Determination of Suitable Pixel Size for Extracting Physiographic Parameter on Iran Maps with Scale of 1:25000

Because of cellular nature of Digital Elevation Model (DEM), it is very important in most of hydrology engineering models. In this model, pixel size is basic factor that is effects on its accuracy.In this study, we notice the role of pixel size changing in calculation of channel length as a most important hydrological parameter. Maps with scale of 1:25000 from five regions of Iran edited and then interpolated in eight different pixel sizes.For appointment of role of pixel size changing, channel extracted is compared with use of D8 model with channel of maps with scale of 1:25000 in different pixel size with a view to length, movement and fractal dimension. Result of this study showed that pattern changes where the pixel size of channel length changes. The results of fractal diminution show that the pixel sizes large than 30 meter have difference geometric pattern with real channels. Also delineation channels errors from 5 and 10 meters was lees and has high accuracy.