Friday, 10 October 2014

QGIS tutorial: Display with color symbology from data defined properties of shape files

How many of you have this problem: you classify your images in ecognition or any other software and exported classified image as shape file. Now you want to view the shape file in a GIS environment and with the same color for each class that was used in eCognition. Manually matching color RGB values from eCognition to shape colors for each class is a tedious task, specially when  you have many classes.

This problem can be solved with a little trick.

  • Do your classification in eCognition. Assign class colors as you wish.

  • Export classification result as shape file with Class attributes.

  • Export classification result as thematic raster. By doing so, you will get a CSV file in which you can get RGB color codes for each class.

  • Load your shape file in GIS (I use QGIS which is free. I am a poor guy therefore I always love free stuffs)

  • Load the CSV file in QGIS

  • Perform attribute Join of shape file and CSV file with key attribute join based on Class. With this you will get RGB values attached in attribute table of shape file.

  • Double click color symbology of shape file and use data defined properties of  color attributes  of shape file.

  • Build an expression for color selecting respective color attribute column from shape file.
import CSV file into QGIS

Attribute of shape file after performing join
Data defined properties
data defined properties for color

data defined properties for color
Normal display without color symbology

Display with color symbology

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