[Portugal] Scribe - A tool to facilitate the creation of Maps with MapServer

[termal12]

http://www.mapgears.com/en/blog/archive/2013-03-12-scribe

Sent to you by termal12 via Google Reader: Scribe - A tool to
facilitate the creation of Maps with MapServer via Planet OSGeo by
Charles-Éric Bourget on 3/12/13

Creating a nice looking map that delivers efficiently its message is
not an easy task. Quite often we are dealing with large datasets that
shouldn’t be displayed at all scale levels or that should be displayed,
but, with a style that changes with the scale level. Every single
detail is important when defining the style of a map. The process is
generally an iterative one and requires a lot of modifications to be
made at each scale levels.

With MapServer, MINSCALEDENOM and MAXSCALEDENOM tags in the LAYER and
CLASS objects are responsible for the scale management. That means that
as soon as an element of the style changes for a given LAYER or CLASS
at a given scale level, a new LAYER or CLASS needs to be created. That
way, we end up rewriting a lot of text to modify only a single
parameter. The task becomes more tedious if a given parameter (the
color of a road for example) is defined within 16 different LAYERs or
so.

That’s where Scribe comes to the rescue. It is a converter tool written
in python that we developed to facilitate the creation of ‘mapfiles’ by
allowing for the use of variables as well as shortcuts for managing
scale levels. This approach is similar to that of Basemaps but simpler
to use and generally less verbose.
Scale management
LAYER { 1-16 { NAME: 'land' TYPE: POLYGON @layerconfig DATA {
1-4: '110m_physical/ne_110m_land' 5-10: '50m_physical/ne_50m_land'
11-16: '10m_physical/ne_10m_land' } CLASS { STYLE { COLOR {
1-6: '#EEECDF' 7-16: '#AAA89B' } OUTLINECOLOR: 200 200 200
OUTLINEWIDTH: @land_ol_width } } } }

In the above example, a LAYER named ‘land’ is created. With the ‘1-16’
tag, this LAYER will be displayed from scale level 1 to 16. Scribe
converts automatically those levels in terms of MINSCALEDENOM and
MAXSCALEDENOM. Furthermore, from level 1 to 4, the data (DATA) has a
resolution of 110m. From 5 to 10, it has a resolution of 50m and from
11 to 16 it has a resolution of 10m. The color (COLOR) changes
according to the scale level too. Using this syntax makes it very easy
to modify a given parameter for one or more scale levels without having
to rewrite or copy and paste a load of text and having to make changes
to a bunch of places.
Defining and using variables
Not only Scribe facilitates scale management, it also allows for
defining and using variables. In the above example, the variables
‘layerconfig’ and ‘land_ol_width’ are called with a ‘@’. Those
variables are defined in the following way:

VARIABLES { layerconfig { GROUP: 'default' STATUS: ON PROJECTION
{{ 'init=epsg:4326' }} PROCESSING: 'LABEL_NO_CLIP=ON'
PROCESSING: 'CLOSE_CONNECTION=DEFER' } land_ol_width: 1 }

The variable ‘layerconfig’ contains some parameters that are used in
the definition of most LAYERs. That means for each LAYER, writing
‘@layerconfig’ outputs all the parameters contained in the variable and
spares the writing of a few lines of text. The next variable
‘land_ol_width’ contains a single value.

Note that in the definition of the ‘layerconfig’ variable, the
PROJECTION tag is followed with two ‘{‘. This syntax is necessary for
tags like PROJECTION, METADATA AND PATTERN which contain no parameter,
only plain text.
Comment blocks
LAYER { 1-16 { NAME: 'land' TYPE: POLYGON @layerconfig DATA {
1-4: '110m_physical/ne_110m_land' 5-10: '50m_physical/ne_50m_land'
11-16: '10m_physical/ne_10m_land' } CLASS { STYLE { COLOR {
1-6: '#EEECDF' 7-16: '#AAA89B' } ##Comments preceded with ##
appear ##in the resulting mapfile. ##Comment blocks between /* */ ## do
not appear in the resulting mapfile. /* OUTLINECOLOR: 200 200 200
OUTLINEWIDTH: @land_ol_width */ } } } }
Run the script
Finally, to run the script, a single command is required, configurable
with a few options:

python scribe.py

The result is a perfectly indented mapfile that includes scale
management and, more than often, way more lines of text than what had
to be written. Therefore, Scribe saves time and makes creating quality
maps easy and user friendly.
Follow Scribe on GitHub
Scribe is a work in progress. New functionnalities will be added in the
future. We invite you to follow its progress and start using it by
visiting its GitHub repository. If you require any further information,
please feel free to contact us.

NOTE: This article was originally posted in French on Simon Mercier’s
blog

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