DIGITAL ELEVATION MODELS (DEM)

INTRODUCTION

The term Digital Elevation Model (DEM) has several meanings and is not always understood correctly or misinterpreted due to the surface it represents or geographic location the DEM data is being used. As used in most of the western hemisphere a DEM has "Bare Earth" z-values at regularly spaced intervals in x (Easting's) and y (Northing's). In some countries the x and y orientation are opposite from the general use with "x" being Northing and "y" Easting. Because of many different horizontal and vertical geodetic reference systems available around the world the necessity to clearly define the 3D geodetic reference system to be utilized is critical to a projects success to obtain the mapping standards defined.

Satellite Imaging Corporation (SIC) provides Orthorectified Satellite Images and Aerial Photography which can be processed for visualization of terrain conditions in 3 Dimensions (3D) or Digital Elevation Model (DEM) which are generated from a variety of resources. In addition to the Orthorectification of Remote Sensed Images, the DEM's are utilized in support of the pre-planning and lay-out of corridor surveys, area developments, seismic line locations, Well sites and Engineering, Construction activities, Environmental and Geologicalpurposes.

3 Meter Digital Surface Model (DSM) for 5 Km Wide Pipeline Corridor

Digital Elevation Model (DEM) — Eritrea, Africa — IKONOS Image

There is a variety of DEM source data available for developed areas and the suitability of this available data is depending on the project specifications. In remote regions around the World, were little or no source Data is available, the DEM can be produced by automatic DEM extraction from stereo satellite scenes, from Satellite sensors such as IKONOS (2-5m resolution), SPOT-5 (5-10m res.) and ASTER (15-25m res.). The DEM can also be provided from Stereo Digital Aerial Photography at various resolutions, depending on the quality and scale of the Aerial Photography.

To represent the earth's surfaces and bathymetry/hydrographic features in a 2D or 3DGeographic Information Systems (GIS) or CAD computer environment relative to existing geodetic horizontal and vertical datum's the reference surfaces have to be clearly defined.

WHAT'S A DEM?

Digital Elevation Models are data files that contain the elevation of the terrain over a specified area, usually at a fixed grid interval over the "Bare Earth". The intervals between each of the grid points will always be referenced to some geographical coordinate system. This is usually either latitude-longitude or UTM (Universal Transverse Mercator) coordinate systems. The closer together the grid points are located, the more detailed the information will be in the file. The details of the peaks and valleys in the terrain will be better modeled with small grid spacing than when the grid intervals are very large. Elevations other than at the specific grid point locations are not contained in the file. As a result peak points and valley points not coincident with the grid will not be recorded in the file.

For practical purpose this "Bare Earth" DEM is generally synonymous with a Digital Terrain Model (DTM).

Quality DEM products are measured by how accurate the elevation is at each pixel and how accurately the morphology is presented. Several factors are important for quality of DEM-derived products:

• Terrain roughness
• Sampling density (elevation data collection method)
• Grid resolution or pixel size
• Interpolation algorithm
• Vertical resolution
• Terrain analysis algorithm

Common uses of DEMs include:

• Extracting terrain parameters
• Modeling water flow or mass movement (for example, landslides)
• Creation of relief maps
• Rendering of 3D visualizations
• Creation of physical models (including raised-relief maps)
• Rectification of aerial photography or satellite imagery
• Reduction (terrain correction) of gravity measurements (gravimetry, physical geodesy)
• Terrain analyses in geomorphology and physical geography

WHAT'S A DSM?

Within the same project area, a Digital Surface Model (DSM) represents the MSL elevations of the reflective surfaces of trees, buildings, and other features elevated above the "Bare Earth".

WHAT'S A TRIANGULATED IRREGULAR NETWORK (TIN)?

TIN's are sets of adjacent, non-overlapping triangles computed from irregularly spaced points with x/y coordinates and z-values. TIN models are used to provide better control over terrain slope, aspect, surface areas, volumetric and cut-fill analysis and generating contours.

The TIN's vector data structure is based on irregularly-spaced point, line and polygon data interpreted as mass points and breaklines and stores the topological relationship between triangles and their adjacent neighbors.

WHAT ARE BREAKLINES?

Breaklines define and control surface behavior in terms of smoothness and continuity. As their name implies, breaklines are linear features. They have a significant effect in terms of describing surface behavior when incorporated in a surface model such as a TIN. Breaklines can describe and enforce a change in the behavior of the surface. Two types of breaklines are included in this layer: hard and soft.

Hard breaklines define interruptions in surface smoothness and are typically used to define streams, ridges, shorelines, building footprints, dams, and other locations of abrupt surface change.

Soft breaklines are used to ensure that known "Z" (elevation) values along a linear feature (such as a roadway) are maintained in a TIN. Soft breaklines can also be used to ensure that linear features and polygon edges are maintained in the TIN surface model by enforcing the breakline as TIN edges. Soft breaklines, however, do not define interruptions in surface smoothness.

WHAT ARE MASS POINTS?

Mass Points are irregularly distributed sample points, each with an x/y location and a z-value, which are used as the basic elements to build a TIN. Each mass point has important, yet equal, significance in terms defining the TIN surface. Ideally, the location of each mass point is intelligently chosen to capture important variations in the surface's morphology.