[GIS] Colouring contour lines in shapefile when lines are not meeting in QGIS

bathymetrycontourpolygonizeqgis

I am doing bathymetry analysis, where I would like to color the contours one by one in the case land is rising (or sea level going down). There is a comprehensive explanation about coloring contours (Colouring areas between (vector) contours?) but it is not working since my contours never meet.
I didn't success to change the .shp file with one contour to polygon correctly: I wanted the "land side" of the contour to become polygon, not other way round.

I have several .shp files of contours about the sea bottom topography, but the proper visualization is still missing.

I am using QGis 2.2.

Best Answer

One fast way for doing visual analysis is to use DEM as starting point and classify it with pseudocolors. You can edit the class ranges and colors etc. to suit your needs.

Styling DEM as pseudocolor

For further analysis you can use the raster calculator. The next screen capture shows how to make a black/white image where pixels with height<10 m are white and pixels above 10 m black. You can continue with gdal_polygonize tool http://www.gdal.org/gdal_polygonize.html and convert this B/W image into polygon shapefile.

enter image description here

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[GIS] Polygons between contour lines in QGIS

isMultiPart() is a method of class QgsFeature. The error 'NoneType' object has no attribute 'isMultipart' means that at least one feature has geometry() of None. It might be a degenerated contour line with length 0, or some other error concerning geometry.

If you have problems using a provided tool, then first check geometry, eg delete polyline having length equal 0, polygon having area equal 0, or check their topology. This problems may or may not raise errors in tools, depending on which error handling the developer has taken into consideration.

In case you have written python code that raises this error, test the QgsGeometry object before using its methods or process the underlying geometry store:

geom = geometry()
if geom:
    if not geom.isEmpty():
        # do something useful

Or more in-depth checking:

geom = geometry()
if geom:
    err = geom.validateGeometry()
    if not err:
        # do something useful
    else:
        print '%d geometry errors detected' % len(err)

qgis – Finding Negative Contours When Creating Contour Lines from DEM

It sounds like you have the original surface model so to identify the depressions use a sink algorithm. Here is a link to filling sinks using QGIS.

I have not used this tool in QGIS, only something similar in ArcGIS so I am not sure of the output. In any case, you could subtract the sink output surface from the original surface. Any greater than zero pixel would be a depression.

Best Answer

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[GIS] Polygons between contour lines in QGIS

qgis – Finding Negative Contours When Creating Contour Lines from DEM

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