I am using ArcGIS10. I need to find the out the area which is below 5000 and above 5000 from the Digital Elevation Model which is available to me. How can I find out this ? My DEM ranges from 8848 to 309 m with the total area of 4100 sq.km.
You will need a 3D Analyst license, but from what I can see the Surface Volume tool provides the area above your reference plane as Area 2D in its output format. You just need to provide its reference plane as being a base_z of 5,000 m. The area below your reference plane will be ( 4,100 - Area 2D ) sq.m.
If this is too slow then @whuber's comment seems to provide a quicker solution:
If you have Spatial Analyst available, simply [use Less Than to] compare the DEM to 5000. The resulting grid will have an attribute table for the values 1 (below 5000) and 0 (above 5000). The [count] field in the table shows how many cells of each type there are. To obtain the areas of each, multiply the counts by the square of the cellsize