Konstantinos G. Nikolakopoulos

SRTM vs ASTER elevation products. Comparison for two regions in Crete, Greece

The Shuttle Radar Topography Mission (SRTM) collected elevation data over 80% of earths land area during an 11‐day Space Shuttle mission. With a horizontal resolution of 3 arc sec, SRTM represents the best quality, freely available digital elevation models (DEMs) worldwide. Since the SRTM elevation data are unedited, they contain occasional voids, or gaps, where the terrain lay in the radar beams shadow or in areas of extremely low radar backscatter, such as sea, dams, lakes and virtually any water‐covered surface. In contrast to the short duration of the SRTM mission, the ongoing Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is continuously collecting elevation information with a horizontal resolution of 15 m. In this paper we compared DEM products created from SRTM data with respective products created from ASTER stereo‐pairs. The study areas were located in Crete, Greece. Absolute DEMs produced photogrammetricaly from ASTER using differentially corrected GPS measurements provided the benchmark to infer vertical and planimetric accuracy of the 3 arc sec finished SRTM product. Spatial filters were used to detect and remove the voids, as well as to interpolate the missing values in DEMs. Comparison between SRTM‐ and ASTER‐derived DEMs allowed a qualitative assessment of the horizontal and vertical component of the error, while statistical measures were used to estimate their vertical accuracy. Elevation difference between SRTM and ASTER products was evaluated using the root mean square error (RMSE), which was found to be less than 50 m.

The contribution of probability theory in assessing the efficiency of two frequently used vegetation indices

Frequency band ratios are often used as vegetation indices in environmental studies as a measure of the amount of vegetation in a digital image. The Simple Vegetation Index (SVI) and the Normalized Difference Vegetation Index (NDVI) are two frequently used vegetation indices and their mathematical form is the basis for the development of other modified vegetation indices, such as TVI, SAVI or SAVI2. Vegetation indices are mainly defined and evaluated empirically. In the present paper, a different approach based on probability theory is developed in order to evaluate the efficiency of SVI and NDVI and to suggest two modified vegetation idices, MSVI and MNDVI. According to the mathematical analysis and experimentation with a Landsat 7 Enhanced Thematic Mapper (ETM) image of an island in western Greece, it is concluded that NDVI provides better results than SVI, since the image of the former has a much broader brightness histogram and the targets of interest are more clearly expressed in the satellite image. The image of MSVI has a broader histogram from those of NDVI and SVI and a more diverse tonality. MNDVI may provide better results than NDVI if the standard deviations of the images of the near infrared (NIR) and red bands vary considerably. It is also concluded that the signal-to-noise ratio of the MSVI image is better than that of the SVI image. The signal-to-noise ratio of the MNDVI image may be better than that of the NDVI image if a proper value for a characteristic parameter in the expression for the MNDVI is chosen.

Combined use of remote sensing, GIS and GPS data for landslide mapping

The term landslide includes a wide range of ground movement, such as slides, falls, flows etc. mainly based on gravity with the aid of many conditioning and triggering factors. Particularly in the last two decades, there is an increasing international interest on the landslide susceptibility, hazard or risk assessments. In this paper a combined use of Remote Sensing, GIS and GPS data is presented. The area of study is Lefkas Island in the Ionian Sea, Greece. The Ionian Sea presents a very big seismicity and the Landslides phenomena are very often. During the last twenty-five years more than 20 big landslides have been recorded on Lefkas Island. The latest landslides have been recorded in August 2003 as a consequence of a 6.1R earthquake. Multispectral and multitemporal satellite data were used for landslide detection. More especially we have used Landsat MSS, Landsat TM & ETM data and Terra ASTER data. Because most of the landslides are mostly bare of vegetation they present high reflectance. As a result big landslides can be detect from satellite data. ASTER stereo-pairs have been used for the creation of two DTM’s. The images cover an area of 60X60 km. The images have been received in the near infrared (0,78-0,86μm) part of the spectrum with a spatial resolution of 15m. The first stereo-pair was acquired on January 2000 and the second one on August 2003 a few days after the earthquake and the landslides. The same GCP’s were used for the creation of the two DTM’s. In some cases the landslides can be detected as a difference of the two DTM’s. All the landslides have been mapped in-situ using a GPS receiver. Finally all the above data have been integrated in a GIS and detailed landslides maps have been produced. Keywords-component: Landslides, satellite data, DTM, GPS stero-pairs, GIS.

Monitoring the urban expansion of Athens using remote sensing and GIS techniques in the last 35 years

During the last thirty-five years the capital of Greece has suffered from an enormous internal immigration. Its population has overpassed the five millions and today almost the half population of Greece is squeezed in Athens metropolitan area. Because of the significant increase of population, the urban expansion in the basin of Athens was also excessive and in some cases catastrophic. Buildings have covered all the free places, new roads have been constructed, the drainage networks have been covered or disappeared and a lot of changes have been occurred to the landforms. The construction of the new airport (Elefterios Venizelos) at the beginning of this decade created a new commercial and urban pole at the eastern part of Athens and the constructive activity has been moved to new areas around the airport. Our aim was to detect and map all the changes that occurred in the urban area, estimate the urban expansion rate and the human interferences in the natural landscape, using GIS and remote sensing techniques. We have used satellite images from three different periods (1973, 1992, 2002) and topographic maps of 1:25.000 scale. The spatial resolution of all the satellite images ranges from 5 to 10 meters and is it acceptable for the monitoring and mapping of the urban growth. Supervised classification and on screen digitizing methods have been used in order to map the changes. Finally the qualitative and quantitative results of this study are presented in this paper.

Quality assessment of ten fusion techniques applied on Worldview-2

Abstract Fusion or merging or pansharpening is the digital processing that combines the high spatial and multispectral information to obtain a fused multispectral image that retains the spatial information from the high resolution panchromatic image, as well as the spectral characteristics of the lower resolution multispectral image. Many fusion algorithms were developed and tested on different commercial satellite data such as Ikonos and Quickbird, however, the efficacy of these algorithms is weakly assessed on the new satellite images of Worldview-2 with very high spatial resolution. In the present study ten well known fusion techniques namely Color Normalized (CN), Ehlers, Gram-Schmidt, High Pass Filter (HPF), Local Mean Matching (LMM), Local Mean and Variance Matching (LMVM), Modified IHS (Modihs), Pansharp, PCA and Wavelet are used for the fusion of Worldview-2 panchromatic and multispectral data. Fused images were evaluated for spectral and spatial fidelity using visual inspection and different quality indexes such as the correlation coefficient (CC), ERGAS, the Universal Image Quality Index (Q), the Expanded Quality Index (Q4expand) and the Entropy values. Under each quality metric the fusion algorithms were ranked and best performances were indentified. It has been proved that all fusion algorithms have a different performance with the new multispectral bands of Worldview-2 in comparison to the classical ones. It has also been proved that almost all quality indices present a very strong dependency to the spatial resolution of the input data. The Q4expand quality index is proposed as the main quality discriminator since it gave the most robust results independently of the spectral and spatial input resolution. Pansharp algorithm took the lead in spatial enhancement while Ehlers and HPF are ranked in the first places for spectral preservation.

Updating the 1:50.000 topographic maps using ASTER and SRTM DEM. The case of Athens, Greece

The 1:50.000 topographic maps present a nominal horizontal accuracy of 20 meters and a nominal vertical accuracy of 10 meters with 90% confidence. The data were in most cases extracted with photogrammetric techniques from aerial stereo-photographs during the 80s. The usual update rate for these maps ranges from ten to twenty years. The Advanced Spaceborn Thermal Emission and Reflection Radiometer (ASTER) offers along-track stereoscopic viewing capability. Its viewing geometry is suitable for DEM generation even without the use of ground control points. Recent studies have proved that in this case the vertical accuracy of DEM is about 20m with 95% confidence. The horizontal geolocation accuracy appears to be limited by the spacecraft position accuracy which is considered to be better than 50 m. Other studies have shown that the use of GCPs resulted in a plannimetric accuracy of 15 m and in a near pixel size vertical accuracy. The Shuttle Radar Topography Mission (SRTM), used an Interferometric Synthetic Aperture Radar (IFSAR) instrument to produce a near-global digital elevation map of the Earths land surface with 16 m absolute vertical height accuracy at 30 meter postings. An SRTM 3-arc-second product (90m resolution) is available for the entire world. In this paper we examine the possibility of updating the 1:50.000 topographic maps using ASTER and SRTM DEMs. The area of study is the broader area of Athens, Greece. Presupposing, that the horizontal and vertical accuracy of the ASTER and SRTM DEM is similar to the relative accuracies of the DEM from digitized contours, optical comparison of the DEMs and statistical analysis (difference, correlation) can immediately prove if there is any need for update to the topographic maps. A DEM from digitized contours from the 1:50.000 topographic maps was created and compared with ASTER and SRTM derived DEMs. Almost three hundreds points of known elevation have been used to estimate the accuracy of these three DEMs. The resulted accuracy of the SRTM and ASTER derived DEMs was satisfactory, therefore they are considered as suitable for updating 1:50.000 topographic maps.

Comparison of six fusion techniques for SPOT5 data

Image fusion is a technique used to integrate the geometric detail of a high-resolution panchromatic (Pan) image and the color information of a low-resolution multispectral (MS) image to produce a high-resolution MS image. During the last twenty years many methods such as Principal Component Analysis (PCA), Multiplicative Transform, Brovey Transform, IHS Transform have been developed producing good quality fused images. Despite the quite good optical results many research papers have reported the limitations of the above fusion techniques. The most significant problem is color distortion. Another common problem is that the fusion quality often depends upon the operator’s fusion experience, and upon the data set being fused. No automatic solution has been achieved to consistently produce high quality fusion for different data sets. More recently new techniques have been proposed such as the Wavelet Transform, the Pansharp Transform and the Modified IHS Transform. Those techniques seem to reduce the color distortion problem and to keep the statistical parameters invariable. In this study we compare the efficiency of six fusion techniques and more especially the efficiency of IHS, Modified IHS, PCA, Pansharp, Wavelet and LMM (Local Mean Matching) fusion techniques for the fusion of SPOT5 data. The area of interest is a rural area. A SPOT5 cloud free subscene was used in this comparative study. A 2.5 panchromatic image was fused with a 10m multispectral image. The nearest neighborhood method has been used for the resampling and the final fused images have a 2.5 meter pixel size. For each merged image we have examined: a) the optical qualitative result, b) the statistical parameters of the histograms of the various frequency bands, especially the standard deviation All the fusion techniques improve the resolution and the optical result. The LMM, the Pansharp, and the Wavelet merging technique do not change the statistical parameters of the original images. The Modified IHS provokes minor changes to the statistical parameters than the classical IHS or than the PCA. Keywords-component; Principal Component Analysis (PCA), Pansharp Technique, Wavelet Technique, Modified IHS color distortion, statistical parameters.

Comparative study of fusing ETM data with five different techniques for the broader area of Pyrgos, Greece.

Many image fusion algorithms such as Principal Component Analysis (PCA), Multiplicative Transform, Brovey Transform, IHS Transform and wavelet transform have been presented in order to fuse low-resolution multispectral data with high-resolution panchromatic data. More recently Yun Zhang has presented a new algorithm for the fusion of Landsat ETM and Ikonos data respectively. In this study we compare the efficiency of five of the above fusion techniques and more especially the efficiency of PCA, Multiplicative Transform, Brovey Transform, IHS Transform and Pansharp for the fusion of Landsat ETM data. The area of interest is situated in Western Peloponnese near the city of Pyrgos. The broader region combines at the same time the characteristics of an urban, a coastal and a rural area. A Landsat 7 ETM cloud free subscene taken in the morning of July 28, 1999, was used in this comparative study. For each fused image we have examined: a) the optical qualitative result, b) the statistical parameters of the histograms of the various frequency bands, especially the standard deviation c) the amplitude spectrums of the frequency bands. All the fusion techniques improve the resolution and the optical result but according to the statistical analysis the Brovey and the Multiplicative techniques do not improve the contained information in the fused images. The IHS fusion technique seems to have the best optical result, increases the sum of the contained information but provoke changes to the colors of the original RGB image. The PCA fusion technique seems better in discriminating between the coastal zone, the urban area and the rural area and maintains the natural colors. The Pansharp fusion technique gives the best results without changing at all the statistical parameters of the original multispectral image.

Comparing a DTM created with ASTER data to GTOPO 30 and to one created from 1/50.000 topographic maps

In this paper we present the use of ASTER data for the creation of a Digital Terrain Model (DMT) of high accuracy. The area of study is Milos Island in the Aegean Sea. For the creation of the DTM we used an ASTER stereo-pair dating August 07, 2002. The images cover an area of 60X60 km. The images have been received in the near infrared (0,78-0,86μm) part of the spectrum with a spatial resolution of 15m. We created a DMT a 30m-pixel size. Then we compared the DTM with the following DTMs: a) A GTOPO 30 DTM b) A DTM created from digitized contours of 1/50.000 scale topographic maps. We first made an optical comparison of the DTMs. Then, we proceed to statistical control of the histogram values of the three DTMs. From the first two controls we conclude that the GTOPO30 DTM is less accurate than the other two DTMs and we proceed in more detailed control only for the DTM from the digitized contours and for the DTM from the ASTER stereo-pair. We verified the DTMs accuracy using different points of well-known elevation. All the results demonstrated that DTMs derived from ASTER data have better accuracy than the other two DTMs.

Pansharp vs. wavelet vs. PCA fusion technique for use with Landsat ETM panchromatic and multispectral data

In this study we compare the efficiency of PCA, Pansharp and Wavelet fusion techniques for the fusion of Landsat ETM data. A Landsat 7 ETM cloud free subscene was used in this comparative study. The nearest neighborhood method has been used for the resampling and the fused images have a 15 meters pixel size. For each merged image we have examined: a) the optical qualitative result, using an ASTER vnir image with 15 meters resolution for comparison, b) the statistical parameters of the histograms of the various frequency bands, especially the standard deviation. All the fusion techniques improve the resolution and the optical result. The PCA merging technique seems better in discriminating between the coastal zone, the urban area and the rural area, maintains the natural colors but corrupts the statistical parameters. The Pansharp and the Wavelet merging technique give the best results without changing at all the statistical parameters of the original images.