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bharathaithal when you have just a simple aerial image (ortophoto) with only 3 bands in the visible spectrum (red, green, blue) you have limited possibilities to obtain a detailed and accurete classification. For example, if you don't have at least 1 NIR bad, you can't make a good distinction beetwen a river and a forest, using the traditional pixel-based image classification. It will be a major problem with the spectral mixture, most of the classes will have very similar specral responses, so the best way is to use an Object-based image classification method > eCognition software. Then, using eCognition, you can classify the river using not only it's spectral caracteristics, but also other caracteristics (lenght, widith, area, etc) versus the vegetation, at different levels of image segmentation. So, in conclusion, when you have only a simple aerial photo (3 visible bands) and you want to obtain a detailed classification, you ca't do it without auxiliar information (cadastral maps, LIDAR data, aditional infrared bands), even when you use an Object-based image classification. The traditional pixel-based image classification is excluded from the start for this job ! Good luck!

ransara you can learn more about basic Remote Sensing just by a simple "search" in this forum, and you will find here a lot of books and information. Also, you can search on Google. You have an aerial foto (orthophoto) of a city and you whant to make a land use classification ? How big is the image ? Even with the most powerfull image classification software that you have ( eCognition ! ) you will still have some problems making an automated clasification of the aerial photo. But you can use eCognition segmentation tool to create a layer of shapefile polyons of the contours of the buildings, roads, water bodies, etc on your aerial image and then do a manual calssification (in ArcGIS or in eCognition), if your image is not too big and you have a lot of time to work on it. Good luck, my friend !