[QGIS Commit] r11053 - docs/trunk/english_us/gis_introduction

svn_qgis at osgeo.org svn_qgis at osgeo.org
Mon Jul 13 06:31:35 EDT 2009

Author: dassau
Date: 2009-07-13 06:31:34 -0400 (Mon, 13 Jul 2009)
New Revision: 11053

finished section 4

Modified: docs/trunk/english_us/gis_introduction/datacapture.tex
--- docs/trunk/english_us/gis_introduction/datacapture.tex	2009-07-13 08:26:32 UTC (rev 11052)
+++ docs/trunk/english_us/gis_introduction/datacapture.tex	2009-07-13 10:31:34 UTC (rev 11053)
@@ -16,5 +16,450 @@
+In the previous two topics we looked at vector data. We saw that there are
+two key concepts to vector data, namely: \textbf{geometry} and
+\textbf{attributes}. The
+geometry of a vector feature describes its \textbf{shape} and
+\textbf{position}, while the
+\textbf{attributes} of a vector feature describe its \textbf{properties}
+(colour, size, age etc.).
+In this section we will look more closely at the process of creating and
+editing vector data - both the geometry and attributes of vector features.
+\subsection{How does GIS digital data get stored?}
+Word processors, spreadsheets and graphics packages are all programs that let
+you create and edit digital data. Each type of application saves its data
+into a particular file format. For example, a graphics program will let you
+save your drawing as a '.jpg' JPEG image, word processors let you save your
+document as an '.odt' OpenDocument or '.doc' Word Document, and so on.
+Just like these other applications, GIS Applications can store their data in
+files on the computer hard disk. There are a number of different file formats
+for GIS data, but the most common one is probably the 'shape file'. The name
+is a little odd in that although we call it a shape file (singular), it
+actually consists of at least three different files that work together to
+store your digital vector data, as shown in Table \ref{tab:shapefile}. 
+%% Note: xdvi does not show white text on black background but it works!
+\caption{The basic files that together make up a 'shapefile'.}\medskip
+ \label{tab:shapefile}
+ \begin{tabular}{|p{3cm}|p{13cm}|}
+ \hline
+ \rowcolor{black}
+ \textcolor{white}{\textbf{Extension}} &
+ \textcolor{white}{\textbf{Description}} \\
+ \hline .shp & The geometry of vector features are stored in this file. \\
+ \hline .dbf & The attributes of vector features are stored in this file. \\
+ \hline .shx & This file is an index that helps the GIS Application to find
+features more quickly. \\
+When you look at the files that make up a shapefile on the computer hard
+disk, you will see something like Figure \ref{fig:shapefiles}. If you want to share
+vector data stored in shapefiles with another person, it is important to give
+them all of the files for that layer. So in the case of the trees layer shown
+in \ref{fig:shapefiles}, you would need to give the person trees.shp,
+trees.shx, trees.dbf, trees.prj and trees.qml.
+   \begin{center}
+   \caption{The files that make up a 'trees' shapefile as seen in the
+computer's file manager.}
+   \includegraphics[clip=true, width=\textwidth]{shapefiles}
+Many GIS Applications are also able to store digital data inside a
+In general storing GIS data in a database is a good solution because the
+database can store \textbf{large amounts} of data \textbf{efficiently} and
+can provide data to
+the GIS Application quickly. Using a database also allows many people to work
+with the same vector data layers at the same time. Setting up a database to
+store GIS data is more complicated than using shapefiles, so for this topic
+we will focus on creating and editing shapefiles.
+\subsection{Planning before you begin}
+Before you can create a new vector layer (which will be stored in a
+shapefile), you need know what the geometry of that layer will be (point,
+polyline or polygon), and you need to know what the attributes of that layer
+will be. Let's look at a few examples and it will become clearer how to go
+about doing this.
+\minisec{Example 1: Creating a tourism map}
+Imagine that you want to create a nice tourism map for your local area. Your
+vision of the final map is a 1:50 000 toposheet with markers overlaid for
+sites of interest to tourists. First, let's think about the geometry. We know
+that we can represent a vector layer using point, polyline or polygon
+features. Which one makes the most sense for our tourism map? We could use
+points if we wanted to mark specific locations such as look out points,
+memorials, battle sites and so on. If we wanted to take tourists along a
+route, such as a scenic oute through a mountain pass, it might make sense to
+use polylines. If we have whole areas that are of tourism interest, such as a
+nature reserve or a cultural village, polygons might make a good choice.
+As you can see it's often not easy to know what type of geometry you will
+need. One common approach to this problem is to make one layer for each
+geometry type you need. So, for example, if you look at digital data provided
+by the Chief Directorate : Surveys and Mapping, South Africa, they provide a
+river areas (polygons) layer and a rivers polyline layer. They use the river
+areas (polygons) to represent river stretches that are wide, and they use
+river polylines to represent narrow stretches of river. In Figure
+\ref{fig:tourism} we can see how our tourism layers might look on a map if we
+used all three geometry types.
+   \begin{center}
+   \caption{A map with tourism layers. We have used three different geometry
+types for tourism data so that we can properly represent the different kinds
+of features needed for our visitors, giving them all the information they
+   \includegraphics[clip=true, width=0.8\textwidth]{TourismMap3GeometryTypes}
+\minisec{Example 2: Creating a map of pollution levels along a river}
+If you wanted to measure pollution levels along the course of a river you
+would typically travel along the river in a boat or walk along its banks. At
+regular intervals you would stop and take various measurements such as
+Dissolved Oxygen (DO) levels, Coliform Bacteria (CB) counts, Turbidity levels
+and pH. You would also need to make a map reading of your position or obtain
+your position using a GPS receiver.
+To store the data collected from an exercise like this in a GIS Application,
+you would probably create a GIS layer with a point geometry. Using point
+geometry makes sense here because each sample taken represents the conditions
+at a very specific place.
+For the attributes we would want a \textbf{field} for each thing that describes the
+sample site. So we may end up with an attribute table that looks something
+like Table \ref{tab:attrtable}.
+%% Note: xdvi does not show white text on black background but it works!
+\caption{Drawing a table like this before you create your vector layer will
+let you decide what attribute fields (columns) you will need. Note that the
+geometry (positions where samples were taken) is not shown in the attribute
+table - the GIS Application stores it separately!}\medskip
+ \label{tab:attrtable}
+ \begin{tabular}{|p{2.3cm}|p{1cm}|p{1cm}|p{1cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|}
+ \hline
+ \rowcolor{black}
+ \textcolor{white}{\textbf{SampleNo}} &
+ \textcolor{white}{\textbf{pH}} &
+ \textcolor{white}{\textbf{DO}} &
+ \textcolor{white}{\textbf{CB}} &
+ \textcolor{white}{\textbf{Turbidity}} &
+ \textcolor{white}{\textbf{Collector}} &
+ \textcolor{white}{\textbf{Date}} \\
+ \hline 1 & 7 & 6 & N & Low & Patience & 12/01/2009 \\
+ \hline 2 & 6.8 & 5 & Y & Medium & Thabo & 12/01/2009 \\
+ \hline 3 & 6.9 & 6 & Y & High & Victor & 12/01/2009 \\
+\subsection{Creating an empty shapefile}
+Once you have planned what features you want to capture into the GIS, and the
+geometry type and attributes that each feature should have, you can move on
+to the next step of creating an empty shapefile. 
+The process usually starts with choosing the 'new vector layer' option in
+your GIS Application and then selecting a \textbf{geometry type} (see Figure
+\ref{fig:newshape}). As we covered in an earlier topic, this means choosing
+either point, polyline or polygon for the geometry. 
+   \begin{center}
+   \caption{Creating a new vector layer is as simple as filling in a few
+details in a form. First you choose the geometry type, and then you add the
+attribute fields.}
+   \includegraphics[clip=true, width=0.6\textwidth]{NewVectorLayer}
+Next you will add fields to the attribute table. Normally we give field names
+that are short, have no spaces and indicate what type of information is being
+stored in that field. Example field names may be 'pH', 'RoofColour',
+'RoadType' and so on. As well as choosing a name for each field, you need to
+indicate how the information should be stored in that field - i.e. is it a
+number, a word or a sentence, or a date? 
+Computer programs usually call information that is made up of words or
+sentences '\textbf{strings}', so if you need to store something like a street name or
+the name of a river, you should use string for the field type.
+The shapefile format allows you to store the numeric field information as
+either a whole number (\textbf{integer}) or a decimal number
+(\textbf{floating point}) - so you
+need to think before hand whether the numeric data you are going to capture
+will have decimal places or not.
+   \begin{center}
+   \caption{After defining our new layer's geometry and attributes, we need
+to save it to disk. It is important to give a short but meaningful name to
+your shapefile.}
+   \includegraphics[clip=true, width=0.6\textwidth]{SaveAs}
+The final step (as shown in Figure \ref{fig:saveas}) for creating a shapefile
+to give it a name and a place on the computer hard disk where it should be
+created. Once again it is a good idea to give the shapefile a short and
+meaningful name. Good examples are 'rivers', 'watersamples' and so on.
+Let's recap the process again quickly. To create a shapefile you first say
+what kind of geometry it will hold, then you create one or more fields for
+the attribute table, and then you save the shapefile to the hard disk using
+an easy to recognise name. Easy as 1-2-3!
+\subsection{Adding data to your shapefile}
+So far we have only created an empty shapefile. Now we need to \textbf{enable
+in the shapefile using the 'enable editing' menu option or tool bar icon in
+the GIS Application. Shapefiles are not enabled for editing by default to
+prevent accidentally changing or deleting the data they contain. Next we need
+to start adding data. There are two steps we need to complete for each
+\textbf{record} we add to the shapefile:
+\item Capturing geometry
+\item Entering attributes 
+The process of capturing geometry is different for points, polylines and
+To \textbf{capture a point}, you first use the map pan and zoom tools to get to the
+correct geographical area that you are going to be recording data for. Next
+you will need to enable the point capture tool. Having done that, the next
+place you click with the \textbf{left mouse button} in the map view, is where you want
+your new point \textbf{geometry} to appear. After you click on the map, a window will
+appear and you can enter all of the \textbf{attribute data} for that point (see
+Figure \ref{fig:enterattr}). If you are unsure of the data for a given field you
+can usually leave it blank, but be aware that if you leave a lot of fields
+blank it will be hard to make a useful map from your data!
+   \begin{center}
+   \caption{After you have captured the point geometry, you will be asked to
+describe its attributes. The attribute form is based on the fields you
+specified when you created the vector layer.}
+   \includegraphics[clip=true, width=0.5\textwidth]{EnterAttributeValues}
+To \textbf{capture a polyline} the process is similar to that of a point, in that you
+need to first use the pan and zoom tools to move the map in the map view to
+the correct geographical area. You should be zoomed in enough so that your
+new vector polyline feature will have an appropriate scale (see Topic 2:
+Working with Vector Data for more details on scale issues). When you are
+ready, you can click the polyline capture icon in the tool bar and then start
+drawing your line by clicking on the map. After you make your first click,
+you will notice that the line stretches like an elastic band to follow the
+mouse cursor around as you move it. Each time you click with the \textbf{left
+mouse button}, a new vertex will be added to the map. This process is shown in
+Figure \ref{fig:linecapture}. 
+   \begin{center}
+   \caption{Capturing lines for a tourism map. When editing a line layer, the
+vertices are shown with circular markers which you can move about with the
+mouse to adjust the line's geometry. When adding a new line (shown in red),
+each click of the mouse will add a new vertex.}
+   \includegraphics[clip=true, width=\textwidth]{LineCapture}
+When you have finished defining your line, use the \textbf{right mouse
+button} to tell
+the GIS Application that you have completed your edits. As with the procedure
+for capturing a point feature, you will then be asked to enter in the
+attribute data for your new polyline feature.
+The process for \textbf{capturing a polygon} is almost the same as capturing a
+polyline except that you need to use the polygon capture tool in the tool
+bar. Also, you will notice that when you draw your geometry on the screen,
+the GIS Application always creates an enclosed area.
+To add a new feature after you have created your first one, you can simply
+click again on the map with the point, polyline or polygon capture tool
+active and start to draw your next feature.
+When you have no more features to add, always be sure to click the 'allow
+editing' icon to toggle it off. The GIS Application will then save your newly
+created layer to the hard disk.
+\subsection{Heads-up digitising}
+As you have probably discovered by now if you followed the steps above, it is
+pretty hard to draw the features so that they are \textbf{spatially correct} if you do
+not have other features that you can use as a point of reference. One common
+solution to this problem is to use a raster layer (such as an aerial
+photograph or a satellite image) as a backdrop layer. You can then use this
+layer as a reference map, or even trace the features off the raster layer
+into your vector layer if they are visible. This process is known as
+'heads-up digitising' and is shown in Figure \ref{fig:headsupdigi}.
+   \begin{center}
+   \caption{Heads-up digitising using a satellite image as a backdrop. The
+image is used as a reference for capturing polyline features by tracing over
+   \includegraphics[clip=true, width=\textwidth]{HeadsUpDigitising}
+\subsection{Digitising using a digitising table}
+Another method of capturing vector data is to use a digitising table. This
+approach is less commonly used except by GIS professionals, and it requires
+expensive equipment. The process of using a digitising table, is to place a
+paper map on the table. The paper map is held securely in place using clips.
+Then a special device called a 'puck' is used to trace features from the map.
+Tiny cross-hairs in the puck are used to ensure that lines and points are
+drawn accurately. The puck is connected to a computer and each feature that
+is captured using the puck gets stored in the computer's memory. You can see
+what a digitising puck looks like in Figure \ref{fig:digitable}.
+   \begin{center}
+   \caption{A digitising table and puck are used by GIS professionals when
+they want to digitise features from existing maps.}
+   \includegraphics[clip=true, width=\textwidth]{digitising_table}
+\minisec{After your features are digitised}
+Once your features are digitised, you can use the techniques you learned in
+the previous Topic to set the symbology for your layer. Choosing an
+appropriate symbology will allow you to better understand the data you have
+captured when you look at the map.
+\subsection{Common problems / things to be aware of}
+If you are digitising using a backdrop raster layer such as an aerial
+photograph or satellite image, it is very important that the raster layer is
+properly georeferenced. A layer that is georeferenced properly displays in
+the correct position in the map view based on the GIS Application's internal
+model of the earth. We can see the effect of a poorly georeferenced image in
+Figure \ref{fig:refgoodbad}.
+   \begin{center}
+   \caption{The importance of using properly georeferenced raster images for
+heads-up digitising.  On the left we can see the image is properly
+georegistered and the road features (in orange) overlap perfectly. If the
+image is poorly georeferenced (as shown on the right) the features will not
+be well aligned. Worse still, if the image on the right is used as a
+reference when capturing new features, the newly captured data will be
+   \includegraphics[clip=true, width=\textwidth]{GeoreferencingGoodVsBad}
+Also remember that it is important that you are zoomed in to an appropriate
+scale so that the vector features you create are useful. As we saw in the
+previous topic on vector geometry, it is a bad idea to digitise your data
+when you are zoomed out to a scale of 1:1000 000 if you intend to use the
+data you capture at a scale of 1:50 000 later.
+\subsection{What have we learned?}
+Let's wrap up what we covered in this worksheet:
+\item \textbf{Digitising} is the process of capturing knowledge of a
+feature's \textbf{geometry} and \textbf{attributes} into a \textbf{digital
+format} stored on the computer's disk.
+\item GIS Data can be stored in a \textbf{database} or as \textbf{files}.
+\item One commonly used file format is the \textbf{shapefile} which is
+actually a group of three or more files (.shp, .dbf and .shx).
+\item Before you create a new vector layer you need to plan both what
+\textbf{geometry} type and \textbf{attribute} fields it will contain.
+\item Geometry can be point, polyline or polygon.
+\item Attributes can be \textbf{integers} (whole numbers), \textbf{floating
+points} (decimal numbers), \textbf{strings} (words) or \textbf{dates}.
+\item The digitising process consists of \textbf{drawing} the geometry in the
+map view and then entering its attributes. This is repeated for each feature.
+\item \textbf{Heads-up digitising} is often used to provide orientation
+during digitising by using a raster image in the background.
+\item Professional GIS users sometimes use a \textbf{digitising table} to
+capture information from paper maps.
+\subsection{Now you try!}
+Here are some ideas for you to try with your learners:
+\item Draw up a list of features in and around your school that you think would be
+interesting to capture. For example: the school boundary, the position of
+fire assembly points, the layout of each class room, and so on. Try to use a
+mix of different geometry types. Now split your learners into groups and
+assign each group a few features to capture. Have them symbolise their layers
+so that they are more meaningful to look at. Combine the layers from all the
+groups to create a nice map of your school and its surroundings!
+\item Find a local river and take water samples along its length. Make a careful
+note of the position of each sample using a GPS or by marking it on a
+toposheet. For each sample take measurements such as pH, dissolved oxygen
+etc. Capture the data using the GIS application and make maps that show the
+samples with a suitable symbology. Could you identify any areas of concern?
+Was the GIS Application able to help you to identify these areas?
+\subsection{Something to think about}
+If you don't have a computer available, you can follow the same process by
+using transparency sheets and a notebook. Use an aerial photo, orthosheet or
+satellite image printout as your background layer. Draw columns down the page
+in your notebook and write in the column headings for each attribute field
+you want to store information about. Now trace the geometry of features onto
+the transparency sheet, writing a number next to each feature so that it can
+be identified. Now write the same number in the first column in your table in
+your notebook, and then fill in all the additional information you want to
+\subsection{Further reading}
+\url{http://www.k12science.org/curriculum/waterproj/S00project/miami2000/miamiriverfinal.html} - A school project to assess water quality in their local river.
+The QGIS User Guide also has more detailed information on digitising vector
+data in QGIS.
+\subsection{What's next?}
+In the section that follows we will take a closer look at raster data to
+learn all about how image data can be used in a GIS.

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