PE&RS January 2004

Direct Georeferencing

Direct Georeferencing Activities in Italy

Italy is a severe test site for surveyors and sensors because of its topography that is composed of long Mediterranean coasts, long chains of high mountains, many lakes, plain terrain and rivers. Very often, a mix of those various topographic elements show up in one mapping project. For example, many mountains have landslides, and many rivers are in danger of flooding because their basin is partially obstructed by buildings. Therefore, Italy needs good knowledge and frequent monitoring of its territory. Consequently, direct georeferencing is the optimal enabling technology that allows forsuch high frequency surveying.

Direct georeferencing is a well-established technology in Italy. Its first application was about 10 years ago when it was used with the airborne hyperspectral sensors MIVIS and CASI. Then, an Optech ALTM 1210 - the first airborne laser scanner or lidar – was used to acquire elevation data. Airborne laser scanners compute elevation data based upon laser scans and direct georeferencing data. The first application of direct georeferencing with aerial frame cameras was done about five years ago.

Currently there are four airborne laser scanners operated by as many companies; there is one Optech ALTM 1210, two Optech ALTM 3033, and one Saab TopEye. The owner of one ALTM 3033 also has the availability of the Toposys sensors thanks to an agreement between the two companies: it is used for urban applications. Two companies have entered the direct photogrammetry market using film cameras; one owns an Applanix POS AV 510 system and the other owns five POS AV 510 systems. The latter company also recently announced the purchase of a Leica ADS40; the first one sold in Europe.

The market for direct georeferencing is rapidly evolving and growing. Furthermore, in the last two years many examples have appeared that involve the integration of data from different mapping sensors: airborne lidar and airborne images, airborne and terrestrial lidar, airborne lidar and bathymetric soundings and many other more complex examples.

In this article, we will briefly introduce some interesting industrial and scientific direct georeferencing projects that were carried out in Italy.

Examples of Italian Industrial Projects
The Terraitaly City™ project deals with the realization of the digital color orthophoto of the 110 most important Italian cities. Images were acquired with a Leica RC30 camera equipped with a 300 mm focal length and integrated with an Applanix POS AV 510 system. The average scale of the photographs is 1:9000 with a ground pixel size of around 0.10 m. The City project used purely directly georeferenced images and is now concluded; the company that carried out the work will continue enlarging the set of the covered cities. They will also produce the directly oriented digital color orthophoto of the whole Italian territory with a ground pixel size of 1 meter; this is an update of a previously existing product and the company plans to perform frequent updates of this 1 meter orthophoto.

Another project was carried out by a major Italian mobile phone company that commissioned the production of 3D vector maps of approximately 100 of the most important Italian cities in order to support their network’s planning. The maps were produced by photogrammetry using the Terraitaly City™ images and, due to the slightly relaxed accuracy requirements it was possible to use directly oriented images without any GCPs.

Moreover, the basin authority of the Eastern part of Italy commissioned the lidar surveying of the five main rivers of that area and their tributaries. The survey has a total length of 644 kilometers and is 1200 km wide. The airborne survey was performed with an Optech ALTM 3033 sensor coupled with a Rollei DB44 digital camera. The main deliverables are the lidar DSM, whose density is 4 points per square meter for the inner part of the acquired buffer and 1 point outside, and the digital orthophoto with a 0.40 m ground sample distance. Notably, bathymetric surveys were performed of the river bottoms with a directly georeferenced Reson Seabat 9001 multibeam sonar capable of a sounding density of 2 points per square meter. The survey will be concluded in March 2004.

In the Campania region, the local basin authority commissioned the survey of all the region’s coasts, which are 400 kilometers long, for monitoring coastal erosion and water quality. It was performed in 2000 by an Optech ALTM 1210 with a CASI-1 hyperspectral sensor: both the sensors were coupled with the GPS/IMU device, so that the CASI-1 images are directly and precisely georeferenced. The delivered products were the lidar DTM with a density of 1 point per square meter and a hyperspectral image with 15 channels and a ground pixel size of 2 meters.

Finally, the Provincia of Torino commissioned a lidar survey of all the roads of its territory with the aim of performing noise propagation simulations and planning the placing of noise-reduction barriers. The survey project was finished this year and was performed with a Saab TopEye sensor. It covered a buffer of 500 m wide and 350 km long. The average point density within the whole buffer was 1 point per square meter and 2 for a narrower, inner part of the buffer. Features were extracted from the lidar data, such as roads and buildings, in order to produce a vector 3D map to be used as a base for the simulation.

Examples of Italian Research Projects
In the period from 2000 to 2002, two research projects were chaired by the University of Trieste, which focused on MMVs and on quick and low-impact methods for acquiring data for the road cadastre. The final product was the construction of an Italian vehicle equipped with digital cameras, and an integrated inertial sensor, together with some other accessories.

In the period from 1998 to 2000, the University of Pavia chaired a project on laser scanning which was joined by more than ten Italian universities that worked on different issues such as data filtering, feature extraction, true orthophotos and quality assessment of data. Many datasets were acquired with the sensors Toposys I and ALTM 1210 above Pavia’s test site, which was created for that occasion. For the period 2003 to 2004, another project known as Integrated inertial positioning systems in aerial Photogrammetry is being chaired by the University of Pavia. Even if other projects on this subject were run elsewhere in the world, it is worth organizing a new one because it is necessary to have a vast sampling in order to derive reliable conclusions about the new methodology with the focus on setting the necessary procedures for map compilation.

The project will focus on the following main issues:

• Assessment of precision and reliability of direct photogrammetry as a function of the image scale and of the focal length: three image scales and two focal lengths will be considered.
• Assessment of the stability of calibration and its dependence on the type of the coordinates used.
• Verification of the extent to which it is possible to perform map compilation with exterior orientation parameters given by direct sensor orientation and what kind of refinements are necessary to use it for other, more demanding, applications.
• Definition and testing of techniques that can be applied to reduce residual parallaxes and make manual stereoplotting more comfortable.
• Definition and tuning of intrinsic quality check methods and estimation of their reliability.
• Definition of guidelines for taking aerial images with GPS/IMU aiding.

A qualifying part of the project is Pavia’s test site, which already existed and has been enlarged and enriched. The site is 6 x 4 square kilometers wide and more than 170 artificial GCPs have been painted and measured with GPS. There are also 62, well measured, natural GCPs. Over the test site a complex program of four flights was performed.

Two flights were performed with the 150 mm film camera. Each of them is composed of three sub-blocks characterized by the scales 1:5000, 1:9000 and 1:18000. The first two blocks have a complex structure: one ordinary strip is flown twice, back and forth; two cross strips are executed at the head and tail of the block; each of them is re-flown in reverse at the end. Two flights were done with the 300 mm film camera. Each of them is composed of only two sub-blocks with the scales 1:5000 and 1:9000. They have the same structure of the previously described flights.

This concludes the summary of the Italian direct georeferencing activities. The authors hope they have succeeded in describing the intense and interesting activities, which are running in this field in Italy. They would also be pleased if the project itself, the associated dataset or the test site could be an opportunity for collaboration with other researchers.

Authors
Dr. Vittorio Casella and Dr. Riccardo Galetto
DIET - Universita’ di Pavia

Edited by Dr. Mohamed M.R. Mostafa, Applanix Corporation.

Top

Home