Digital Elevation Model (DEM) represent the bare earth Z- values at regularly spaced intervals in X (Eastings) and Y (Northings). However, the grid spacing, datum, coordinate system, data formats and other characterstics may vary widely.
The term DEM (Digital Elevation Model) is frequently being used for the digital representation of any topographic surface or any part of earth’s surface. Mostly, it refers to a regular grid of spot heights on the earth surface. The grid size or the distance between adjacent grid points is an important parameter for DTM.

In a DTM/DEM, masspoints and break lines are captured to represent the shape of the terrain. Mass points are irregularly spaced each with X and Y location and Z values typically ( but not always) used to form a TIN. When generated manually, mass points are ideally chosen to depict the most significant variations in the slope or aspects of TIN triangles.The masspoints are representing the bare Z-value (spot height with X,Y and Z information) of the earth surface.
Breaklines are captured in the areas where there is a break or change in slope or continuity.Breaklines are captured on ridgelines, valleys, top and bottom of the slope.These are also used for roads, ditches.All masspoints and breaklines are measured from Aerial Photographs in stereo mode to achieve the  high accuracy on the terrain.

Uses of DEM:-DEM’s are application specific and they are used as input for:-

• Generation of elevation models and contours.
• Orthorectification process of aerial photography.
• Visualizing, analyzing and modeling purposes.
• Modelling of hydrographical networks & functions.
• ;Expansion Model.
• Slope Analysis.
• Battle planning.