International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
      
  
EVALUATION OF ASTER GDEM v.2 USING GPS CHECKPOINTS, OSGB DEM 
VALUES AND PHOTOGAMMETRICALLY DERIVED DEMS 
Haval Sadeq, Jane Drummond, Zhenhong Li. 
School of Geographical and Earth Sciences, College of Science and Engineering, University of Glasgow, G12 8QQ, UK. 
(jane.drummond@glasgow.ac.uk). 
Commission IV, WG IV/6 
KEYWORDS: ASTER GDEM, accuracy, bias, test-sites, slope-effect, coastal-cffect. 
ABSTRACT: 
A 2010 study examining ASTER GDEM v1 data revealed accuracies of 12-25m and strong negative discrepancy biases compared to 
precise GPS observations, in several test sites in China. Rather than further investigating these, with the advent of ASTER GDEM v2 
a new series of tests, also using precise GPS observations but also 
other DEMs, was performed. In these tests better than the expected 
17m accuracies were found (RMSE values of 3.9m to 15.3m) and no strong biases. 
1. BACKGROUND 
A study, with colleagues (Li, et al., 2012) revealed accuracies 
of 12-25m in five Chinese test areas when comparing 
ASTER GDEM vl values with high accuracy GPS check 
points. This accuracy was poorer than expected, but also 
exposed a strong negative bias in most of the test areas. For 
purposes of comparison SRTM data of the test areas were 
also investigated revealing no negative bias. The test areas 
were coastal, agricultural, steep /mountainous and high 
plateau. We proposed several reasons for the bias including 
landcover effects, the lack of a water mask and other 
systematic errors in the data processing. It was planned to 
look at the data further in more extensive areas in China and 
three test sites in the UK (mountainous, coastal and 
agricultural). However the very recent advent (October, 
2011) of ASTER GDEM v2 encouraged us to immediately 
investigate these new data, particularly with respect to the 
negative bias, using the three British test areas, only. 
Comparisons were planned with GPS check points, 10m 
DTMs supplied by the national mapping organization 
(Ordnance Survey of Great Britain or OSGB) and in-house 
generated digital photogrammetric DTMs. Investigation has 
shown improved accuracy (15.2m, 8.7m and 3.7m, 
respectively) in the three sites. This paper will present the 
findings and details of our validation with regard to the three 
test areas and the four data sources (ASTER GDEM v2; 
OSGB Profile DTMs; digital photogrammetry; high accuracy 
GPS). 
2. TEST AREAS 
The test areas are Plockton (forest /mountainous), 
Caerlaverock Merse (coastal/ salt-marsh/pasture), Wicken 
Fen (low lying/ inland/arable). They are located in northern, 
central, and southern Britain, as follows (shown by their 
approx. centrepoints’ Lat/Long): 
Lat Long 
Plockton 57.33°N 5.61°W 
Caerlaverock Merse 54.98°N 3.54°W 
Wicken Fen 52.33°N 0.31°E 
The three areas are well known to the authors. Precise GPS 
observations had been gathered in Plockton in 2007, and 
these are utilised. The GPS derived coordinates of these 
points are shown in Table 1, with their ASTER, OSGB and 
photogrammetric heights (derived from digitally processed 
1:14000 scale RC20 aerial photography, flown 2004); all 
heights are on the ODN vertical datum (that used by OSGB). 
3. METHODOLOGY 
For all three areas ASTER GDEM v2 was compared with 
OSGB Profile DTMs. The ASTER GDEM v2 is supplied at 1 
arc-second resolution (approx. 30x15m at the UK's latitude), 
with geographical coordinates based on the WGS84 ellipsoid 
and height values based on the EGMO96 geoid. The 
coordinates used by ASTER are the same as the Google Earth 
reference system, importantly with heights, for the UK, some 
30m different from those based on the WGS84 ellipsoid, 
according to Lemoine et al. (1998). The difference between 
the EGM96 and ODN is small, being about 80cm over the 
British Isles (Stillman, 2009). The data sets were imported 
into ArcMap having been appropriately labelled for their 
original planimetric coordinates (ASTER: Lat, Long on 
WGS84; OSGB Profile: BNG). The project in ArcMap was 
set up with BNG planimetric coordinates, thus, on import to 
ArcMap, ASTER Lat, Long values were displayed as BNG 
coordinates. The height correction, ensuring both terrain 
models were based on the same vertical datum (ODN), 
involved a simple -80cm shift to the ASTER data. Thus all 
terrain models were approximately the same resolution, and 
the same coordinate frame and vertical datum. 
ASTER GDEM v2 and OSGB Profile terrain models were 
processed to provide ‘difference maps’ (Figs la, 3a, 4a). The 
average difference in each case is: 0.8m, 0.4m and 4.9m, and 
considerably less than in our previous study using ASTER 
GDEM v1 data (typically 20 — 30m). Histograms of the 
distribution of values from each of the ‘difference maps’ do 
not show the negative bias previously identified (Figs 2c, 3c, 
4c) The greatest differences tend to be found in steep, 
forested areas and the smallest differences in the areas of 
pastureland, with arable land intermediate; but, anomalously, 
in the third test site (Wicken Fen) very high differences were 
found in low-lying (below sea-level, but inland) arable land. 
Table 1 shows the E,N and elevation values of the fifteen 
precise GPS points, the height value of those same points 
from the ASTER GDEM v2, the OSGB Profile and the 
(incomplete) photogrammetric terrain models, giving 
RMSE’s of 7.3, 2.1 and 3.7m, respectively.