Lancaster University GIS Course 2010

Lancaster University Masterclass in Open Source GIS: January
2010

Databases and Web Mapping the Open Source way

Introduction
There is considerably more to web mapping than Google Mashups. While the approach outlined in this
section is a bit more complicated, it's a lot more powerful, flexible and interesting, and you have
complete control over your maps.
There are three parts to this section: databases, map servers and web servers. It's not possible to go into
full detail about any of these areas in the time available, but hopefully you will go away with an
overview of how things work and an idea about where to go for more information.
Hopefully, this section will allow you to load data into a spatial database and learn how to work with
and interrogate it. You will then use a map server and web server to display it in a webbased map over
suitable base data. There will be brief digressions into displaying your data in a desktop GIS, and into
using web mapping services. Finally we'll look at some of the more advanced and interesting new
developments in web mapping.

Databases
The database we are going to use is PostgreSQL (http://www.postgresql.org), which is a serverbased
multiuser database with support for storing spatial vector data such as points, lines, and polygons. The
difference between serverbased databases such as PostgreSQL and filebased databases such as
Microsoft Access is that there is complete separation between the data, stored on the server, or host, and
the software used to access the data on the remote pc, or client. The software used to access the data is
usually one of the following:
1.PSQL a command line package bundled with the server installation
2.PgAdmin3 with a graphical user interface allowing access to tables, management tasks and queries
3.PhpPgAdmin a webbased user interface similar to PgAdmin3
4.Desktop GIS packages such as Quantum GIS and GvSIG
5.Map Servers
PostGIS is an extension for PostgreSQL that provides additional functionality such as spatial queries
(within, distance from, inside and so on) and coordinate systems. These allow you to do complex spatial
queries and management of your data without needing any additional software.

Basics
Data is stored on the HOST server in a DATABASE, and accessed using a USERNAME, PASSWORD,
and PORT. The HOST parameter is likely to be an IP address (numbers of the form 192.168.3.23 which
form the unique address of the server on the network), a name, or “localhost” if you are connecting

directly from the server. The port parameter is usually the default 5432.
Each program needs these 5 parameters in order to connect to the database. The Detailed configuration
of user security (who is allowed to access which database, and using which computer) is possible using
textbased config files on the host.
The way of communicating with the data in a PostgreSQL database is by using Structured Query
Language, or SQL. This is textbased, and humanreadable, with a very precise syntax and grammar.
We will cover a small subset of SQL when we learn how to query the data in a later section.
PostGIS has to be installed after PostgreSQL. It creates additional tables in the database to store
information on coordinate systems. PostGIS can either be installed on an individual database, or in the
template from which all new databases are created.

Loading data into PostgreSQL
There are several ways of loading spatial data into PostgreSQL. The most common are two command
line packages called OGR2OGR and SHP2PGSQL. We will use SHP2PGSQL, which is bundled with
the PostGIS installation. As the name suggests it only works with shape files.
SHP2PGSQL converts the data in a shapefile into a stream of SQL commands which are then loaded
using PSQL. These two steps can be concatenated into a single command.
As well as the 5 connection parameters, SHP2PGSQL requires the following pieces of information:
1.The location and name of the shapefile
2.The coordinate system of the data in SRID form (4326 for lat/long)
3.The name of the table you wish to load it into
The syntax is as follows:
$ shp2pgsql /location/of/your/shapefile.shp tablename s coordinate system W latin1 | psql d database
U username W h host p port
The shp2pgsql W parameter sets the encoding of the database to match the encoding of the shape file
and isn't always necessary. The psql W parameter prompts you for a password before processing the
command.
Note the case of U and W, the lowercase parameters mean something different!
You can check that the data has loaded using PgAdmin3 this will show you the textbased attributes of
the data. The geometry of the data will be stored in the geometry column in the table, this is called
the_geom if you use SHP2PGSQL or wkb_geometry if you use OGR2OGR. The different names do
not matter, as the postgisenabled database automatically contains a table called geometry_columns that
lists the geometry column used by each table, and the coordinate system in use. This is automatically
filled in by the SHP2PGSQL or OGR2OGR program.
TASK: Use PgAdmin3 to check your shapefile has loaded, and also to check that the
geometry_columns table has been filled in.

Querying data
The tabular data in PostgreSQL can be queried using SQL as described above. The basic syntax is as

follows:
SELECT columns FROM table WHERE some condition is met;
A guide to using SQL can be found on the PostgreSQL cheatsheet provided, but in brief:
•SELECT * FROM table;
This will select all the data in all the fields from a given table.
•Logical and Arithmetic operators can be used in the WHERE condition to choose data that meets a
given parameter (or parameters).
•In the WHERE condition, text is enclosed in commas 'like this' but numeric data is not.
•The wildcard character % can be used in the WHERE condition LIKE, for example to find all names
that end in 's':
SELECT * FROM table_with_names_in WHERE firstname LIKE ('%s');
TASK: Use the PgAdmin3 query window to select all counties that begin with L.

Spatial Querying
In a PostGISenabled database, it is possible to construct complex spatial queries, like buffers,
intersections, unions, as well as management functions like reprojecting your data into a new
coordinate system. The syntax is an extension to SQL, and many examples can be found in the PostGIS
cheatsheet and manual. (http://postgis.refractions.net/download/postgis1.4.1.pdf)
Some examples:
1.Find the total extent of a layer:
SELECT st_extent(the_geom) FROM tblcounties;
2.Find out which county a particular point is in:
SELECT name_2 FROM tblcounties WHERE st_within(geomfromtext('POINT(1.3 54.26)', 4326),
the_geom);
3.Find out which counties adjoin Lancashire:
SELECT a.name_2 FROM tblcounties a, tblcounties b WHERE b.name_2 = 'Lancashire' AND
a.the_geom && b.the_geom;
Note that this uses a couple of additional concepts. Firstly, to answer this question a table needs to be
compared to itself, otherwise known as a selfjoin. This necessitates the use of aliases, so PostgreSQL
knows which copy of the table it is doing each command to. In this case the aliases are assigned in the
SELECT part of the statement and then used to refer to the table throughout the query. The && symbol
is shorthand for checking that two geometries overlap.
TASK: Find out which county the above point is in, then find out which counties overlap it.

Map Servers
A map server is a program that lives on a host server and accepts requests from a client computer (web
or deskbased) and returns geographic data from a variety of defined data sources. These can be raster

(basemapping) or vector (points, lines and polygons).
In its most simple form that map server packages up all the defined data sources and returns a simple
flat image to the client. Given additional parameters, the map server can return an image of a limited
subset of the data sources, and for a given bounding box or coordinate system.

Types of Map Server
There are two main open source map servers available. These are:
1.Minnesota Mapserver (usually abbreviated to Mapserver). This is a cgi executable, ie a program that
runs when required by a web server. Configuration is done using a text file called a map file.
2.Geosever. This is a javabased package with a web interface for configuration.
There are strengths and weaknesses to both packages. It is possible to run both packages on the one
server, returning data from the same data sources. In this course we will just be using Minnesota
Mapserver.

Installation
Detailed installation instructions are beyond the scope of this tutorial, but information can be found for
all major operating systems at the mapserver website (http://mapserver.org/). In brief, the program
comprises the main executable, mapserv, which must be placed in a location accessible to the web
server, and a series of utility tools for working with maps and data.

Map File Syntax
The map file is a text file with a .map extension. It must live in a location accessible to the web server.
It comprises three basic sections, of which the second two are subsets of the first.
1.Map
This defines global configuration options such as the name, the maximum extent, where to find the
data, and supporting files, and the level of error reporting.
2.Web
This contains the locations of the folders where mapserver stores temporary files and images.
3.Layer
Each data source in the map is defined as a layer, it's type, and connection information are defined.
Below is a very simple map, with one single layer, showing the hierarchy of the sections. Note that each
section ends with the keyword END. Note also that, although mapserver is not casesensitive, it is
standard practice to put keywords in capitals and text strings in quotation marks.
If a map is to be used in a dynamic fashion, such as on a web site, it normally has a number of
additional sections such PROJECTION, which defines it's coordinate system, and CLASS, which
contains the styling information for a layer.
MAP
NAME "sample"
STATUS ON
SIZE 600 400

SYMBOLSET "../etc/symbols.txt"
EXTENT -180 -90 180 90
UNITS DD
SHAPEPATH "../data"
IMAGECOLOR 255 255 255
FONTSET "../etc/fonts.txt"

#
# Start of web interface definition
#
WEB
IMAGEPATH "/ms4w/tmp/ms_tmp/"
IMAGEURL "/ms_tmp/"
END

#
# Start of layer definitions
#
LAYER
NAME 'global-raster'
TYPE RASTER
STATUS DEFAULT
DATA bluemarble.gif
END
END

Checking a map file
There are a number of ways to check that the basic syntax of a map file is correct:
1.Shp2img
This is a utility bundled with most mapserver installations that generates an image from a map file. The
syntax is as follows:
$shp2img -m /location/of/your/mapfile.map -o test.png
2.Checking using a browser
This method is more complex but will generate an error message rather than a blank image in the event
of an error, so can be more informative. The basic syntax is as follows, but note that this differs slightly
from operating system to operating system:
http://yourserver.com/cgibin/mapserv?map=/full/path/of/your/mapfile.map&mode=map

Connecting to a PostgreSQL data source using Mapserver
Mapserver uses the 5 parameters defined above to connect to a PostGISenabled PostgreSQL data
source. The CONNECTIONTYPE parameter must also be used, and set to PostGIS. Additionally the
DATA parameter must include an SQL query that selects some or all of the geometry from the
specified table. Note that the SELECT keyword is omitted from the query, and all the query terms must
be in lowercase.
LAYER
NAME "province"
STATUS ON
TYPE POLYGON

CONNECTIONTYPE POSTGIS
CONNECTION "host=localhost port=5432 dbname=canada user=postgres
password=postgres"
DATA "the_geom from province"
END

Task: Find the extent of the counties data. In the template map file provided, add a connection to
the counties data, change the extent, and check it using shp2img and using a web browser.

Web Servers and Web Pages

Introduction
A web server is a program that sits on a host and responds to remote requests from a client pc using http
(hypertext transfer protocol). This is normally in the form of a web page that is then displayed in a
browser. The web page simply needs to be stored in a location accessible to the web server.
The simplest form of web page generally has the suffix html (it is written in hypertext markup
language). The client requests a page using its URL (Uniform Resource Locator usually the name of
the web site, plus the directory the page is stored in, plus the name of the page) and the web server
sends that page to the client. The client browser renders the page, and sends off further requests for the
additional resources referred to in the page, such as images, or styling information.

Basic structure of a web page
Web pages have a very simple predefined structure. Like map files, they contain a hierarchy of
elements that perform different functions, in this case defined by opening and closing tags. There can
be only one set of each tags. The basic structure is as follows:
<html>

<head>
<title> My Home Page </title>
</head>

<body>
HELLO WORLD!
</body>

</html>

The <head> section is used to tell the browser information about the page and how it should be run. In
addition to the title (which will appear in the title bar of the browser, it may contain metadata about the
language of the page, and links to external files for styling or scripting.
In the <body> section of the page, everything outside of a pair of angle brackets will be displayed on
the web page. As well as plain text, such as in the example above, other attributes (keywords in angle
brackets) can be used for styling, formatting text as a link to another web page, or showing an image.

By using “view source” in a web browser, you can view the structure of any web page.
Task: Make a simple web page showing some text and display it in your browser

Scripting
Scripting is a way of getting a web page to do more than display static text or images. For example, it is
used to create forms to allow you to submit data to a web site, and to display data from a database or
map.
There are many scripting languages available, but there are only two types:
1.Serverside
In this case the web page contains instructions that are interpreted using additional software on the web
server. The result is then formatted as a standard web page and rendered as such by the client (so “view
source” shows you the rendered structure rather than the original script). Examples of this are pages
that display data from a database. The usual language (and software) used for this is PHP (PHP
Hypertext Processor this is not a mistake). These pages usually have the suffix php. This has the
disadvantage that the page must be submitted back to the server for information (such as a filledin
form) to be processed.
2.Clientside
In this case the browser interprets the instructions after it has received the web page from the server.
The most common language used is javascript. The script is either placed in the <head> element of a
standard html web page, in it's own <script> tags, or in an additional file requested by the browser
(these will usually have the suffix js). This method allows for considerable functionality without the
need to resubmit the page back to the server, but it is more limited.
Task: Look at the phpinfo.php file in your web directory using a text editor, then run it in a
browser.

And finally... Web Mapping

Introduction
There are a number of different open source web mapping options, generally using either client or
serverside scripting to work. We are going to concentrate on openlayers (http://openlayers.org), which
is written in javascript. The information below is largely taken from the excellent opengeo openlayers
workshop (http://workshops.opengeo.org/openlayersintro/), which also goes into more advanced
mapping.

Making a map
You don't really need to know much/any javascript to use openlayers in its basic form, but you do need
to respect the syntax and the order in which the components are declared and used.
This works in the following way:
1. The OpenLayers library is loaded using the script tag.
2. There is a positioning element (<div>) in the body of the page, with the id “map” that contains

the map.
3. A map is created using JavaScript.
4. A layer is created using JavaScript, and then added to the map.
5. The map zooms out as far as possible.
6. The JavaScript function gets called only after the body tag’s onLoad event.

For example:
<html>
<head>
<script src="http://www.openlayers.org/api/OpenLayers.js"></script>
<script type="text/javascript">
var map;
function init() {
map = new OpenLayers.Map('map');
mylayer = new OpenLayers.Layer.MapServer( "World Map",
"http://localhost/cgibin/mapserv.exe",
{map: 'C:/world_mapfile.map'});
map.addLayer(mylayer);
map.zoomToMaxExtent();
}
</script>
</head>
<body onload="init()">
<div id="map" style="width: 600px; height: 300px"></div>
</body>
</html>

Step 1 (red): Getting the OpenLayers Library
<script src="http://www.openlayers.org/api/OpenLayers.js"></script>
The URL points to the location of the JavaScript file that loads OpenLayers.

Step 2 (orange): Positioning the map in the web page
<div id="map" style="width: 600px; height: 300px"></div>
The <div> tag is a container with the id “map” for the map that we are creating in our page. Later,
when we initialize the JavaScript map object, we will give it this id.
The size of the map is determined by the size of the element that contains it. Here, we have set the size
of the map to be 600 pixels by 300 pixels.

Step 3 (yellow): The Map Object
map = new OpenLayers.Map('map');
There are lots of "maps" in this statement! This line creates a new OpenLayers.Map object with the to
go in the div 'map' and makes it a variable called map so it can be referred to elsewhere in the code.
The OpenLayers.Map object, tells the openlayers javascript that we are using the Map class. This allows
us to access a whole library of prebuilt code without needing to write the code ourselves.

Step 4 (blue): Creating a Layer
mylayer = new OpenLayers.Layer.MapServer( "World Map",
"http://localhost/cgibin/mapserv.exe", {map: 'C:/world_mapfile.map'} );
map.addLayer(mylayer);
Like a Mapserver map file, OpenLayers organizes a single map into several layers or data sources. In
this case, we are adding a new object of the Mapserver layer class, telling it the location of the
mapserver executable, the location of the map file, and the layer we want from it (“World Map”). We
are then assigning it the variable name mylayer, which is then used to load the layer in the map.
Different layer classes need different parameters to be given. Examples of many different layer types
can be seen on the openlayers web site (http://openlayers.org/dev/examples/)

Step 5 (purple): Positioning the Map
map.zoomToMaxExtent();
This code tells the map to zoom out to its maximum extent, which by default is the entire world. It is
possible to set a different maximum extent.
OpenLayers maps need to be told what part of the world they should display before anything will
appear on the page. Calling map.zoomToMaxExtent() is one way to do this. Alternative ways that offer
more precision include using zoomToExtent() and setCenter().

Step 6 (green): Loading the Map
<body onload="init()">
In order to ensure that your document is ready, it is important to make sure that the entire page has been
loaded before the script creating the map is executed.
We ensure this by putting the mapcreating code into a function, and then calling it only when the
<body> element is fully loaded.
Task: Copy the openlayers.html file in your web directory to openlayers_ms.html and adapt to
use your mapserver file then display the results in the web browser.

Base Mapping
Displaying your data in a web map is one thing, but it looks much better when overlain on some base
mapping or satellite data. Openlayers can access base mapping from a variety of different mapping and
imagery providers, such as Google, Yahoo, Microsoft, OpenStreetmap and so on.
Overlaying data onto base mapping brings an additional layer of complexity into the mix as the base
mapping and your data may not necessarily be in the same coordinate system. In fact it is quite unlikely
that they will be, as your data is likely to be smaller in scale and the base mapping is likely to be world
wide.
Commercial base layers tend to be in Spherical Mercator coordinates, which has the EPSG code 900913
(it looks like GOOGLE as an aide memoire). Your data can be projected either serverside by the map
server or clientside by the openlayers code.

However, for simplicity, we will choose a base layer that does not require reprojection in this case.
Task: Edit the base layers example in your web directory to use your layer and examine the other
components of the file to see how additional layers and map controls are included.

Beyond Openlayers
There are a number of alternative web mapping options than what have been demonstrated here. They
all have strengths and weaknesses and different approaches.
MapGuide Open Source (http://mapguide.osgeo.org) provides a prebuilt web server and web mapping
application with a full Graphical User Interface for building your maps (in php or .net) without needing
to know any code.
Mapfish (http://mapfish.org/) builds on openlayers but adds additional libraries to allow you to build
more sophisticated applications.
Mapchat (http://mapchat.ca/) uses mapserver and kamap, an alternative to openlayers and adds the
ability to do realtime chat, like a geospatial instant messenger.
Featureserver (http://featureserver.org/) is another cgi executable that works with openlayers to provide
full read and write access to your data sources. In other words you can allow webbased editing of your
postgresql data, such as adding and deleting features or changing attribute text.

This time it really is final
That's all folks!

Lancaster University GIS Course 2010

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