Name____________________________________
NR 143 Intro to GIS
UVM/RSENR
Instructor: Ernie Buford
Assignment 1: Vector Topology
The power of GIS lies in its ability to link spatial data and attribute data. Locational data tell
where features are. While attribute data tell what features are. Topological data tell where features
are in relation to one another. Topology is a mathematical procedure that determines spatial
properties and relationships, including: area of polygons, connectivity of arcs (i.e. lines), direction of
an arc, length of an arc, and adjacency (contiguity) of polygons. This assignment should help you to
understand the fundamentals behind GIS topology – how it is established, what defines it, and how to
interpret it. You should also gain an understanding of the components of a data layer, its features, and
how they are linked to attributes.
Much of this assignment is based on Lesson 2 from the book “Understanding GIS” which is part of
the course readings (that can be found in the syllabus on the course web page). All questions are
worth 1/2 point unless otherwise indicated (10 pts. total). This assignment is due in one week.
Work independently. Answer the questions in this document using Microsoft Word and then save the
file as a PDF document. When complete, upload the file to the NR 143 web site (homework upload).
Exercise 1: How to Generate Topology
Arc-node topology expresses the relationship between arcs and nodes (connection between two
arcs). It defines length, direction, and connectivity for arcs (i.e. lines).
The map of roads below shows seven numbered nodes (the circled numbers), and six numbered arcs.
1. Use the table to list the from-node and the to-node of each arc (1 point). This table expresses
directionality.
Arc
From
Node
To
Node
1
2
3
4
5
6
Note that one or more arcs can share a node; but by definition, a node cannot exist without an arc. In
the table below, list the arcs you would traverse to get from node 6 to node 1 (in the figure above).
Also, indicate your direction of travel across each arc (i.e., beginning at the from-node and traveling
to the to-node, or vice versa). This demonstrates arc connectivity.
2. List the path from Node 6 to Node 1 (1 point) in the table below:
Arc #
5
Direction
‘+’ = from-node to to-node
‘–’ = to-node to from-node
3. Arcs connect if they share a _____________________?
The next part of this exercise illustrates polygon-arc topology which expresses the relationship
between arc features and the polygon features for which arcs create boundaries. It defines area and
adjacency (contiguity) of polygons. Using the first table below, define each polygon (indicated by
circled numbers) in the figure below by listing the arcs (i.e. lines) that connect to create it. Record the
number of each arc in the table. Then in the second table (on the next page), for each arc, list the
polygons to the left and right sides. Note: arrows indicate the direction of the arcs. This demonstrates
polygon adjacency.
Note: Always start with the lowest numeric valued arc and continue in a clockwise direction.
4. Using the table below, define each polygon (circled numbers in figure) by listing the arcs that
connect to create it. Record the number of each arc in the table (1 point).
Polygon
No. of Arcs
List of Arcs
1
0
0
2
3
4
Note, the direction of the arc determines which arc is identified as the” left” and which is the” right”
polygon. Think of direction as if you are driving down a one way street (that is the direction of the
line). When you look out the driver’s window, you are looking at the left polygon and when you look
out the passenger’s side window, you are looking at the right polygon.
5. From the above figure, for each arc, list the polygons to the left and right sides (in the table
below). Note, arrows indicate the direction of the arcs. (1 point)
Arc
Left Polygon
Right Polygon
1
2
3
4
5
6
In addition to the polygons drawn in a layer, there is a background polygon called the Universe
Polygon (the software needs to keep track of what’s not data too).
6. In the figure above, which polygon (number) is the Universe Polygon?
7. Which arcs have polygon # 1 to their left?
8. How many nodes does an arc have?
9. Two polygons are adjacent if they share an _____________________?
10. In the figure on the previous page, what symbol represents the “to-node”?
11. How many arcs are in the figure below?
Now I have added an intersecting arc.
12. How many arcs are in this new figure above?
Although ArcGIS doesn’t use these exact topology rules, this assignment should give you an idea of
the information maintained by the software for each GIS data layer.
Exercise 2: Linking Attributes to Geographic Features in a GIS
Use the identifier to connect features with their attributes. In the first part of this exercise, you will
connect descriptive attributes with their corresponding geographic features. Then you will perform a
manual relate, that is, combine attributes from two data layers.
13. Using the PARCEL layer (figure below), complete the attribute information in the boxes
connected to the three parcels in the figure by referring to the Assessor’s Parcel List at the bottom
of the page for the attribute information. Notice that, in this case, the PARCEL-ID is used to relate a
parcel to its attributes in the Assessor’s Parcel List. (1 point)
PARCEL-ID: 122
PARCEL NO.:
OWNER:
ZONING:
PARCEL-ID: 124
PARCEL NO.:
OWNER:
ZONING:
PARCEL-ID: 127
PARCEL NO.:
OWNER:
ZONING:
A data layer may have more than one attribute table. Information in attribute tables can be related or
joined by having a matching column (called a “Field”) in the two different attribute tables.
14. Now “relate” the attributes in the tables below by DRAWING LINES (use the Drawing
Tools to draw lines, Insert > Shapes > Line) to connect common parcel numbers; that is, use
the parcel number as the relate item. (1 point)
This is what the ArcGIS software does when it performs a RELATE between two attribute tables. A
JOIN physically combines two attribute tables into one attribute table, a RELATE just maintains a
pointer linking appropriate attributes between two tables (but keeps the attribute tables separate). If
you were to JOIN the above two tables, you could perform a query on all parcels on Orange Street
with a Zoning code of R-2, that is, you could query the attributes in both data layer attribute tables.
This topology assignment is based on the “old” Arc/INFO (pre ArcGIS software version) data layers
called “coverages”. ArcGIS’s new geodatabase format is “object-oriented”, that is, each feature (i.e.
point, line or polygon) is a single independent object and is treated as an object (rather than a bunch
of connected lines making up a polygon, for instance). So topology within an ArcGIS geodatabase is
based on a set of user-selected rules (e.g. polygons must not overlap) that apply to all objects (i.e.
vector features such as points, lines and polygons) in a data layer (called a feature class). The
geodatabase is smarter than coverages in that the software keeps track of the myriad of files for each
data layer and a single geodatabase can contain, that is, keep track of many GIS data layers and other
types of data (pictures, rasters, layouts, external tables, etc.) as well. Think of a geodatabase as a big
container or file folder that keeps all your project files together in one place.
That’s it. When done, upload your PDF file (with your answers) to the NR 143 web site (Course
Info >> Upload Homework)! Alternatively, you could print out this assignment, write down your
answers, and hand it in. Deadline is next Thursday, February 9.