[Qgis-user] creating vector layer from slope map with few classes

[Shinoto, PD Dr. Maria]

Hi Richard, 

this was great, a big thanks to you as well. Classifying was the first problem, and thanks to your step by step explanation, it worked well, easy, quick, producing exactly the results I wished for. 

Cheers to you and the community, 
Maria


> Am 30.03.2022 um 20:36 schrieb Richard McDonnell <richard.mcdonnell at opw.ie>:
> 
> Hi Maria,
> There is an option to reclassify a raster called Reclassify by Table, which should create the classified raster you are looking for.
> 
> You can find the Reclassify by Table in the Processing Toolbox, you can then do the following.
> 1.      Select the Raster you want
> 2.      Under Reclassification table click on the button with the three dots.
> a.      You need to add a row for each class you want, by clicking Add Row
> b.      For each row set the Minimum and Maximum pixel Values and the new Value
> c.      Click OK once you are happy
> 3.      Set your output no data value, in your case it can be zero
> 4.      Set Range boundaries as you require
> 5.      Set your Output data type, I would, based on your description select Byte
> 6.      Set location to which you want to save your Raster.
> 
> To polygonise the data then is just a simple matter of using the QGIS Polygonize tool, which again can be accessed from the Processing Toolbar. Its default settings should create the output you are looking for.
> Hope the above goes some way to helping you.
> 
> Regards,
> 
> Richard
> 
> 
> 
> ——
> Richard McDonnell MSc GIS, FME Certified Professional
> FRM Data Management
> 
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>> -----Original Message-----
>> From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Shinoto, PD
>> Dr. Maria via Qgis-user
>> Sent: 29 March 2022 22:25
>> To: qgis forum <qgis-user at lists.osgeo.org>
>> Subject: [Qgis-user] creating vector layer from slope map with few classes
>> 
>> Hi,
>> 
>> I have created a slope map from a DTM.
>> 
>> I have limited the symbology to three meaningful classes and wish to create a
>> vector layer with polygons of three classes from that. The conditions are One-
>> Channel-Pseudocolor, Channel1 (Gray), interpolation discrete, modus
>> "fortlaufend" -- not identical interval or quantile, 3 classes with values <=20, 20 -
>> 40, > 40.
>> 
>> The classes show up in the layer design panel symbology tab, but the layer
>> browser still (thankfully!) shows the greyscale ramp.
>> 
>> I wish to create a vector layer with polygons from the 3 classes.
>> 
>> All solutions I have looked up work under slightly different condition. After all I
>> do not know how to get the data of the DTM into three classes like "Flat-Gentle-
>> Steep"; I think the vectorization should work easily then.
>> 
>> Can anybody hint me to a solution?
>> 
>> 
>> Thanks and best wishes,
>> Maria
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