Abstract
Abstract. Height information is a basic part of topographic mapping. Only in special areas frequent update of height models is required, usually the update cycle is quite lower as for horizontal map information. Some height models are available free of charge in the internet; for commercial height models a fee has to be paid. Mostly digital surface models (DSM) with the height of the visible surface are given and not the bare ground height, as required for standard mapping. Nevertheless by filtering of DSM, digital terrain models (DTM) with the height of the bare ground can be generated with the exception of dense forest areas where no height of the bare ground is available. These height models may be better as the DTM of some survey administrations. In addition several DTM from national survey administrations are classified, so as alternative the commercial or free of charge available information from internet can be used. The widely used SRTM DSM is available also as ACE-2 GDEM corrected by altimeter data for systematic height errors caused by vegetation and orientation errors. But the ACE-2 GDEM did not respect neighbourhood information. With the worldwide covering TanDEM-X height model, distributed starting 2014 by Airbus Defence and Space (former ASTRIUM) as WorldDEM, higher level of details and accuracy is reached as with other large area covering height models. At first the raw-version of WorldDEM will be available, followed by an edited version and finally as WorldDEM-DTM a height model of the bare ground. With 12 m spacing and a relative standard deviation of 1.2 m within an area of 1° x 1° an accuracy and resolution level is reached, satisfying also for larger map scales. For limited areas with the HDEM also a height model with 6 m spacing and a relative vertical accuracy of 0.5 m can be generated on demand. By bathymetric LiDAR and stereo images also the height of the sea floor can be determined if the water has satisfying transparency. Another method of getting bathymetric height information is an analysis of the wave structure in optical and SAR-images. An overview about the absolute and relative accuracy, the consistency, error distribution and other characteristics as influence of terrain inclination and aspects is given. Partially by post processing the height models can or have to be improved.
Highlights
The time consuming and partially expensive generation of large area covering digital height models (DHM) can be avoided by use of free of charge or commercially available DHM if they meet the requirements
Figure 1: frequency distribution of discrepancies of Shuttle Radar Topography Mission (SRTM) Digital Surface Models (DSM) against reference DEM in open areas and normal distribution based on RMSZ, SZ and Normalized Medium Absolute Deviation (NMAD), test area Warsaw
In open areas Altimeter Corrected Elevation 2 (ACE2) Global Digital Elevation Model (GDEM) sometimes has the advantage of lower bias, caused by height correction, but without the bias SZ and NMAD are better for SRTM
Summary
The time consuming and partially expensive generation of large area covering digital height models (DHM) can be avoided by use of free of charge or commercially available DHM if they meet the requirements. The absolute and relative accuracy is important, the point spacing, definition as Digital Terrain Models (DTM) with points on the bare ground or as Digital Surface Models (DSM) with heights of the visible surface – on top of vegetation and building –, homogeneity, support by error map, actuality and percentage of blunders, advantages and disadvantages, limitations in special areas as cities, steep mountains or deserts as well as required postprocessing are important. On the other hand based on very high resolution satellite imagery height accuracy and details may exceed the specification for height models from survey administrations. Height models from survey administrations are usually DTM with the advantage of reduced update requirement because vegetation and buildings are changing usually faster as the terrain itself with the exception of areas with strong erosion or subsidence in mining areas. The reference of the DHM as ellipsoidal or geoid height has to be respected, but a transformation based on geoid is simple
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