Need to stick to some scales in QGIS ?
My colleague, Michaël, would like to be able in QGIS to force the canvas scales to some predefined ones. Since we now have project scales (and QGIS pre-defined scales) in the scale selector, we could use them, and add a simple checkbox near the combobox Stick to predefined scales.
The aim is to force the canvas to render only at these scales. For example, if a QGIS user uses the rectangle zoom tool, and he should go to 1/56003, he would instead land on 1/50000, the closest pre-defined scale.
He made a little python script as a proof of concept to illustrate it:
# -*- coding: utf-8 -*- # Import the PyQt and QGIS libraries from PyQt4.QtCore import * from PyQt4.QtGui import * from qgis.core import * class forcedScale(): # pre-defined scales # has to be read from project properties predefinedScales = [ 5000, 10000, 25000, 50000, 100000, 250000, 500000 ] # mapCanvas mc = None def init( self ): ''' Initialize class instance ''' mc = iface.mapCanvas() self.mc = mc self.mc.scaleChanged.connect(self.setScale) def setScale( self, scale ): targetScale = min( self.predefinedScales, key=lambda x:abs(x-scale) ) self.mc.zoomScale( targetScale )
In this script, he hard coded the scales, but obviously we should get them from the project properties.
Run this script in the QGIS Python console and initialize with
fs = forcedScale() to stick to scales.
A discussion has been opened in QGIS-Dev mailing list : http://osgeo-org.1560.x6.nabble.com/Force-canvas-to-predefined-scales-help-needed-to-improve-a-script-td5201783.html