[GIS] Creating an overview polygon from a Atlas Print Layout map extent in canvas

atlasgeometry-generatorprint-composerqgisqgis-print-layouts

How do you create multiple polygons from a QGIS Print Composer or Print Layout map extent (i.e. a polygon that are the exact area shown in a map within a print composer/layout)?

Ultimately I would like to do this for an entire atlas so I would like to do this in as automatic a way as possible, but I would even just be content with a rather manual solution too!

Best Answer

A manual solution would be:

In print composer item properties > extents, find the xmin, xmax, ymin and ymax values of the map item.

Use the Geometry by expression tool to create a rectangular polygon. Substitute the actual values into this expression:

make_polygon( geom_from_wkt( 
  'Polygon(( xmax ymax, xmax ymin, xmin ymin, xmin ymax, xmax ymax ))' 
))

Related Solutions

[GIS] Save Print/Map QGIS composer view as PNG/PDF using Python (without changing anything in visible layout)

I recommend using the following script by Tim Sutton as a base, it worked well for me:- http://kartoza.com/how-to-create-a-qgis-pdf-report-with-a-few-lines-of-python/

Just pass an empty dict to the composition.loadFromTemplate() function instead of the 'substitution map' he uses, since you're not interested in that functionality.

Also, since you asked about "export as image" rather than "export as PDF", you have to do a bit more work than use the exportAsPDF() member function.

Below is a modified version of Tim's code that should do the trick, working from an external Python script. Set the DPI, and project+composer file variables as required.

(If you are using the Python console within QGIS rather than doing this as an external script, you can get the current map canvas using the qgis.iface and don't need to do use all the project loading etc code).

import sys
from qgis.core import (
    QgsProject, QgsComposition, QgsApplication, QgsProviderRegistry)
from qgis.gui import QgsMapCanvas, QgsLayerTreeMapCanvasBridge
from PyQt4.QtCore import QFileInfo
from PyQt4.QtXml import QDomDocument

gui_flag = True
app = QgsApplication(sys.argv, gui_flag)

# Make sure QGIS_PREFIX_PATH is set in your env if needed!
app.initQgis()

# Probably you want to tweak this
project_path = 'project.qgs'

# and this
template_path = 'template.qpt'

# Set output DPI
dpi = 300

canvas = QgsMapCanvas()
# Load our project
QgsProject.instance().read(QFileInfo(project_path))
bridge = QgsLayerTreeMapCanvasBridge(
    QgsProject.instance().layerTreeRoot(), canvas)
bridge.setCanvasLayers()

template_file = file(template_path)
template_content = template_file.read()
template_file.close()
document = QDomDocument()
document.setContent(template_content)
ms = canvas.mapSettings())
composition = QgsComposition(ms)
composition.loadFromTemplate(document, {})
# You must set the id in the template
map_item = composition.getComposerItemById('map')
map_item.setMapCanvas(canvas)
map_item.zoomToExtent(canvas.extent())
# You must set the id in the template
legend_item = composition.getComposerItemById('legend')
legend_item.updateLegend()
composition.refreshItems()

dpmm = dpi / 25.4
width = int(dpmm * composition.paperWidth())
height = int(dpmm * composition.paperHeight())

# create output image and initialize it
image = QImage(QSize(width, height), QImage.Format_ARGB32)
image.setDotsPerMeterX(dpmm * 1000)
image.setDotsPerMeterY(dpmm * 1000)
image.fill(0)

# render the composition
imagePainter = QPainter(image)
composition.renderPage(imagePainter, 0)
imagePainter.end()

image.save("out.png", "png")

QgsProject.instance().clear()
QgsApplication.exitQgis()

[GIS] Capturing coordinates of map extents in QGIS print composer atlas

Answering my own question here. The following steps in QGIS 2.10.1 Pisa on Windows 10 helped me answer the question:

Let's say your polygons are in a vector layer called "foo.shp" and you've loaded them into your QGIS project. We're trying to find out the "extents" of each polygon, which are the coordinates of the corners of a squares/rectangles exactly big enough to fit each polygon in.
To obtain these extents for each polygon in foo.shp select "Polygon from Layer Extent" from the menu called Vector -> Research Tools. In the resulting dialogue, choose foo.shp as the input layer and check the boxes labelled "Calculate Extent for each feature separately" and (if you want to see the outcome) "Add result to canvas".
Then choose an output layer: you'll need to create a shapefile in /path-to-my/project/ to save the output in. Let's call it "bar.shp".
Once you've done this, select OK to run the operation. You will then have a new vector layer called "bar" which displays the data on the extents of the your original polygons.
To see this data for "bar" open the layer's Attribute Table. The full coordinates of the polygons can be exported from there.

Best Answer

Related Solutions

[GIS] Save Print/Map QGIS composer view as PNG/PDF using Python (without changing anything in visible layout)

[GIS] Capturing coordinates of map extents in QGIS print composer atlas

Related Question