We also used ERDAS Imagine to classify pixels. In ERDAS, the classification process was more tedious but generally worked the same. In the Raster tab, under the Classification group, click the Unsupervised button, then select Unsupervised Classification.
I performed an Unsupervised Classification using an aerial of the UWF Campus. I made five classes, named them and designated colors for each class:
1. Grass – green
2. Trees – dark green
3. Buildings/Road – grey
4. Shadows – black
5. Mixed - orange
The process was long but I am happy with the results. Additionally, I created another column or field for "Area" and then I totaled the area as "Permeable" or "Impermeable" and listed the percentages of each on my final map.