Integration of orthophotographic and sidescan sonar imagery: an example from Lake Garda, Italy 
: Giuseppe Gentili (CISIG), Via Argini 101, PARMA, Italy 
David C. Twichell, and Bill Schwab (USGS), Branch of Atlantic Marine Geology, WOODS HOLE, MA, USA 
KEY WORDS: Sidescan sonar, Orthophoto, Hyperspectral data, Data integration, Lakes 
ABSTRACT 
Digital orthophotos of Lake Garda basin area are available at the scale of up to 1:10,000 from a 1994 high altitude ( average scale 
of 1: 75,000 ) air photo coverage of Italy collected with an RC 30 camera and Panatomic film. In October 1994 the lake bed was 
surveyed by USGS and CISIG personnel using a SIS 1000 Sea-Floor Mapping System. Subsystems of the SIS-1000 include high 
resolution sidescan sonar and sub-bottom profiler. The sidescan imagery was collected in ranges up to 1500m, while preserving a 
50cm pixel resolution. The system was navigated using differential GPS. The extended operational range of the sidescan sonar 
permitted surveying the 370km? lake area in 11 days. Data were compiled into a digital image with a pixel resolution of about 2m 
and stored as 12 gigabytes in exabyte 8mm tape and converted from WGS84 coordinate system to the European Datum (ED50) and 
integrated with bathymetric data digitized from maps.The digital bathymetric model was generated by interpolation using commer- 
cial software and was merged with the land elevation model to obtain a digital elevation model of the Lake Garda basin.The 
sidescan image data was also projected in the same coordinate system and seamed with the digital orthophoto of the land to produce 
a continuous image of the basin as if the water were removed. Some perspective scenes were generated by combining elevation and 
bathymetric data with basin and lake floor images. In deep water the lake’s thermal structure created problems with the imagery 
indicating that winter or spring is best survey period. In shallow waters, < 10 m, where data are missing, the bottom data gap can 
be filled with available images from the first few channels of the Daedalus built MIVIS, a 102 channel hyperspectral scanner with 
20 channel bands of 0.020 um width, operating in the visible part of the spectrum. By integrating orthophotos with sidescan 
imagery we can see how the basin morphology extends across the lake, the paths taken by the lake inlet along the lake bed and the 
areal distribution of sediments. An extensive exposure of debris aprons were noted on the western side of the lake. Various 
anthropogenic objects were recognized: pipelines, sites of waste disposal on the lake’s bed, and relicts of Venitian and Austrian (?) 
boats. 
1.0 Introduction 
  
Being able to “see” the floor of bodies of water as if the water Re 
were not there is of interest to many research scientists from a 
variety of disciplines and also to governmental agencies 
working on land use problems near water bodies. 
Occasionally a geologic formation that is found on land cannot 0 
be traced with confidence to the other side of a body of water 3 
without some knowledge of the configuration of its bed. Often : 
much can be learned about the dynamics of coastal sediment 
formation by being able to trace onshore features to morpholo- 
gical characteristics of the bed. A variety of environmental pro- 
blems can be addressed much more successfully if the shore 
data can be correlated with the images of the offshore bottom. S 
The project described here was undertaken to answer some of e 
these questions. 
2.0 The lake Garda project S 
The CISIG had contemplated the possibility of carrying out a 
project of trying to merge images of lake beds with onshore 
aerial photography for some time but it was only the advent of 
extended coverage sidescan sonar instrumentation and the de- 
velopment of a collaborative agreement with the US Geologi- 
cal Survey that made this possible. 
Choice of Lake Garda as project area was dictated by the size amo 4 Ve a iie B iio. 
of the lake, the largest in Italy, and by the fact that another 
EEE» SEAT Ss also Figure 1 - Location of Lake Garda project area and cities refe- 
An outline of the project area is given in Figure 1. minced iine ext 
The lake Garda is a Quaternary-age glacial lake that formed 
along the geomorphic expression of a major Cenozoic-age 
LAZISE 
SIRMIONE 
  
  
  
  
along the lake bed to the peninsula of Sirmione, thus dividing 
thrust fault which was associated with the formation of the the southern lobe of the lake into two parts, a deeper one on 
Alps (Petrucci and Valloni, 1981). The fault extends along the the west, and a relatively shallow segment on the east. The 
eastern shore of the lake and is believed to continue south succession of quaternary glaciations and their successive retre 
289 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996