An Assessment of the SRTM Topographic Products - Jet Propulsion ...

CHAPTER 2. ABSOLUTE HEIGHT ERROR 32
2.2 DTED Level 2 Comparison
Table 2.2 summarizes the continental average results for the DTED 2 comparisons, while Figure 2.7
summarizes the global performance in a histogram. The results for all the continents are roughly
consistent, with the exception of South America, which shows a significantly higher mean deviation.
In order to investigate the source of large 90% errors, we plot in Figure 2.8 the behavior of
the 90% error against the median and standard deviation of the height difference. We also present
in Tables 2.3–2.6 the detailed values for each cell. The largest differences are due to mean shifts
between the two data sets. Since the SRTM data is continentally adjusted against the ocean, and
because the kinematic GPS data did not show these large deviations, it is possible that the average
errors may be present in the DTED 2 data, rather than the SRTM data.
On the other hand, Figure 2.8 shows that large errors can also occur due to large RMS differences.
These may be due to degraded SRTM performance over regions of large slopes, such as Afghanistan
(see next chapter).
Continent Average Median 90% Diff STD RMS
Africa 2.44 2.30 8.80 4.68 5.53
Eurasia -0.07 -0.09 8.07 4.50 5.36
N.America 1.38 1.40 7.67 4.01 4.88
S.America 11.36 11.20 16.88 4.60 12.77
Table 2.2: Statistics for the height difference between SRTM and DTED level 2. All quantities are
in meters.
Cell Average Median 90% Diff STD RMS
n09e007 2.85 2.77 6.55 2.96 4.11
n21e031 2.73 2.20 7.92 4.52 5.28
n21e039 4.97 4.48 11.82 5.45 7.38
n21e040 -0.07 0.34 12.17 7.10 7.10
n22e048 1.72 1.70 5.54 3.36 3.77
Average 2.44 2.30 8.80 4.68 5.53
Table 2.3: Statistics for the height difference between SRTM and DTED level-2 data for Africa.
All quantities are in meters. The cell name has the format n(latitude)e(longitude) denoting the
south-west corner of the SRTM height cell