12 GDAL

lightmare edited this page 2018-07-17 13:09:19 +02:00

Table of Contents

• Installation
• Parameters
• Styling
• Usage
• Python
• XML
• C++
• Further References

Mapnik's PluginArchitecture supports the use of different input formats.

This plugin supports the GDAL library in order to read a lot of spatial geo raster data from multiple formats.

Installation

Make sure that running python scons/scons.py shows the following line

Checking for gdal-config --libs... yes
Checking for gdal-config --cflags... yes

To check if the gdal plugin built and was installed correctly you can do:

    >>> from mapnik import DatasourceCache as c
    >>> 'gdal' in c.plugin_names()
    True

Parameters

parameter	value	description	default
file	string	file of the raster to be read
base	string	base path where to search for file parameter
band	integer	request for a specific raster band index, -1 means all bands. Note that a band read from a single band raster gets interpreted as Grayscale if band=-1 is specified while they retain their original value when explicitly referenced with the "band" parameter. This affects effectiveness of RasterColorizer	-1
nodata	double	allow setting nodata value on the fly (will override value if nodata is set in data)
shared	boolean	Whether to open the dataset in shared mode. Not recommend to enable true unless you can guarantee that one on thread accesses a given file at the same time (unlikely given the way that most tiling software works with mapnik). It is only beneficial therefore if you are rendering multiprocess and single threaded and are working in low memory situation. Internally it directs Mapnik to call GDALOpenShared instead of GDALOpen which means that GDAL will consult its global cache of datasets. GDAL will try to return a copy of the dataset if access from multiple threads (but this defeats the purpose of sharing)	false

Styling

To style a layer from GDAL use the RasterSymbolizer

Usage

This plugin in Mapnik >= 0.7.0 supports reading overviews created with gdaladdo. Parallel creation of GDAL datasources from TIFF files in a multi-threaded program requires libgdal >= 2.0.2.

Python

    style=Style()
    rule=Rule()
    rule.symbols.append(RasterSymbolizer())
    style.rules.append(rule)
    map.append_style('Raster Style',style)
    lyr = Layer('GDAL Layer from TIFF file')
    lyr.datasource = Gdal(base='/path/to/your/data',file='raster.tif')
    lyr.styles.append('Raster Style')

XML

    <!-- NOTE: must be in the same SRS as your map-->
    <Layer name="GDAL Layer from TIFF file">
    	<StyleName>raster</StyleName>
    	<Datasource>
    		<Parameter name="type">gdal</Parameter>
    		<Parameter name="file">/path/to/your/data/raster.tiff</Parameter>
    	</Datasource>
    </Layer>

C++

Plugin datasource initialization example code can be found on PluginArchitecture.

A GDAL datasource may be created as follows:

    {
        parameters p;
        p["type"]="gdal";
        p["file"]="/path/to/your/data/raster.tiff";
    
        set_datasource(datasource_cache::instance()->create(p));
    
        layer lyr("GDAL Layer from TIFF file");
        lyr.add_style("raster");
        m.add_layer(lyr);
    }

Further References