QGIS Plugins planet

February 24, 2011 Nathans QGIS and GIS blog

One of the cool things I love about QGIS is finding stuff that you didn’t know it could do, well not just itself but plugins that you didn’t know about.

Today my discovery was in how to generate contour lines from a point layer.

First install the contour plugin for qgis via the plugin installer. Just search for “contour”
Once installed open a vector point layer in QGIS. Make sure the point layer has a field that you can use for elevation.
One I prepared earlier
Then select from the menu: Plugins->Contour->Contour
Fill in the information
Details form (The above setting will generate 0.5m contours)
Press OK
Results
Results from plugin
Profit??

The resulting contours will have a field that contains the label and z value for each contour line, you can then just label or color them how you wish.

Note: There is a bug with QGIS memory layers where the fields don’t show up in dropdown or attribute browsers, a simple fix is just to make the layer editable and then non editable then the fields will be there.

The contour layer is a QGIS memory layer so remember to save it to disk eg a shapefile before you close you will loose your new fancy contour layer.

Happy mapping :)

Filed under: MapInfo, Open Source, qgis Tagged: gis, mapping, Open Source, python, qgis, shapely

by Nathan at 7:00 AM under gis , mapinfo , mapping , open source , python , qgis , shapely (Comments)

Getting total length of selected lines in QGIS via python

February 23, 2011 Nathans QGIS and GIS blog

The other day I was trying to get the total length of the some selected lines in QGIS. In MapInfo I would just do

Select Sum(ObjectLen(obj,”m”)) from Selection

however QGIS doesn’t have the ability (yet..) to run SQL queries like this, but you can do it via python.

The following is a little python snippet that will get the total length of the selected objects. (You will need to have shapely installed to use)

from shapely.wkb import loads
def getLength():
    layer = qgis.utils.iface.mapCanvas().currentLayer()
    total = 0
    for feature in layer.selectedFeatures():
        geom = feature.geometry()
        wkb = geom.asWkb()
        line = loads(wkb)
        total = total + line.length
    return total

print getLength()

EDIT:Or as Peter asked in the comments, yes you can just call length on the geometry:

def getLength():
    layer = qgis.utils.iface.mapCanvas().currentLayer()
    total = 0
    for feature in layer.selectedFeatures():
        geom = feature.geometry()
        total = total + geom.length()
    return total

print getLength()

Just copy and past that into your QGIS python console and call getLength() when ever you need the total length.

Note: I have found the QgsGeometry .legnth() function to be unstable in the past and it has crashed my QGIS instance. Just test it first, if not you can always use the shapely method.