.. Author: gislite .. Title: Display layers by attribute category

Display layers by attribute category
====================================

In the previous example, the layers consisted of a set of features from
the same dataset, rendered using the same style. However, you may not
want to plot all features contained in a given dataset, or you may not
want to plot all features in the same way. For example, making a world
map containing all the roads for each dataset would take quite a while
and the resulting map would be too cluttered to be very useful. The same
symbols are used to render maps produced by country roads and highways,
and there is no distinction between graphical functions; in the real
world, the difference is obvious.

View the map
------------

Classes allow you to distinguish between features based on attributes
and features based on classes. The previous diagram uses classes because
each layer must have at least one class. At its simplest, the default
class includes every feature of the dataset. In this case, the layer has
a class and each feature in that class is rendered in the same way.

However, you usually want to avoid using only the default classes and
choose classification features instead, for the two reasons mentioned
earlier: First of all, you may not want to draw each feature, and
second, you may want to use different symbols, colors or sizes to
present some features with different attributes.

By using non-spatial attribute information in the data, you can create a
map like this:

.. image:: http://webgis.cn/cgi-bin/mapserv?map=/owg/mfk3.map&layer=country&layer=weq&mode=map

or:

.. image:: http://webgis.cn/cgi-bin/mapserv?map=/owg/mfa3.map&layer=states_poly&layer=states_line&mode=map


Map file
--------

Here is the map file MapFile ( ``mfa3.map`` ):


.. literalinclude:: ./mfa3.map
   :lineno-start: 1


Mapfile for layer classification

The structure of the map file, through the object, looks like this

.. raw:: html

   <pre>
                                     MAP
         (states_poly) LAYER----------|---------LAYER (states_line)
                         |                      |
          (land) CLASS---|---CLASS (water)      |-CLASS
                     |       |                    |
               STYLE-|       |-STYLE              |-STYLE
   </pre>


The changes in the documents are as follows:


`Open the file:diff_mfa3_mfa2.html <./diff_mfa3_mfa2.html>`_

.. aa literalinclude:: ./xx_diff_mfa3_mfa2.htmp



Attribute description in Mapfile
--------------------------------

MapFile still has only two layers, but polygon layers are subdivided
into two broad categories. Take a look at the additional parameters:

::

   CLASSITEM "NAME"

This keyword is used to specify which properties are used by the
detached class object. In this example, the property is ``NAME`` . If
you open the Shapefile of the database file associated with the
attribute, you will see that there is a column (property) called
``NAME`` .

The easiest (and fastest) way to define a class using an expression to
determine which MapServer the class contains is to use a string
comparison. You can use the hierarchy keyword ``CLASSITEM`` to identify
the attribute name that will be used for the classification function.
Then specify that the comparison string is expressed using class-level
keywords. It is good practice to quote strings to ensure that characters
are interpreted correctly. This is shown in the code snippet below. The
``CLASSITEM`` attribute expresses the value of the string that will be
associated with each feature of the dataset. If the expression string
matches the same ``CLASSITEM`` value, the function will be included in
this class. Although fast and easy to use, this method is not very
flexible, since the specified string expression must be passed through
``CLASSITEM`` to determine the matching attribute value.

::

   12         CLASSITEM "NAME"
   13         CLASS
   14             NAME "China"
   15             EXPRESSION "CHINA"
   16             STYLE
   17                 COLOR 232 30 30
   18             END
   19         END
   20         CLASS
   21             NAME "Others"
   22             STYLE
   23                 COLOR 198 198 255
   24             END
   25         END

Shapefile database records are stored in DBF files. You can open a
spreadsheet program like Openoffice.org Compute, or a GIS software such
as QGIS on the desktop. If the data has metadata (as it should), it can
be known from the content of the metadata file. You can also use
``ogrinfo`` to display basic property information in a Shapefile - look
back at Example 1.1 (after the last few lines, “LayerSRS WKT:” displays
property names and types).

::

   EXPRESSION "CHINA"

For each class, specify the property value to use. This is the simplest
form of expression. Expressions can be more complex than this, allowing
a regular comparison expression or logical expression.

Classification based on comparative expressions
-----------------------------------------------

Allows for the classification of more complex functions, based on a
logical expression of one or more attribute values. ``CLASSITEM`` does
not need to be specified (actually, it will be ignored if so far).
Keyword expressions introduce logical expressions, which are delimited
within parentheses. The syntax is simple: a logical expression consists
of the property name enclosed in square brackets, a comparison operator
and the value. For example, the following code compares the number of
values attributed to the population with the value 100000:

::

   EXPRESSION ( [POPULATION] < 100000 )

It will include a feature with less than 100,000 population attribute
values. Like C and Perl, MapServer uses different comparisons for
strings as well as comparisons for numbers, and you have to take care of
observing the differences. If an attribute is a string value, then its
reference must be enclosed in quotes and the values must be compared.
Both single and double quotes can be used, but they must match. Consider
the following code:

::

   EXPRESSION ( '[STATE_FIPS]' eq 'MN' )

This will include a feature only if this value attribute ``state_FIPS`` is equal to the string Mn. Logical expressions can be combined
using conjunctions and separation operators and or. Consider the
following example:

::

   EXPRESSION (( [POPULATION] < 100000 ) and ( '[STATE_FIPS]' eq 'MN' ))

This matches features with populations less than 100000 ``STATE_FIPS``
equal to MN.

Note that confusion may occur if a string-valued property contains
numeric strings (eg ``"123"`` ). If you compare strings-valued
properties numerically, there will never be a match ( ``123`` will never
be equal to ``"123"`` ), and there will never be an error. You can learn
more about data types.

Note that MapServer’s mapping file reference documentation has a
misspelled numeric “not equals” operator, and ``!=`` does not display an
exclamation mark ( ``!`` ).

-  Operator, Data Type
-  ``!=``, Numeric
-  ``=``, Numeric
-  ``>``, Numeric
-  ``<``, Numeric
-  ``>=``, Numeric
-  ``<=``, Numeric
-  ``and``, Logical
-  ``or``, Logical
-  ``eq``, String
-  ``ge``, String
-  ``gt``, String
-  ``le``, String
-  ``lt``, String
-  ``ne``, String

Note that although a class must be defined with a single method, each
class can use a different method at a level.