System for the generation of digital terrain elevation data (DTED) from compressed arc digitized raster graphics (CADRG) images

There are many applications for which there is a strong need to analyze digital terrain elevation data (DTED). However, the availability of DTED is limited. For some areas of the world, DTED is not available at all, and for other areas it may be only partially available. This introduces significant problems if analysis is required for areas of constrained DTED availability. On the other hand, Compressed Arc Digitized Raster Graphics (CADRG) maps provide a guaranteed source of digital data that contains a limited amount of elevation information, namely elevations that occur along the contours in the maps. Unfortunately, CADRG maps typically contain a significant amount of additional visual information that obscures features of interest, making difficult the automated location of contours and the subsequent transformation into DTED. This paper documents a system that has been developed for the purpose of transforming CADRG images into DTED. From a system design standpoint, this transformation is based on two principal components that consist of an automated image processing subsystem and a graphical user interface (GUI). In the initial stages, the GUI is used to select a CADRG map region and to specify appropriate image processing parameters such as contour colors and contrast. The image processing that is applied to CADRG maps consists of algorithms that seek contours based on color, local linearity, feature thinness, and morphological connectedness. As the result of applying the image processing algorithms, a set of contours is extracted from the image which may then be interactively edited using the GUI. The purpose of the interactive editing step is to assure that the final contours used to generate DTED are complete and properly connected, and that their elevations have been assigned. After this has been accomplished, DTED is then generated using a weighted averaging method based on local contour information. The results presented demonstrate that the system is capable of processing all variants of CADRG maps, including Topographic Line Maps (TLM), Joint Operations Graphics (JOG), Tactical Pilotage Charts (TPC), Operational Navigation Charts (ONC), Jet Navigation Charts (JNC), and Global Navigation Charts (GNC). Furthermore, test cases involving areas for which there is existing DTED demonstrate consistency between the existing DTED and elevation data produced by the system from a CADRG map covering the same region.