I want to carry out LULC change detection of landsat images. Do I need to convert DN to radiance or reflectance values?
The following article may help:
Klemas,V. (2011) Remote sensing of wetlands: Case studies comparing practical techniques. Journal of Coastal Research, 27, 418-427.
You should guarantee that your orthorectification and the atmospheric correction are corect and using same method.
The approach outlined in Li et al http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5422912&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5422912
Details the process for converting from DN to atmospherically corrected, view angle corrected BRDF corrected surface reflectance. Such an approach ensures that any changes detected are due to on-ground change rather than an artefact of the sun-sensor geometry/timing/atmospheric conditions at the time of observation.
However it's worth noting that if you're trying to detect change between two very different land cover types i.e. forest vs bare ground this can be achieved using a more basic radiometric correction routine, however, more subtle changes in land cover such as increases or decreases in the amount of vegetation cover necessitate an accurate surface reflectance correction methodology.
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