45% of the world changed, with 29% replaced completely!!!!

"ACE - Altimeter Corrected Elevations" is a brand new GDEM at 30" resolution, which for the first time has globally combined the best available ground truth with a unique global database of satellite altimeter derived heights. This model, "ACE -Altimeter Corrected Elevations" demonstrates radically improved spatial accuracy and resolution over large parts of the globe, in particular over Africa and South America.

The independent altimeter based dataset, which contains over 100 million individual height estimates, has enabled the identification and correction of gross errors in individual ground truth datasets. Source data of low spatial resolution have been replaced with greatly improved altimeter generated surfaces. Additionally, the use of this satellite derived near-global dataset of very high vertical accuracy, referenced to a single datum, has allowed the identification, measurement and correction of the vertical offsets between adjacent tiles of DTED data, widely reported by users of previous models. The result is a model in which a significant proportion of the earth's land surface is represented by a completely new dataset: this is especially true of areas for which previous models have drawn on Digital Chart of the World. Adjoining ground based datasets of good quality have also been adjusted for vertical misplacement.

The detailed assessment of dataset quality undertaken as part of the programme to create the ACE GDEM has allowed the inclusion of a unique additional information matrix: a data quality indicator is being constructed for every land pixel of ACE, together with detailed source information. This will allow the user, for the first time, to make an informed judgement on expected performance of the ACE GDEM for the desired application. This quality matrix will be released with the first full release of ACE.

These two height maps show two different representations of the upper Amazon basin. The ACE heights in this areas are 100% altimeter derived heights. Note the vast increase in detail of the river network. These picture use the same colour scale for fairne ss of comparison.

These imaginary features are typical of the sort of errors that can be found in the previous GDEMs. the image on the left shows where two different data sources have been joined together to produce a rather striking difference as you move over the area. T he cause of the stripping in the centre tiles is not known. The image on the right shows the result of using a data source with extremely sparse data and then using mathematical techniques to fill in the gaps. A single height measurement was used in an ar ea with very little data. When the interpolation routines have then used this point, a series of spline functions surround the point. These can be observed by looking at height profiles across the area.

The "dog" region in the Sudan

This 3x3 degree region of the Sudan was one of the first serious errors that we detected in the previous GDEMs. further analysis revealed that it does in fact exist in all GDEMs although it is indeed a completely imaginary feature. As can be seen from the picture on the left, the previous representations show a huge valley structure up to 200m deep with geometric edges that lie along the 1-degree lat/long grid. As you can see in the picture on the right hand side, the ACE representation (from 100% Altimet er data) shows a gently sloping region instead of such a dramatic feature.

Map showing the regions that have been changed.

Red- Altimetry derived
Green- Data warped to altimeter control arcs
Blue- Data not changed

This map shows very simplistically the changes that have been made to the previously best available GDEM. The decision as to what should happen to each 1-degree tile was based on many factors. eg. Data that is deemed as good quality is left in the ACE dat aset as it was before. Data which contains high frequency information but has somehow become distorted, would be warped to the control arcs provided by the altimeter etc. Further details of how this was done are to be found in the ACE documentation which can be obtained as part of the ACE CD or can be downloaded from this web site.