response unit in Illinois.
Some traffic accident has been reported to occur on the Interstate 55. You
heard that the Interstate 55 is highly congested at the time of response, so you
made a decision to dispatch helicopters instead. She wants to know where
helipads are located close to the accident site.
Select roads where the
name ends with 55 and is in Illinois state using Query Builder 
. (The selected features are displayed in green here.)
Now we want to find
helipads that are within 50 miles from these selected roads.
Make a target theme
active; a target theme is the helipad.
Choose Select By Theme…
from the Theme menu.
In the Select By Theme
dialog box, choose “Are within distance of” for spatial relation type list;
Select the road theme for selector theme list.
Press
New Set button unless you want to perform the operation over the previous
selection.
The selection is made in
the target theme; 9 helipads are selected.
She also wants to find out where trauma centers
are located near the selected helipad location; Now the helipad theme becomes
the selector theme, and the target theme is the hospital theme. Make hospital
theme active, and choose the Select By Theme from the Theme menu in a view.
29 hospitals have been
found within 30 miles to the selected helipads.
Select features within other features
Suppose you are
interested in evaluating the accessibility of hospitals to air medical services
in the United States. We have a polygon theme showing ten minutes fly circle,
and a point theme for the hospital. This query can be expressed as “is the
hospital completely within a 10 minutes fly circle?”. The target theme is the
hospital, and the selector theme is a 10 minutes fly circle.
We don’t make any select
on the selector theme, so the operation will be performed over all features of
the selector theme.
Make a hospital theme
active (i.e., Specify the target theme.)
Choose the Select By
Theme… from the Theme menu
Press New Set button.
Since we want to look at
relatively inaccessible hospital, that is outside of the 10 minutes fly circle,
you can simply switch the selection by selecting the Switch Selection button 
in a Table view.
The view shows the
hospitals outside of 10 minutes fly circles.
We can narrow down the
selection by selecting hospitals that do not have a landing zone among the
hospitals outside of 10 minutes fly circle. Open Query builder while hospital
theme is still made active. Type in the query in the input box, and press
Select From Set button.
Now we found 4 hospitals
which are relatively inaccessible from the helipad and do not have a landing
zone.
Similarly, suppose you
are interested in the coverage of air medical services in the United States.
You have the city theme that contains the population in 1998. This time, you
want to know how many people are, and are not served by 20 minutes fly circles
from the air medical service helipad. To do this, we have to figure out which
city is within the fly circles and which city is not. City is the target theme,
and the 20 minutes fly circle is the selector theme. In the case of the spatial
relationship between point and polygon, “Intersect” is the same as “completely
within” because point is necessarily either within polygon or not; However,
line can intersect polygon and cannot be completely within the polygon at the
same because of additional dimension of line. For example, I-90 intersects the
city of Buffalo, but is not completely within Buffalo.
The selection is made in
the City theme. You can simply have a statistics of populations served by a 20
minutes fly circle by choosing Statistics from the Field menu while the field
[pop_98] made active.
Suppose you are asked to
provide a map showing the coverage that varies with state, so that policy
makers will be able to evaluate the coverage state by state.
Make a table summarized
by state, which contains the total sum of [pop_98] from the selected features;
Click the state name field to make it active; Choose the Summarize from the
Field menu; Select [pop_98] for field and select Sum for Summarize by, and
press Add button. You can make other summary table that contains the total sum
of [pop_ 98] from all features by selecting the Select None 
button.
As a result, you will
get two summary tables: One is a table for covered population, and the other is
a table for total population by state. Join two tables, where a target table
contains a total population, and a source table contains a covered population.
Join this target table to the State theme table. Make a thematic map (choose
graduated color for legend type, choose covered population for classification
field, and choose total population for total population). The result map shown bellows
makes it easier to identify the state that would require more investment such
as Kensas state.
Select features that
intersect other features
Can we find the state
where interstate 90 passes through?
Select the road using
Query Builder.
Now make a state theme
active.
Choose Select By Theme…
from the Theme menu. Choose “Intersect” for spatial relation type list, and
road theme for selector theme.
Press New Set button.
You can also find the
states adjacent to the selected states by using “Intersect” and setting both
the target and selector theme to state theme. Press New Set.
The “Intersect” relation
between polygon themes is not strict; They are selected just because they share
the border. We can use “have their center in” instead to find polygons that
intersect in a more strict manner. For example, suppose you select Tenesse for
selector theme. Set a 10 minutes fly circle to the target theme (make it active),
and choose Select by Theme from the Theme menu.
Clear the selection in 10 minutes fly
circle. This time choose “have their center in” for Select by Theme spatial
relation type field.
