In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
468 
Lakes 
Area Comparison 
Intersection/Union x 100 
GPS 
Ikonos 
GPS 
Ikonos 
Très 
94,54% 
n/a 
81.05% 
n/a 
Meio 
93,34% 
86,01% 
91,53% 
71,04% 
Sede 
89,50% 
94,41% 
89,16% 
83,85% 
Azul 
94,18% 
96,36% 
92,13% 
93,20% 
Formosa 
n/a 
95,08% 
n/a 
92.66% 
Table 3: Validation of the lake contour extraction using the GPS 
survey and the Ikonos scene. Column 2 and 3 show the results for 
the area comparison; column 4 and 5 show the accuracy obtained 
with the wterjsctum x 100 a pp roac h. 
union 
(a) Landsat lake contours 
Figure 9: Comparison of the contours of three of the six lakes 
using the interpolated Landsat data (left) and the geodetic GPS 
survey data (right). 
are presented in Table 4. The only correlation between the ar 
eas of the lakes and the AW is with the “Pista” lake which has 
dried up since 2000 and the level of significance is p=0,05. Con 
versely, all the lakes are strongly related among themselves with 
a significance of 0,01. This confirms that the trend is statistically 
significant and that we can infer that the lakes are rapidly shrink 
ing. Even lake “Azul” which has kept a much more constant sur 
face area is strongly correlated with all the other lakes (0,601 to 
0,871). Since the AW cannot be said to be correlated with the 
shrinking of the lakes, the meteorological explanation becomes 
much less plausible and the human pressure on the watershed can 
more easily be pinpointed as responsable. 
Table 4: Results of the Spearman’s correlation tests. 
AW Lakes 
Pista Très Meio Sede Azul 
Pista 
Coir. 
*0.329 
Très 
Coir. 
0,209 **0,455 
Meio 
Corr. 
0,209 **0,611 **0,834 
Sede 
Coir. 
0,075 **0,566 **0,735 **0,957 
Azul 
Corr. 
0,259 **0,601 **0,871 **0,866 **0,789 
Formosa 
Corr. 
0,068 **0,524 **0,674 **0,899 **0,897 **0,730 
* Significant at 0,05 
** Significant at 0,01 
4 CONCLUSIONS 
Multi-temporal remote sensing offers countless opportunities for 
monitoring past and present changes in land cover and land use. 
By monitoring the size and shape of water bodies, we can infer on 
human pressure and climate change. In this article we proposed 
an innovative approach for monitoring small lakes using medium 
resolution Landsat data. The approach uses minimum curvature 
interpolation to artificially improve the resolution of the image 
data and produce a much cleaner lake contour that matches the 
actual measured contour with a high success rate (15 validation 
out of 16 with better than 80% and 10 better than 90%). Us 
ing posterior probability of a maximum likelihood classifier, we 
were able to systematically extract contours from six lakes for 
50 different dates with ease and good matching of control data. 
The Modified Normalized Difference Water Index (MNDWI) did 
not perform well for these small shallow lakes with the presence 
of aquatic vegetation. The water balance using the Thomthwaite 
approach is well suited for area with limited climatological infor 
mation and provides valuable insight on the climatological con 
dition ruling water availability. In this study, the water balance 
could not be statistically correlated (Spearman’s correlation) to 
the shrinking of six small lakes in Northern Minas Gerais, Brazil. 
ACKNOWLEDGEMENTS 
The authors are thankful to the Forestry Institute of Minas Gerais 
for providing the Ikonos data and field support. We are most 
thankful to Thiago Alencar Silva and Thais Amaral for their help 
and support. 
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