Diploma of Environmental Monitoring & Technology
Study module 3
Vector data
MSS025005A
Produce Site Maps
(PSM)
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Produce site maps (PSM) SM 3
Vector data
VECTOR DATA
Points
Lines
Polygons
VECTOR DATA IN LAYERS
Editing vector data
Scale and vector data
Symbology
WHAT CAN WE DO WITH VECTOR DATA IN A GIS?
Using GIS as a map
Using GIS to display spatial change
Using GIS to display temporal change
COMMON PROBLEMS WITH VECTOR DATA
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13
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Slivers
Overshoots and undershoots
Vector data relationships
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15
ASSESSMENT & SUBMISSION
16
Knowledge questions
Assessor feedback
Assessment & submission rules
References & resources
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Vector Data
Vector data provides a way to represent real world features within the GIS environment
using different graphic styles either as an overlay to a photo or image, or by vector imagery
alone. A feature is anything you can see on the landscape. Imagine you are standing on the
top of a hill. Looking down you can see houses, roads, trees, rivers, and so on. Each one of
these things would be a feature when we represent them in a GIS Application. Vector
features have attributes, which consist of text or numerical information that describe the
features.
Figure 3.1 – A typical mining landscape (raster) showing mining areas, roads, houses and
water bodies. When we turn these features into points, lines and polygons, we have created
vector data from those raster features.
A vector feature has its shape represented using geometry. The geometry is made up of
individual vertex, the plural of which is vertices. A vertex describes a position in space using
an X, Y and optionally Z axis. Geometries which have vertices with a Z axis are sometimes
referred to as 2.5D since they describe height or depth at each vertex, but not both.
When a feature’s geometry consists of only a single vertex, it is referred to as a point
feature. Where the geometry consists of two or more vertices and the first and last vertex
are not equal, a polyline feature is formed. Where four or more vertices are present, and
the last vertex is equal to the first, an enclosed polygon feature is formed.
A point feature is described by its X, Y and optionally Z coordinate. The point attributes
describe the point e.g. if it is a tree or a lamp post.
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A line is a sequence of joined vertices. Each vertex has an X, Y (and optionally Z) coordinates.
Attributes describe the line.
A polygon, like a line, is a sequence of vertices. However in a polygon, the first and last
vertices are al- ways at the same position.
Figure 3.2 – Visual guide to vector point, polyline and polygon geometries
Looking back at the picture of a landscape we showed you further up, you should be able to
see the different types of features in the way that a GIS represents them now.
Points
The first thing we need to realise when talking about point features is that what we describe
as a point in GIS is a matter of opinion, and often dependent on scale. Let’s look at cities for
example. If you have a small scale map (which covers a large area), it may make sense to
represent a city using a point feature, however as you zoom in to the map, moving towards
a larger scale, it makes more sense to show the city limits as a polygon.
When you choose to use points to represent a feature is mostly a matter of;
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scale (how far away are you from the feature)
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convenience (it’s quicker to create point features than polygon features)
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the type of feature (telephone poles shouldn’t be stored as polygons)
As we show in the figure above, a point feature has an X, Y and optionally, Z value. The X
and Y values will depend on the Coordinate Reference System (CRS) being used. A CRS is a
way to accurately describe where a particular place is on the earth’s surface. One of the
most common reference systems is Longitude and Latitude.
Lines of Longitude run from the North Pole to the South Pole. Lines of Latitude run from the
East to West. You can describe precisely where you are at any place on the earth by giving
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someone your Longitude (X) and Latitude (Y). If you make a similar measurement for a tree
or a telephone pole and marked it on a map, you will have created a point feature.
Since we know the earth is not flat, it is often useful to add a Z value to a point feature. This
describes how high above sea level you are, which, in Australia, is usually referenced to the
Australian height Datum (AHD 71).
Figure 3.3 - Examples of points used in a GIS. Each of these points can have attributes
associate with them such as coordinates, names or flow rates.
Lines
Where a point feature is a single vertex, a line has two or more vertices. The line is a
continuous path drawn through each vertex. When two vertices are joined, a line is created.
When more than two are joined, they form a ’line of lines’, or polyline.
A polyline is used to show the geometry of linear features such as roads, rivers, contours or
footpaths. Sometimes we have special rules for lines in addition to their basic geometry. For
example contour lines may touch (e.g. at a cliff face) but should never cross over each other.
Similarly, lines used to store a road network should be connected at intersections. In some
GIS applications you can set these special rules for a feature type (e.g. roads) and the GIS
will ensure that these polylines always comply with these rules.
If a curved polyline has very large distances between vertices, it may appear angular or
jagged, depending on the scale at which it is viewed. Because of this it is important that
polylines are digitised (captured into the computer) with distances between vertices that
are small enough for the scale at which you want to use the data.
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The attributes of a polyline describe its properties or characteristics, for example a road
polyline may have attributes that describe whether it is surfaced with gravel or tar, how
many lanes it has, whether it is a one way street, and so on. The GIS can use these attributes
to symbolise the polyline feature with a suitable colour or line style.
Figure 3.4 - Line vectors, in this case creek systems in a catchment. The attributes of this could
include flow data or pollutant concentrations. In this example, the middle arm of the creek
shows the individual vertices that were used to make up the length of the creek
Polygons
Polygon features are enclosed areas like dams, islands, Council boundaries and so on. Like
line features, polygons are created from a series of vertices that are connected with a
continuous line. However because a polygon always describes an enclosed area, the first
and last vertices should always be at the exact same place!
As is found with intercepting or adjoining lines, we find that polygons often have shared
geometry, such as boundaries that are in common with a neighbouring polygon (as is found
with two adjoining Council areas). Many GIS applications have the capability to ensure that
the boundaries of neighbouring polygons exactly coincide.
Polygons have attributes as well, which describe each polygon. For example a dam may have
attributes for depth and water quality, or a mine boundary may have time based (temporal)
information such as lease length’s associated with it. The attributes of polygons can be
extremely complex and incorporate a variety of information.
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Figure 3.5 – Example of a polygon. In this case, it is a catchment boundary. This example
shows the vertices used to create the boundary line. Attributes can also be included.
Finally we can show you all of these points, lines and polygons as separate overlays on the
one map window; as can be seen in the figure below.
Figure 3.6 – Points lines and polygons in action showing an application of GIS in catchment
management. This is the simplest use of GIS – it is much more powerful than this!
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The examples included from figures 3.3-6 above show the basics of the three types of vector
data used in GIS, but this is by no means the end of what can be achieved. There is a lot
more that we can do with continuous data and symbology to ‘paint’ the picture displaying
the story you are trying to tell.
Another aspect of working with vector data to consider is that it is commonly used without
raster imagery in the background. The examples above show vector data over raster
imagery to visually support the use of the data, yet a program like Google maps™ is vector
data only! All you need is georeferenced vector data to display.
Vector data in layers
Vector data needs to be managed in a GIS environment. Most GIS applications group vector
features into layers, the data of which is stored in an attribute table (or database if it is
online). Features in a layer have the same geometry type (i.e. points, lines or polygons) and
the same kinds of attributes (e.g. information about what species a tree is for a trees layer).
This is both convenient because it allows you to hide or show all of the features for that
layer in your GIS application with a single mouse click, and an inconvenience because you
end up with lots of data! Let’s look at how the vector data for a creek is displayed in GIS;
Figure 3.7 – Vector data displayed in a GIS program. In this example it is showing the creeks
within a catchment.
The two windows open in this program show the ‘layers’ window (left), and the ‘map’
window (right). The layers pane shows the three creeks as three separate layers. In most
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applications you can access the layers attributes by double clicking an individual layer which
will bring up an attributes data table.
Editing vector data
The GIS application will allow you to create and modify the geometry data in a layer (a
process commonly called digitising). If a layer contains polygons, such as catchments or
dams, the GIS application will only allow you to create new polygons in that layer. Similarly if
you want to change the shape of a feature, the application will only allow you to do it if the
changed shape is correct. For example it won’t allow you to edit a line in such a way that it
has only one vertex.
Remember in our discussion of lines above that all lines must have at least two vertices.
Creating and editing geographic vector data is an important function of a GIS since it is one
of the main ways in which you can create personal data for things you are interested in. Say
for example you are monitoring pollution in a river. You could use the GIS to digitise all
outfalls for storm water drains (as point features). You could also digitise the river itself (as a
line feature). Finally you could take readings of pH levels along the course of the river and
digitise the places where you made these readings (as a point layer). An example of how to
edit a vector layer can be seen in the figure below.
Figure 3.8 – Editing vector data.
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Vector data
As well as creating your own data, there is a lot of free vector data that you can obtain and
use. For example, you can obtain vector data that appears on the 1:250 000 map sheets
from Geoscience Australia.
Scale and vector data
Map scale is an important issue to consider when working with vector data in a GIS. When
data is captured, it is usually digitised from existing maps, or by taking information from
surveyor records and global positioning system devices. Maps have different scales, so if you
import vector data from a map into a GIS environment (for example by digitising paper
maps), the digital vector data will have the same scale issues as the original map.
Creating vector data at the wrong scale can have a significant effect on the end use of the
data. Choose the original map scale very carefully.
Many issues can arise from making a poor choice of map scale. For example using the vector
data in the figure below to plan a wetland conservation area could result in important parts
of the wetland being left out of the reserve! On the other hand if you are trying to create a
regional map, using data captured at 1:1000 000 might be just fine and will save you a lot of
time and effort capturing the data.
Figure 3.9 - Maps with different scales
Symbology
When you add vector layers to the map view in a GIS application, they will be drawn with
random colours and basic symbols. One of the great advantages of using a GIS is that you
can create personalised maps very easily.
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The GIS program will let you choose colours to suite the feature type you can tell it to draw
a water bodies vector layer in blue). The GIS will also let you adjust the symbol used. So if
you have a trees point layer, you can show each tree position with a small picture of a tree,
rather than the basic circle marker that the GIS use when you first load the layer.
Symbology is a powerful feature, making maps come to life and the data in your GIS easier
to understand. In the topic that follows (working with attribute data) we will explore more
deeply how symbology can help the user to understand vector data.
Figure 3.10 - Adjusting the symbology of vector features (default MapWindow™ symbols)
In the GIS, you can use a panel (like the one above) to adjust how features in your layer
should be drawn. When a layer is first loaded, a GIS application will give it a generic symbol,
and after making our adjustments it is much easier to see that our points represent trees.
What can we do with vector data in a GIS?
At the simplest level we can use vector data in a GIS Application in much the same way you
would use a normal topographic map. The real power of GIS starts to show itself when you
apply the data to visually display changes to geographical features such as landscapes, rivers
or lakes and the like. We can start to ask questions like;
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Which houses are within the 100 year flood level of a river?
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Which arm of a river is the most polluted?
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How has the distribution of sea grass changed over time in a lake?
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A GIS is a great tool for answering these types of questions with the help of vector data.
Generally we refer to the process of answering these types of questions as spatial analysis.
In this course, we don’t delve into spatial analysis in too much detail as you are only learning
to use the GIS application to produce an informative and scaled map.
Using GIS as a map
In its most simple form, GIS is no more useful than a hardcopy map, whether it be
topographic, thematic or any other map because in its most simple form, a GIS simply
displays information the same way a map does.
The GIS is different from a map because of the layers. Because we can turn map layers on
and off, or change them, or add and delete layers, GIS is best viewed as an interactive map,
and that has one very powerful outcome – customization.
Being able to customise the displayed imagery allows the information to target the audience
the message is being conveyed to, and that makes GIS infinitely more powerful at displaying
information than a hardcopy map of any type.
The added bonus – a GIS will never go out of date as it can always be updated. Hardcopy
maps are only good for a short period after they are printed. You can see GIS being used as
a map in the figure below;
Figure 3.11 – Example of a GIS map (Google maps™)
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Maps are obviously useful, and there are many more examples to map types than the street
map example used above, but a far more useful aspect of GIS is to show how things change.
Change can occur in two key ways, spatially or temporally.
Spatial change involves changes in area whereas temporal changes involve a change over
time. Typically, displaying changes in time is used to display the change of a feature, which
may or may not cover an area, so temporal change often involves spatial arrangements. It is
the perspective of the change that is different.
Using GIS to display spatial change
As you are being trained to become environmental technicians whose job it is to perform
compliance monitoring or pollutants in the environment, it makes sense that we explore
spatial change involving pollutants in an area. By pollutant, I mean can toads (Bufo marinus).
We can use GIS to explore the distribution of this invasive species in Australia to date by
viewing the map below.
Figure 3.12 – Vector images showing the spatial distribution (actual and potential) of Bufo
marinus. The delineation between the two is defined by a line, not a polygon.
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Using GIS to display temporal change
The best example of using GIS to show change is to use temporal change, as it displays both
spatial and temporal changes which allow the user to make a truly informed decision about
change. Consider the following map which shows how the distribution of change in Cane
toad distribution;
Figure 3.13 – Example of GIS displaying temporal change of a feature (can toad distribution
in Australia over 70 years [source].
The example used here employs the use of vector data only. The map is a vector of Australia
and the distribution of the toads is a continuous vector using geographic coordinates and
date as attributes where the colour changes based on year.
Common problems with vector data
Working with vector data does have some problems. We already mentioned the issues that
can arise with vectors captured at different scales. Vector data also needs a lot of work and
maintenance to ensure that it is accurate and reliable.
Inaccurate vector data can occur when the instruments used to capture the data are not
properly set up, when the people capturing the data aren’t being careful, when time or
money don’t allow for enough detail in the collection process, and so on.
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Slivers
If you have poor quality vector data, you can often detect this when viewing the data in a
GIS. For example slivers can occur when the edges of two polygon areas don’t meet
properly.
Figure 3.14 - Slivers occur when the vertices of two polygons do not match up on their
borders. At a small scale (1 on left) you may not be able to see these errors. At a large scale
they are visible as thin strips between two polygons (2 on right).
Overshoots and undershoots
Overshoots can occur when a line feature such as a road does not meet another road
exactly at an intersection. Undershoots can occur when a line feature (e.g. a river) does not
exactly meet another feature to which it should be connected. The figure below
demonstrates what undershoots and overshoots look like. Because of these types of errors,
it is very important to digitise data carefully and accurately. In the upcoming topic on
topology, we will examine some of these types of errors in more detail.
Figure 3.15 - Undershoots (1) and overshoots (2) happen if a line ends beyond or before the
line it should connect to.
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Vector data relationships
One way of overcoming problems is to have a good understanding of how the data used in vectors relates to the attributes. Use the flowchart
below to further your understanding of how vector data relates to the attributes of the point, line or polygon it is associated with.
Vector Feature
Point
1 vertex
Line
Attributes
>1 vertices
Polygon
Attributes
>2 vertices
Attributes
Latitude
Latitude
Latitude
Longitude
Longitude
Longitude
Height
Height
Height
Figure 3.16 - This diagram shows how GIS applications deal with vector data.
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Assessment & Submission
This section provides formative assessment of the theory. Answer all questions by typing
the answer in the boxes provided. Speak to your teacher if you are having technical
problems with this document.
Knowledge questions
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Type brief answers to each of the questions posed below.
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All answers should come from the theory found in this document only unless the
question specifies other.
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Marks shown next to the question should act as a guide as to the relative length or
complexity of your answer.
1. List three tasks you might perform when assessing a site. 1mk
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2. What is a vector image? 1mk
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3. What is a feature? 1mk
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4. What is a vertex (or vertices)? 1mk
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5. In terms of geometry, what is the difference between a point, a line and a polygon? 6mk
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Click here to enter text.
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6. Why is scale a subjective feature of a point? Provide an example of a feature that could
be described as a point. 4mk
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7. Which datum is used to describe height in Australia? 1mk
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8. What is the minimum number of points required to create a line and polygon? 2mk
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9. How is vector data stored in GIS applications? 2mk
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10. What term is given to creating vector data from a map? 1mk
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11. What type of data is collected when this type of vector creation is performed? 1mk
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12. Why is it important to create vector data at the correct scale? Describe the effect that
scale can have on vector imagery. 4mk
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13. What is symbology? How is it used with vector data? 3mk
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14. Why is a GIS map more useful than a hardcopy map (assuming you have power)? 2mk
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15. What is the difference between spatial and temporal change? 4mk
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16. Using the distribution of the cane toad as an example, how are spatial and temporal
change used to describe this toads distribution? 4mk
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17. Other than incorrect geographic coordinates used, what other common problems are
often associated with vector data? 6mk
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18. How important (subjectively) is geographic accuracy in the creation of vector data? 1mk
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Assessor feedback
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Assessment & submission rules
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Attempt all questions and tasks
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Write answers in the text-fields provided
Submission
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Use the documents ‘Save As…’ function to save the document to your computer using
the file name format of;
Yourname-PSM-SM-3
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email the document back to your teacher
Penalties
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If this assessment task is received greater than seven (7) days after the due date, it may
not be considered for marking without justification.
Results
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Your submitted work will be returned to you within 3 weeks of submission by email fully
graded with feedback.
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You have the right to appeal your results within 3 weeks of receipt of the marked work.
Problems
If you are having study related or technical problems with this document, make sure you
contact your assessor at the earliest convenience to get the problem resolved. The contact
details can be found at;
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www.cffet.net/env/contacts
References & resources
Resources
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www.mapwindow.org/
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www.esri.com/
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http://resources.arcgis.com/en/help/
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www.qgis.org/
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www.gislounge.com/learn-gis-for-free/
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References
Note that some of these resources might be available from your teacher or library
Bailey, D. W. (2003). Practical SCADA for Industry. Sydney: Newnes, Elsevier.
Brimicombe, A. (2010). GIS, Environmental Modelling & Engineering. 2nd Ed. Boca Raton:
CRC press.
Burden, F. E. (2002). Environmental Monitoring Handbook. McGraw-Hill Professional.
Ferrier, R. C. (2010). Handbook of Catchment Management. Oxford: Wiley-Blackwell.
Newton, A. (2007). Forest Ecology and Conservation. Oxford: Oxford University Press.
Schneider, R. R. (2011). MapWindow: Quick Start Tutorial. MapWindow 4.8.6. Edmonton:
Free Software Foundation.
Sutherland, W. (2006). Ecological Census Techniques. 2nd Ed. Cambridge: Cambridge
University Press.
Sutton, T. E. (2009). A Gentle Introduction to GIS. Eastern Cape, South Africa: Chief
Directorate: Spatial Planning & Information, Department of Land Affairs, Eastern
Cape.
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