[QGIS Commit] r11042 - in docs/trunk/english_us/gis_introduction: . attributedata_images

[svn_qgis at osgeo.org]

Author: dassau
Date: 2009-07-11 06:08:12 -0400 (Sat, 11 Jul 2009)
New Revision: 11042

Added:
   docs/trunk/english_us/gis_introduction/attributedata_images/HousesByBalconyRoof.png
   docs/trunk/english_us/gis_introduction/attributedata_images/single_symbol_dialog_point.png
Modified:
   docs/trunk/english_us/gis_introduction/attributedata.tex
   docs/trunk/english_us/gis_introduction/attributedata_images/Makefile
   docs/trunk/english_us/gis_introduction/qgis_style.sty
Log:
worked on section 3


Modified: docs/trunk/english_us/gis_introduction/attributedata.tex
===================================================================
--- docs/trunk/english_us/gis_introduction/attributedata.tex	2009-07-11 08:00:04 UTC (rev 11041)
+++ docs/trunk/english_us/gis_introduction/attributedata.tex	2009-07-11 10:08:12 UTC (rev 11042)
@@ -17,4 +17,524 @@
 
 \subsection{Overview}\label{subsec:overview}
 
+If every line on a map was the same colour, width, thickness, and had the
+same label, it would be very hard to make out what was going on. The map
+would also give us very little information. Take a look at Figure
+\ref{fig:rendercompare} below for example. 
 
+\begin{figure}[ht]
+   \begin{center}
+   \caption{Maps come to life when colour and different symbols are used to
+help you to tell one type of feature from the next. Can you tell the
+difference between rivers, roads and contours using the map on the left?
+Using the map on the right it is much easier to see the different features.}
+\label{fig:rendercompare}\smallskip
+   \includegraphics[clip=true, width=\textwidth]{RenderAsComparison}
+\end{center}
+\end{figure}
+
+In this topic we will look at how attribute data can help us to make
+interesting and informative maps. In the previous topic on vector data, we
+briefly explained that \textbf{attribute data} are used to \textbf{describe
+vector features}. Take a look at the house pictures in Figure
+\ref{fig:houses} below.
+
+The geometry of these house features is a polygon (based on the floor plan of
+the house), the attributes we have recorded are roof colour, whether there is
+a balcony, and the year the house was built. Note that attributes don't have
+to be visible things - they can describe things we know about the feature
+such as the year it was built. In a GIS Application, we can represent this
+feature type in a houses polygon layer, and the attributes in an attribute
+table (see Figure \ref{fig:attrtable}).
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{Every feature has characteristics that we can describe. These can
+be visible things, or things we know about the feature (e.g. year built).}
+\label{fig:houses}\smallskip
+   \includegraphics[clip=true, width=\textwidth]{Houses}
+\end{center}
+\end{figure}
+
+\begin{figure}[h]
+   \begin{center}
+   \caption{A houses layer. House features have attributes that describe the
+houses' roof colour and other properties. The attribute table (lower image)
+lists the attributes for the house areas shown on the map. When a feature is
+highlighted  in the table, it will appear as a yellow polygon on the map.}
+\label{fig:attrtable}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{AttributeTable}
+\end{center}
+\end{figure}
+
+The fact that features have attributes as well geometry in a GIS Application
+opens up many possibilities. For example we can use the attribute values to
+tell the GIS what colours and style to use when drawing features (see Figure
+\ref{fig:houseroofbalc}). The process of setting colours and drawing styles is
+often referred to as setting feature \textbf{symbology}. 
+
+Attribute data can also be useful when creating \textbf{map labels}. Most GIS
+Applications will have a facility to select an attribute that should be used
+to label each feature. 
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{In a GIS Application, we can draw features differently depending
+on their attributes. On the left we have drawn house polygons with the same
+colour as the roof attribute. On the right we colour coded houses according
+to whether they have a balcony or not.}
+\label{fig:houseroofbalc}\smallskip
+   \includegraphics[clip=true, width=\textwidth]{HousesByBalconyRoof}
+\end{center}
+\end{figure}
+
+If you have ever \textbf{searched a map} for a place name or a specific
+feature, you
+will know how time consuming it can be. Having attribute data can make
+searching for a specific feature quick and easy. In Figure
+\ref{fig:housesearch} you can see an example of an attribute search in a GIS. 
+
+Finally, attribute data can be very useful in carrying out \textbf{spatial
+analysis}.
+Spatial analysis combines the spatial information stored in the geometry of
+features with their attribute information. This allows us to study features
+and how they relate to each other. There are many types of spatial analysis
+that can be carried out, for example, you could use GIS to find out how many
+red roofed houses occur in a particular area.  If you have tree features, you
+could use GIS to try to find out which species might be affected if a piece
+of land is developed. We can use the attributes stored for water samples
+along a river course to understand where pollution is entering into the
+stream. The possibilities are endless! In a later topic we will be exploring
+spatial analysis in more detail.
+
+Before we move on to attribute data in more detail, let's take a quick recap:
+
+Features are real world things such as roads, property boundaries, electrical
+substation sites and so on. A \textbf{feature} has a \textbf{geometry} (which
+determines if it is a \textbf{point}, \textbf{polyline} or \textbf{polygon})
+and \textbf{attributes} (which describe the feature).
+This is shown in Figure \ref{fig:attdiagram}. 
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{In a GIS Application, we can also search for features based on
+their attributes. Here we see a search for houses with black roofs. Results
+are shown in yellow in the map, turquoise on the table.}
+\label{fig:housesearch}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{HousesSearch}
+\end{center}
+\end{figure}
+
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{Vector features at a glance.}
+\label{fig:attdiagram}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{attribute_diagram}
+\end{center}
+\end{figure}
+
+\subsection{Attributes in detail}
+
+Attributes for a vector feature are stored in a \textbf{table}. A table is like a
+spreadsheet. Each column in the table is called a \textbf{field}. Each row in the
+table is a record. Table \ref{tab:attributes} shows a simple example of how
+an attribute
+table looks in a GIS. The records in the attribute table in a GIS each
+correspond to one feature. Usually the information in the attribute table is
+stored in some kind of database. The GIS application links the attribute
+records with the feature geometry so that you can find records in the table
+by selecting features on the map, and find features on the map by selecting
+features in the table.
+
+%% Note: xdvi does not show white text on black background but it works!
+\begin{table}[ht]
+\centering
+\caption{An attribute table has fields (columns) and records (in rows)}\medskip
+ \label{tab:attributes}
+ \begin{tabular}{|p{4cm}|p{4cm}|p{4cm}|p{4cm}|}
+ \hline
+ \rowcolor{black}
+ \textcolor{blue}{\textbf{Attribute Table}} &
+ \textcolor{white}{\textbf{Field 1: YearBuilt}} & 
+ \textcolor{white}{\textbf{Field 2: RoofColour}} &
+ \textcolor{white}{\textbf{Field 3: Balcony}} \\
+ \hline Record 1 & 1998 & Red & Yes \\
+ \hline Record 2 & 2000 & Black & No \\
+ \hline Record 3 & 2001 & Silver & Yes\\
+\hline
+\end{tabular}
+\end{table}
+
+Each field in the attribute table contains contains a specific type of data -
+text, numeric or date. Deciding what attributes to use for a feature requires
+some thought and planning. In our house example earlier on in this topic, we
+chose roof colour, presence of a balcony and month of construction as
+attributes of interest. We could just as easily have chosen other aspects of
+a house such as:
+
+\begin{itemize}
+\item number of levels
+\item number of rooms
+\item number of occupants
+\item type of dwelling (RDP House, block of flats, shack, brick house etc)
+\item year the house was built
+\item area of floor space in the house
+\item and so on....
+\end{itemize}
+
+With so many options, how do we make a good choice as to what attributes are
+needed for a feature? It usually boils down to what you plan to do with the
+data. If you want to produce a colour coded map showing houses by age, it
+will make sense to have a 'Year Built' attribute for your feature. If you
+know for sure you will never use this type of map, it is better to not store
+the information. Collecting and storing unneeded information is a bad idea
+because of the cost and time required to research and capture the
+information. Very often we obtain vector data from companies, friends or the
+government. In these cases it is usually not possible to request specific
+attributes and we have to make do with what we get.
+
+\subsection{Single Symbols}
+
+If a feature is symbolised without using any attribute table data, it can
+only be drawn in a simple way. For example with point features you can set
+the colour and \textbf{marker} (circle, square, star etc.) but that is all.
+You cannot
+tell the GIS to draw the features based on one of its properties in the
+attribute table. In order to do that, you need to use either a
+\textbf{graduated}, \textbf{continuous} or \textbf{unique value} symbol.
+These are described in detail in the sections that follow.
+
+A GIS application will normally allow you to set the symbology of a layer
+using a \textbf{dialog box} such as the one shown in Figure
+\ref{fig:symbols}a. In this
+dialog box you can choose colours and symbol styles. Depending on the
+geometry type of a layer, different options may be shown. For example with
+point layers you can choose a \textbf{marker style}. With line and polygon layers
+there is no marker style option, but instead you can select a \textbf{line
+style} and \textbf{colour} such as dashed orange for gravel roads, solid
+orange for minor roads,
+and so on (as shown in \ref{fig:symbols}b). With polygon layers you also
+have the option of setting a \textbf{fill style} and colour.
+
+\begin{figure}[ht]
+\centering
+\caption{Setting the symbology of a vector layer}\label{fig:symbols}
+   \subfigure[When using simple symbols, the feature is drawn without using
+an attribute to control how it looks. This is the dialog for point features.]
+   {\label{subfig:poisymbol}\includegraphics[clip=true, width=0.45\textwidth]{single_symbol_dialog_point}}\goodgap
+   \subfigure[There are different options when defining simple symbols for
+polyline and polygon features.]
+    {\label{subfig:polsymbol}\includegraphics[clip=true, width=0.45\textwidth]{single_symbol_dialog_lines}}\goodgap
+\end{figure}
+
+\subsection{Graduated Symbols}
+
+Sometimes vector features represent things with a changing numerical value.
+Contour lines are a good example of this. Each contour usually has an
+attribute value called 'height' that contains information about what height
+that contour represents. In Figure \ref{fig:housesearch} earlier in this
+topic we showed
+contours all drawn with the same colour. Adding colour to the contours can
+help us to interpret the meanings of contours. For example we can draw low
+lying areas with one colour, mid-altitude areas with another and
+high-altitude areas with a third.
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{The height attribute of contours can be used to separate the
+contours into 3 classes. Contours between 980m and 1120m will be drawn in
+brown, those between 1120m and 1240m in green and those between 1240m and
+1500m in purple.}
+\label{fig:gradcoldialog}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{graduated_colour_dialog}
+\end{center}
+\end{figure}
+
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{Our map after setting graduated colours for our contours.}
+\label{fig:gradcontours}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{graduated_contours}
+\end{center}
+\end{figure}
+
+Setting colours based on discrete groups of attribute values is called
+Graduated Symbology in QGIS. The process is shown in Figure
+\ref{fig:gradcoldialog} and \ref{fig:gradcontours}. \textbf{Graduated symbols
+are most useful when you want to show clear
+differences between features with attribute values in different value ranges}.
+The GIS Application will analyse the attribute data (e.g. height) and, based
+on the number of classes you request, create groupings for you. This process
+is illustrated in Table \ref{tab:gradcolor}.
+
+%% Define colors for the table
+%% From http://oregonstate.edu/~peterseb/tex/samples/color-package.html
+%% Note: xdvi does not show white text on black background but it works!
+\begin{table}[ht]
+\centering
+\caption{Graduated colour breaks up the attribute value ranges into the
+number of classes you select. Each class is represented by a different
+colour.}\medskip
+ \label{tab:gradcolor}
+ \begin{tabular}{|p{8cm}|p{8cm}|}
+ \hline
+ \rowcolor{black}
+ \textcolor{white}{\textbf{Attribute Value}} &
+ \textcolor{white}{\textbf{Class and Colour}} \\
+ \hline 1 & \colorbox{Apricot}{Class 1} \\
+ \hline 2 & \colorbox{Apricot}{Class 1} \\
+ \hline 3 & \colorbox{Apricot}{Class 1} \\
+ \hline 4 & \colorbox{PineGreen}{Class 2} \\
+ \hline 5 & \colorbox{PineGreen}{Class 2} \\
+ \hline 6 & \colorbox{PineGreen}{Class 2} \\
+ \hline 7 & \colorbox{Magenta}{Class 3} \\
+ \hline 8 & \colorbox{Magenta}{Class 3} \\ 
+ \hline 9 & \colorbox{Magenta}{Class 3} \\
+\hline
+\end{tabular}
+\end{table}
+
+\subsection{Continuous Colour Symbols}
+
+In the previous section on Graduated Colour symbols we saw that we can draw
+features in discrete groups or classes. Sometimes it is useful to draw
+features in a \textbf{colour range} from one colour to another. The GIS Application
+will use a numerical attribute value from a feature (e.g. contour heights or
+pollution levels in a stream) to decide which colour to use. Table
+\ref{tab:contcolor} shows how the attribute value is used to define a
+continuous range of colours.
+
+%% Define colors for the table !! not done yet !!
+%% From http://oregonstate.edu/~peterseb/tex/samples/color-package.html
+%% Note: xdvi does not show white text on black background but it works!
+\begin{table}[ht]
+\centering
+\caption{Continuous colour symbology uses a start colour (e.g. light orange
+shown here) and an end colour (e.g. dark brown shown here) and creates a
+series of shades between those colours.}\medskip
+ \label{tab:contcolor}
+ \begin{tabular}{|p{8cm}|p{8cm}|}
+ \hline
+ \rowcolor{black}
+ \textcolor{white}{\textbf{Attribute Value}} &
+ \textcolor{white}{\textbf{Colour (no classes or grouping)}} \\
+ \hline 1 &  \\
+ \hline 2 &  \\
+ \hline 3 &  \\
+ \hline 4 &  \\
+ \hline 5 &  \\
+ \hline 6 &  \\
+ \hline 7 &  \\
+ \hline 8 &  \\
+ \hline 9 &  \\
+\hline
+\end{tabular}
+\end{table}
+
+Using the same contours example we used in the previous section, let's see
+how a map with continuous colour symbology is defined and looks. The process
+starts by setting the layers properties to continuous colour using a dialog
+like the one shown in Figure \ref{fig:contcoldialog}.
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{Setting up continuous colour symbology. The contour height
+attribute is used to determine colour values. Colours are defined for the
+minimum and maximum values. The GIS Application will then create a gradient
+of colours for drawing the features based on their heights.}
+\label{fig:contcoldialog}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{continuous_colour_dialog}
+\end{center}
+\end{figure}
+
+After defining the minimum and maximum colours in the \textbf{colour range}, the
+colour features are drawn in will depend on where the attribute lies in the
+range between minimum and maximum. For example if you have contour features
+with values starting at 1000m and ending at 1400m, the value range is 1000 to
+1400. If the colour set for the minimum value is set to orange and the colour
+for  the maximum value is black, contours with a value of close to 1400m will
+be drawn close to black. On the other hand contours with a value near to
+1000m will be drawn close to orange (see Figure \ref{fig:contourmap}).
+
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{A contour map drawn using continuous colour symbology.}
+\label{fig:contourmap}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{continuous_colour_contours}
+\end{center}
+\end{figure}
+
+\subsection{Unique Value Symbols}
+
+Sometimes the attributes of features are not numeric, but instead
+\textbf{strings} are
+used. 'String' is a computer term meaning a group of letters, numbers and
+other writing symbols. Strings attributes are often used to classify things
+by name. We can tell the GIS Application to give each unique string or number
+its own colour and symbol. Road features may have different classes (e.g.
+'street', 'secondary road', 'main road' etc.), each drawn in the map view of
+the GIS with different colours or symbols. This is illustrated in Table
+\ref{tab:unicolor}.
+
+%% Note: xdvi does not show white text on black background but it works!
+\begin{table}[ht]
+\centering
+\caption{Unique attribute values for a feature type (e.g. roads) can each
+have their own symbol.}\medskip
+ \label{tab:unicolor}
+ \begin{tabular}{|p{8cm}|p{8cm}|}
+ \hline
+ \rowcolor{black}
+ \textcolor{white}{\textbf{Attribute Value}} &
+ \textcolor{white}{\textbf{Colour class and symbol}} \\
+ \hline Arterial route &  \\
+ \hline Main road &  \\
+ \hline Secondary road &  \\
+ \hline Street &  \\
+\hline
+\end{tabular}
+\end{table}
+
+Within the GIS Application we can open /choose to use Unique Value symbology
+for a layer. The GIS will scan through all the different string values in the
+attribute field and build a list of unique strings or numbers. Each unique
+value can then be assigned a colour and style. This is shown in Figure
+\ref{fig:univaldialog}.
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{Defining unique value symbology for roads based on the road type.}
+\label{fig:univaldialog}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{unique_value_dialog}
+\end{center}
+\end{figure}
+
+When the GIS draws the layer, it will look at the attributes of each feature
+before drawing it to the screen. Based on the value in the chosen field in
+the attribute table, the road line will be drawn with suitable colour and
+line style (and fill style if its a polygon feature). This is shown in
+Figure \ref{fig:uniroads}.
+
+\begin{figure}[ht]
+   \begin{center}
+   \caption{A roads vector layer symbolised using a unique value per road
+type.}
+\label{fig:uniroads}\smallskip
+   \includegraphics[clip=true, width=0.5\textwidth]{unique_value_roads}
+\end{center}
+\end{figure}
+
+\subsection{Things to be aware of}
+
+Deciding which attributes and symbology to use requires some planning. Before
+you start collecting any \textbf{GeoSpatial} data, you should ensure you know what
+attributes are needed and how it will be symbolised. It is very difficult to
+go back and re-collect data if you plan poorly the first time around.
+Remember also that the goal of collecting attribute data is to allow you to
+analyse and interpret spatial information. How you do this depends on the
+questions you are trying to answer. Symbology is a visual language that
+allows people to see and understand your attribute data based on the colours
+and symbols you use. Because of this you should put a lot of thought into how
+you symbolise your maps in order to make them easy to understand.
+
+\subsection{What have we learned?}
+
+Let's wrap up what we covered in this worksheet:
+
+\begin{itemize}
+\item Vector features have \textbf{attributes}
+\item Attributes \textbf{describe} the \textbf{properties} of the feature
+\item The attributes are stored in a \textbf{table}
+\item Rows in the table are called \textbf{records}
+\item There is \textbf{one record per feature} in the vector layer
+\item Columns in the table are called \textbf{fields}
+\item Fields represent \textbf{properties} of the feature e.g. height, roof
+colour etc.
+\item Fields can contain \textbf{numerical}, \textbf{string} (any text) and
+\textbf{date} information
+\item The attribute data for a feature can be used to determine how it is
+\textbf{symbolised}
+\item \textbf{Graduated colour} symbology groups the data into discrete classes
+\item \textbf{Continuous colour} symbology assigns colours from a colour
+range to the features based on their attributes
+\item \textbf{Unique value} symbology associates each different value in the
+chosen attribute column with a different symbol (colour and style)
+\item If the attribute of a vector layer is not used to determine its symbology, it
+is drawn using a \textbf{single symbol} only
+\end{itemize}
+
+\subsection{Now you try!}
+
+Here are some ideas for you to try with your learners:
+
+\begin{itemize}
+\item Using the table that you created in the last topic, add a new column for the
+symbology type you would use for each feature type and have the learners
+identify which symbology type they would use (see Table \ref{tab:featuretype}
+for an example).
+\item Try to identify which symbology types you would use for the following types
+of vector features:
+\begin{itemize}
+\item points showing pH level of soil samples taken around your school
+\item lines showing a road network in a city
+\item polygons for houses with an attribute that shows whether it is made of brick,
+wood or 'other' material.
+\end{itemize}
+\end{itemize}
+
+%% Note: xdvi does not show white text on black background but it works!
+\begin{table}[ht]
+\centering
+\caption{An example of a table that defines the feature types and the kind of
+symbology you would use for each.}\medskip
+ \label{tab:featuretype}
+ \begin{tabular}{|p{4cm}|p{3cm}|p{9cm}|}
+ \hline
+ \rowcolor{black}
+ \textcolor{white}{\textbf{Real world feature}} &
+ \textcolor{white}{\textbf{Geometry Type}} & 
+ \textcolor{white}{\textbf{Symbology Type}} \\
+ \hline The school flagpole & Point & \textbf{Single Symbol} \\
+ \hline The soccer field & Polygon & \textbf{Single Symbol} \\
+ \hline The footpaths in and around the school & Polygon & Have your learners
+count the number of learners using each footpath in the hour before school
+and then use \textbf{graduated symbols} to show the popularity of each footpath \\
+ \hline Places where taps are located & Point & \textbf{Single Symbol} \\
+ \hline Classrooms & Polygon & \textbf{Unique Value} based on the grade of
+the learners in the classroom \\
+ \hline Fence & Polyline & Have your learners rate the condition of the fence
+around your school by separating it into sections and grading each  section
+on a scale of 1-9 based on its condition. Use \textbf{graduated symbols} to
+classify the condition attribute. \\
+ \hline Classrooms & Polygon & Count the number of learners in each classroom
+and use a \textbf{continuous colour symbol} to define a range of colours from
+red to blue. \\
+\hline
+\end{tabular}
+\end{table}
+
+\subsection{Something to think about}
+
+If you don't have a computer available, you can use transparency sheets and a
+1:50 000 map sheet to experiment with different symbology types. For example
+place a transparency sheet over the map and using different coloured koki
+pens, draw in red all contour lines below 900m (or similar) and in green all
+lines above or equal to 900m. Can you think of how to reproduce other
+symbology types using the same technique?
+
+\subsection{Further reading}
+
+\textbf{Website}: \url{http://en.wikipedia.org/wiki/Cartography#Map_symbology}
+
+The QGIS User Guide also has more detailed information on working with
+attribute data and symbology in QGIS.
+
+\subsection{What's next?}
+
+In the section that follows we will take a closer look at data capture. We
+will put the things we have learned about vector data and attributes into
+practice by creating new data.
+

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--- docs/trunk/english_us/gis_introduction/attributedata_images/Makefile	2009-07-11 08:00:04 UTC (rev 11041)
+++ docs/trunk/english_us/gis_introduction/attributedata_images/Makefile	2009-07-11 10:08:12 UTC (rev 11042)
@@ -18,12 +18,14 @@
     graduated_contours.eps\
     HousesByBalcony.eps\
     HousesByRoofColour.eps\
+    HousesByBalconyRoof.eps\
     Houses.eps\
     HousesSearch.eps\
     RenderAsColour.eps\
     RenderAsComparison.eps\
     RenderAsGrey.eps\
     single_symbol_dialog_lines.eps\
+    single_symbol_dialog_point.eps\
     unique_value_dialog.eps\
     unique_value_roads.eps\
  

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===================================================================
--- docs/trunk/english_us/gis_introduction/qgis_style.sty	2009-07-11 08:00:04 UTC (rev 11041)
+++ docs/trunk/english_us/gis_introduction/qgis_style.sty	2009-07-11 10:08:12 UTC (rev 11042)
@@ -46,7 +46,7 @@
 	\graphicspath{{./finalpix/}{../finalpix/}{images}}
 
 % color package for dvips
-\usepackage[dvips]{color}
+\usepackage[usenames, dvips]{color}
 
 % author-year-citation paket natbib (for cited literature)
 \usepackage[numbers,square]{natbib}