Changing certain attributes and creating a new field for combined data

Question

I have this combined dataset with building heights divided up over several columns ''Height'' ''Building_h'' and ''Building_L. My end goal is to combine them all in one new column. There are however, several problems:

1. All columns are strings. I am aware of the to_real() function, but it doesn't work as some of the values have 'm' behind it to indicate that they are in meters. How can I change them?

2. I already used regexp_replace( "height" , 'm.*', ' ') in the field calculator to get rid of the 'm' behind meters and got those into a new field called 'without M', but now only the cases that had m are in this field. They are also still in the field they were originally in, so I still cannot apply the to_real().

3. I want to use with ''WithoutM'' field to add all the other values to as well.

How do I do this?

4. Then there is ''Building_l'' which has the number of levels per building. And so, this is a different way of expressing building height than ''height'' and ''building_h'' do. If I say, for example that an average building level is 3 meters; is there a way I can multiply the value for ''building_l'' by 3 and also put this into the ''withoutM'' field?

5. Last problem is related to the fact that several buildings have a value in more than one column. For example in ''height'' and ''building_l'', how should I handle this?

Answer 1

You can use field calculator.

• Open field calculator.
• Select update existing field, select your target field ("withoutM").
• Go to Function editor, select new file and name your new function.
• On the right side type in this code:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')

def my_copy(value1, value2, feature, parent):
  value2 = value2.replace("m", "")
  num_value2 = float(value2)
  if num_value1 != NULL:
      pass
  else:
      return float(num_value2)

For field with number of levels change else statement to:

  else:
      return float(num_value2) * 3

• Click load and go back on Expresion tab.
• Type in: my_copy("your_destination_field", "your_source_field") In your case you would replace "your_destination_field" with "withoutM".

This code will check if your destination field is empty, in that case it will copy value from source field.

• Run this for each of your source fields except where you have number of levels. For that you just change last line of code to :

  return value2 * 3

..for 3 m high floors..

Maybe not the smoothest solution but I think it should work.