GIS Exercise - spring 2011

Transcription

1 GIS Exercise - spring 2011 Maria Antonia Brovelli Laura Carcano, Sara Lucca, Marco Minghini, Luana Valentini Introduction to a MapServer-based webgis I. Introduction of webgis exercise and an overview A simple MapServer application consists of: Map File - a structured text configuration file for your MapServer application. It defines the area of your map, tells the MapServer program where your data is and where to output images. It also defines your map layers, including their data sources, projections and symbologies. It must have a.map extension or MapServer will not recognize it. The Mapfile is the heart of MapServer. It defines the relationships between objects, points MapServer to where data are located and defines how things are to be drawn. Geographic Data - MapServer can utilize many geographic data source types. The default format is the ESRI shapefile. Many other data formats can be supported; this is discussed further below in Adding data to your webgis. HTML Pages - the interface between the user and MapServer. They normally sit in Web root. In its simplest form, MapServer can be called to place a static map image on an html page. To make the map interactive, the image is placed in an html form on a page. In this first practice lesson on MapServer, you will deal with a CGI (Common Gateway Interface) application. CGI is a specific server-side protocol that transfers GIS Exercise Spring

2 the web client requests to the web server, and the web server answers back to the client. This is the so-called static web-mapping because every request implies the creation of a new process: the consequence is a progressive occupation of the server RAM which results in poor performances. Furthermore the interactivity of the web client is considerably limited, since every operation (even a simple zoom in or zoom out) must pass through the entire process. In other words CGI programs are stateless : every request they get is new and they don t remember anything about the last time that they were hit by your application. For this reason, every time your application sends a request to MapServer, it needs to pass context information (what layers are on, where you are on the map, application mode, etc.) in hidden form variables or URL variables. In your webgis project you will instead learn to build an interactive web-mapping application, that overcomes the problems of CGI by adding interactive client-side viewers and/or server-side CGI extensions. In particular you are going to build an user-friendly webgis with p.mapper framework, which allows to create nicer and more interactive interfaces. Anyway, this is just to show you an example of what can be done with the data you have used in the first ArcGIS practice lecture. The basic elements that you will be required to insert in you webgis project are: - a map - a layer list - a reference map - a legend - a scalebar The required basic functionalities are: - pan - zoom in - zoom out GIS Exercise Spring

3 - query - current scale - current map extents Additionally, p.mapper framework will allow you to insert more advanced functionalities, such as the possibility of using a zoom box, a continuously updated cursor position, a clickable reference map, and others that you will see during the p.mapper-related exercise lessons. II. Example: file structure of a MapServer-based webgis An example of a CGI-based webgis can be found at our server: bin/mapserv11?map=/u/www_webgis/giscourse11/completo/ethex.map&zoomsize=2&program=/cgi- bin/mapserv11&web_imagepath=/usr/local/mapserver_tmp/&web_imageurl=/tmp/&utente=completo Data When starting the design of a webgis, it is important to know in advance about the data that are going to be used. In this way it is possible to decide which webgis system is more suited for the project and to check if the data need some pre-processing. In this exercise we will first take a look to the data that will be used for our webgis project. You may load them in a desktop GIS (for instance, ArcGIS) to check what they represent, in which reference system they are, which are their spatial extents, their attributes, their formats, and all the other information that will be useful for the webgis exercise. All the needed data are on our server: under the directory utentexx where xx is the number of your working group (e.g. utente01, utente02, etc ). If not otherwise specified, the coordinate and reference system (CRS) for all the data is Gauss-Boaga Roma40 (an italian CRS). GIS Exercise Spring

4 As you can see, data of this typical MapServer-based webgis are stored in a single directory, with a subdirectory tree designed to keep its files in order. These files are: 1 - Those needed to build the web interface (HTML and CSS files, icons, fonts, and so on) 2 - Geographic data (in data folder) 3 - MapServer-related configuration files (mapfile, symbols and font set files) The data folder contains: ESRI shapefile format ( shpf folder): municipalities.shp GIS Exercise Spring

5 lakes.shp railways.shp GaussBoaga\stations.shp stations.shp (CRS: UTM WGS84) [Note: Each shapefile has other files associated to it with the same name and different extensions (.shx /.sbx /.sbn /.dbf).] TIFF raster format ( raster folder): nw.tif ne.tif sw.tif se.tif [Note: Each TIFF file has a world (.wld) file associated that contains information about how to georeference it.] MapInfo format ( ogr folder): alluvional-cones.tab [Note: Each MapInfo file has other files associated to it with the same name and different extensions (.dat /.id /.map).] EPSG file ( proj folder): epsg [Note: This file contains information about various CRSs; even though it does not have an extension, it is a normal text file and it can be opened with any text editor.] On the other side: HTML files: ethex.html : MapServer (CGI) template file query.html : template file for displaying query results on the municipalities layer query_f.html : template file for displaying query results on the stations layer header.html : header of the municipalities query results page header_f.html : header of the stations query results page GIS Exercise Spring

6 footer.html : footer of any query results page attr_mun.html : page used to display municipalities attributes attr_stat.html : page used to display stations attributes [Note: for the moment we don t enter into details about the template files, also because you are going to develop a different (p.mapper-based) solution. The HTML files listed above are useful only as examples of MapServer CGI-based solution. We use them simply as tools to visualize the changes made to the mapfile that you are going to create during this exercise lesson] While inside the graphics folder you have: CSS file ( css folder): styles.css: cascading stylesheet containing styles for the HTML pages GIS Exercise Spring

7 Fonts ( fonts folder): The fontset.txt file will be used by MapServer to associate each font to a keyword (or alias ). For example, if you choose the alias times to refer to the Times New Roman font, whenever you will need to tell MapServer to use that particular font it will be sufficient to use the keyword times. [Note: Aliases are case-sensitive] Icons ( icons folder): GIS Exercise Spring

8 Symbols ( symbols folder): Pay attention to the symbols file (\graphics\symbols\symbols.sym). This file is going to contain the definition of the symbols used in the webgis to represent some vector features. These definitions could also be included directly in the mapfile (when you define the style for a layer, you can also define a symbol for it), but it is usually preferable to use a standalone file so that it can be copied and reused for other webgis projects. Moreover, using a separate file will result in a better structure for our webgis. GIS Exercise Spring

9 LICENSE This document is realised under the following license: Creative Commons, Attribution Noncommercial - Share Alike, 3.0 Creative Commons, Attribuzione - Non commerciale - Condividi allo stesso modo, 3.0 More information Use conditions Legal Code (the full license) GIS Exercise Spring