Home Articles Multi-Sensor Survey – A Complete Data Set as Source of Numerous Applications

Multi-Sensor Survey – A Complete Data Set as Source of Numerous Applications


Peter Goellner
Head of Representative Office Geokosmos International Representative Office in Germany
[email protected]

Bad Soden (Germany)
Oranien str., 13
D-65812 Bad Soden, Germany
Tel.: +49 6196 655 081
Fax: +49 6196 655 010

Abstract
Geokosmos is an acknowledged leader in the air survey market with its core activity based on technological effectiveness – application of the state-of-the-art technologies of the 3D aerial, terrain scanning and bathymetry in concert with the digital aerial photography; productivity – full service on surveying of any scope in the shortest time and the ability to conduct its tasks in any part of the world. In 2006, Geokosmos started the field works focussed on multi-purpose aerial survey of a city territory using aircraft with onboard equipment consisting of LiDAR system ALTM 3100, large-format digital camera Vexcel UtraCam-D, and two inclined digital metric cameras Rollei AIC-modular-LS. That was the first practical application of such complex equipment, demonstrating different data collection technologies. The combined sensors provided the data for multi-purpose use. Besides creating orthophoto imagery, calculating digital terrain (DTM) and elevation (DEM) models, there is a set of oblique images, which allow the creation of 3D virtual models of urban territories. For the ambitious project, the specifications as well as key parameters and major results are provided. Several application areas for the complex equipment are also described. Main focus is given to the sensor application for the purpose of urban and infrastructure planning and management. The combined sensor equipment allows to increase the productivity of the survey as well as reducing dramatically the volume of terrestrial surveying works which result in significant saving.

Introduction
Since its foundation (1993) Geokosmos has been steadily developing application of up-to-the-date technologies of geospatial data collection and processing. In recent years one of the most important spheres of Geokosmos activities has been aerial survey using a laser scanner (LiDAR) and digital cameras. Area application of an UltraCam-D large scale camera has been started since 2006 in addition to used aerial laser scanners and Rollei AIC-modular-LS (H25) medium digital cameras. Having such excellent technical capability and experience the company became concerned with performance gain. On the other hand, modern consumers of geospatial data show interest in new forms of such data presentation and in obtaining complex regular survey materials, which contain, in addition to traditional forms, such products as 3D models and oblique photographs. First of all, it refers to urban territories including housing developments and territories of large manufacturing facilities. On such prerequisites in 2006 Geokosmos launched an investment project of a complex survey of a large Russian regional and industrial center – Nizhny Novgorod and adjacent areas.

Project purposes and main characteristics
The project purpose at the first stage may be shortly worded as obtaining of initial georeferenced data combining high accuracy, multiple forms and a large volume. Information obtained as a result of survey should be suitable for its multi-purpose application by regional or city administration and individual state and private companies. It was supposed that survey data processing would result in creation of multiple GIS products used as an efficient tool in urban planning and property complex management. The total project area is about 2,200 km2. It includes territory within the city limits (including urban territory), and non-urban areas or areas of industrial facilities adjacent to the city line. The task of the first project stage was to survey the territory and obtain all necessary data for the following output materials and products:

  • digital ground map, scale 1:10000
  • digital ground map, scale 1:2000 (for a part of the territory)
  • digital ground map, scale 1:1000 (for a part of the territory),
  • Orthophotomaps of the same,
  • Digital elevation model,
  • Slant photographs,
  • 3D models of developed areas.

Equipment complex
Based on the set task the following equipment was prepared and installed on board of An30: UltraCam-D full-scale digital air survey camera for vertical aerial photography; 2 Rollei AIC-modular-LS (H25) digital metric middle scale cameras for oblique photography from the left and right boards ( ALTM 3100 Airborne Laser scanner (LiDAR) for lidar survey

UltraCam-D large-scale digital aerial camera UltraCam-D specifications:

  • The picture size is 11500 x 7500 pixel
  • The back focal distance is 100 mm
  • The pixel size is 0.009 mm
  • The photometrical resolution is 12 bits
  • The exposure range is from 1/500 sec to 1/60 sec
  • The compensation of motion aberration is 50 pixels
  • The minimum interval between exposures is 1 sec
  • The on-board storage capacity is 2,692 photographs

Rollei AIC-modular-LS (H25) specifications:

  • The back focal lens distance is 82 mm
  • The picture size is 5440 ? 4080 pixel
  • The pixel size is 0.009 mm
  • The shutter type is central
  • The minimum exposure is 1/1000 sec
  • The photometrical resolution is 16 bits
  • The minimum interval between exposures is 4 sec
  • Photogrammetric calibration had been performed before cameras were installed on the board

ALTM 3100 on-board laser scanning system specifications:

  • The height range (?) is from 80 m to 3,500 m
  • The pulse frequency is from 33 kHz to100 kHz
  • The scan swatch width is up to 0.93 ? ?
  • The vertical accuracy is 1/2000 ? ?
  • The elevation accuracy is 15 cm at ?=1200 m
  • 25 cm at ?=2000 m
  • 35 cm at ?=3000 m
  • The pulse frequency is up to 70 Hz
  • The number of registered returns is 4
  • The beam spread is 0.0003 rad or 0.0008 rad

The used airplane – ?n30 – is a special photographic survey airplane, which is widely used for such purposes in Russia. ALTM 3100 camera and LiDAR have been installed in special holes for surveying in airplane bottom UltraCam-D digital camera has been installed on a gyro-stabilized platform, and ALTM3100 – on a specially designed hard platform. Middle format metric cameras have been installed on specially designed small side holes on the bottom from starboard and portside. A special platform (Fig. 1) has been designed for this purpose, which rigidly fixes cameras with relation to the aircraft body in a required position to the hole to ensure a set angle. Rollei cameras have been designed to make oblique photographs with obliquity of 50 degrees, UltraCam-D camera is designed for plane surveying and further creation of an orthophotomap and an outline of a digital map, ALTM laser scanner is designed to obtain digital surface models, digital elevation models and further creation of 3D models.

Aerial survey parameters
The aerial survey parameters have been designed basing on the set up goals and tasks, objective conditions (flight altitude over the city) and the need for mutual coordination of survey parameters for different equipment components. The following parameters have been chosen:

  • the flight altitude is 800 m;
  • the flight speed is 320 km/h;
  • the distance between routes is 405 m;
  • the route configuration is two sets of routes in mutually normal directions.

These values of general parameters for aerial survey using the UltraCam-D camera ensured pixel size equal to 72 mm and capture bandwidth equal to 828 m. The forward overlap was 60 %, the lateral overlap was 51%. Designed values of aerial survey for perspective (oblique) surveying using the Rollei AIC-modular-LS (H25) cameras were the following:

the tilt angle (to the vertical) was 50 °;
the photograph orientation is a long side along the route axle;
the size pixel afield

– for a frame near side is 11 cm;
– for a frame middle is 14 cm;
– for a far side of the frame is 20 cm; the lateral overlap is 40%
the interval between exposures is 4 sec
the forward overlap
– for the near side frame near side is 40%
– for a frame middle is 53 %
– for a far side of the frame is 66 %.
The following parameters of LiDAR surveying were ensured at the designed flight altitude:

An average error in determining plane coordinates in determining laser return points in relation to the nearest GPS station is not more than 0.4 m;

An average error in determining altitude coordinates in determining laser return points in relation to the nearest GPS station is not more than 0.15 m;

An average point density for each route is not less than 1.4 for 1 m2.

Processing of survey materials and results
Processing of survey data included the following main operations:
Post – processing of flight GPS and IMU measurements;
Post-processing of UltraCam-D photographs from level 0 to level 3;
Calibration of angle units of cameras exposition:
UltraCam-D and Rollei AIC-modular-LS cameras;
Processing of LiDAR data and creation of 3D cloud model and digital elevation model;
Orthorectification of UltraCam-D pictures and orthophotomap mosaic;
Creation of a MultiVision project with oblique photographs;
Creation of a textured 3D-model using a laser points cloud and oblique photographs.

Fig. 2. shows a photograph made with a UltraCam-D camera and its blow up, which makes it possible to judge about quality and details of a photograph. Fig. 3. Digital aerial photograph received with UltraCam-D camera and its blow up. An oblique photograph and its blow up are given in Fig. 4. Angle parameters of UltraCam-D and Rollei AIC-modular-LS cameras expositions were determined by photographic triangulation for small-size blocks. A unit with 4 routes by 11 photographs was created for an UltraCam-D camera. Points, easily identified in oblique photographs were selected and measured manually. No control survey points of detail, but 4 check points were used for block leveling. Root-mean-square values of discrepancies at check points were (m): 0.054 0.035 0.133 correspondingly for X, Y, H. Exposure angle elements for the Rollei AIC-modular-LS camera were determined by the unit – 43 photographs, 4 routes, two routes for each camera. Well- recognizable points of a photogrammetric network built basing on UltraCam-D photographs were used as control survey points and check points. All in all 15 control survey and 10 check points were used. Root-mean-square values of discrepancies at control survey points were (m): ): 0.170 0.148 0.098; at check points – 0.396 0.342 0.458.

Obtained parameters of angle exposure were further used for geo fixing and processing of horizontal and oblique photographs. Within the framework of this project Geokosmos together with MultiVision has carried out a pilot project of preparation and integration of oblique photographic surveying data and other information into the MultiVision system to master the technology with using oblique digital aerial photographs in combination with a digital orthophotomap as an information basis for visual space data. Data of a small volume, which include 211 oblique and 100 horizontal photographs, were used for this purpose. Photographs made with the UltraCam-D camera using the digital elevation model created basing on laser scanning data were used to create a digital orthophotomap and its accuracy was estimated, which is characterized by a mean-root-square error of about 7 sm. An orthophotomap with further assessment of its accuracy was created to control accuracy and quality of geo fixing of oblique photographs. Its accuracy is characterized by error up to 70 sm.

Angle elements of oblique photographs absolute orientation were determined using the SpecialOne (MultiVision) program by manual measurement of common points in the orthophotomap and oblique photographs. 211 photographs have been orientated. Then a project in the MultiVision environment was created, which provided for the following:
creation of a catalogue structure, copying of raster data,
conversion of the digital elevation model file and photograph files,
data encoding using MultiVision special features.

The MultiVision created project was used to assess system operations with oblique photographs including:
accuracy assessment (absolute accuracy is characterized by an error not more than 1.0 m);
possibility to measure object dimensions;
option of creating 3D models of objects;
data export to ARC Gis

analysis of convenience of oblique photograph interpretation in the environment.
Fig. 5 and 6 show examples of measurement taking on oblique photographs for creation of a building 3D model. Fig. 6 gives an idea abut interpretation possibilities of oblique photographs as compared with an orthophotomap. The obtained set of data of a multi-purpose aerial survey made it possible to make a step towards creations of textured 3D models created basing a cloud of points obtained as a result of LiDAR survey. Having significant experience in this sphere and a possibility of 3D GModeler, Geokosmos has designed software for texturing of 3D models of a cloud of such points with phototexture of oblique photographs and elaborated the technology of such model creation. Fig. 7 shows an example of a 3D model built using project data.

Summary
Geokosmos has successfully carried out an innovation project of complex survey of urban areas based on aerial survey advanced technologies with simultaneous use of an onboard LiDAR, a large-scale digital camera and two middle-format cameras used for making oblique photographs.

This first example of practical application of such complicated airborne equipment proved a possibility of successful integration of different technologies and facilities used to make survey from an airplane. Technological processes of complex aerial survey and further processing of obtained data were improved during the project implementation.

The obtained results are characterized by high accuracy and information content and may be efficiently used for different purposes. Combination of such different methods of aerial survey helps to obtain multiple output data for multi-purpose application, including an information system, which works with oblique photographs and 3D models in addition to traditional products (an orthophoto, a digital map, a digital elevation model). The implemented approach makes it possible to create a set of geospatial data, which are an efficient tool for urban land planning and management.