Keywords: Mobile Mapping, Range Data, Sensor Calibration, INS / GPS
Abstract: Laser mapping has become quite popular in recent days due to it’s capability of
providing information directly in three dimension. However, the present laser mapping systems
are either air-borne or ground-based (on a static platform). We cannot achieve detail information
from air-borne system, though it has it’s own suitability for applications like DEM generation.
Ground based static systems are not suitable for larger area mapping purpose.
The vehicle-borne laser mapping system (VLMS) use laser scanners for three dimensional data
acquisition, CCD cameras for texture information, GPS, INS and odometer for positioning
information. The data obtained by this system could be a good resource for developing urban 3-D
database, which has numerous applications in the field of virtual reality, car navigation,
computer games, planning and management.
In this paper, we discuss about the system architecture of VLMS, calibration of sensors,
integration of all these sensors and positioning devices for direct geo-referencing.
1 Introduction
The mobile mapping technology has been developed since late 1980’s. The development of the
mobile mapping system became possible due to the availability of GPS signal to the civilian
community. This system is capable of observing the objects at closer range, thus giving greater
details. As per our knowledge, so far, the vehicle based mobile mapping system are based on
CCD cameras (in combination with video camera in some cases) for data acquisition.
Combination of GPS with either INS or Gyro is used for navigation purpose. For reference of
some of these systems, refer GPS-VanTM (Bossler et al, 1991), VISAT-Van (Schwarz et al,
1993, Li et al, 1994), TruckMapTM (Pottle, 1995), KiSS (Hock et al, 1995), GPS Vision (He,
1996) and GeoMaster (Tamura et al, 1998).
In our system, we have used laser scanners as the main data acquisition device. The system is
supplemented by CCD cameras for texture information and as usual combination of GPS, INS
and odometer is used for position and attitude information.
2 System Configuration
The mobile mapping system is shown in figure 1. It consists of four CCD cameras, four laser
scanners, Hybrid Inertial Survey System (HISS) which consists of DGPS, INS and electronic
odometer for position and orientation data.
Figure 1. Mobile Mapping Survey Vehicle and the Sensors on top of the vehicle
2.1 HISS (The GPS/INS Integration)
The Hybrid Inertial Survey System (HISS) is the integration of GPS, INS and Odometer. The
integrated system has an accuracy of about one-meter in horizontal plane. The system once
initialized can work either on DGPS/INS mode or Odometer/INS mode. The switching between
the two modes is automatic, which is based on PDOP (if PDOP > 4.0) and number of visible
satellites (if no. of sat < 4). The preference is always given to DGPS/INS mode. We have
conducted some tests to see the behavior of the DGPS alone and DGPS/INS results at stationary
mode (while the vehicle is not moving). For details on HISS and it’s field test results, refer
(Tamura et al,1998).
2.2 Laser Scanner
Four laser scanners are fixed on top of the vehicle at four different directions as shown in figure
1. The purpose of using four units is to capture the data from different viewing angle so that as
much of data can be extracted as possible even when the target is occluded by pedestrians or
moving vehicles. The laser scanner head rotates around it’s head (shown in figure 2). It provides
the distance from the laser head to the target. This distance is later converted into 3-D
coordinates. The configuration of laser scanner is given in Table 1.
Figure 2. CCD and Laser sensor units
| Meas. Distance Range |
50m(reflection rate 20%) |
| Accuracy of Range Measurement |
3cm(1) |
| Scanning Range |
300° |
| Angular Range Step |
0.25° |
| Frequency |
10Hz |
Table 1. Laser Scanner configuration
2.3 CCD Camera
Four CCD cameras are used with parabolic reflecting mirrors. Each unit of CCD camera and
Laser scanner are housed in a single frame. Table 2 shows the configuration of the CCD camera.
| Photograph Element
| 1/2 inch color CCD
|
| Resolution
| 659x494
|
| Output
| RGB
|
Trigger
shutter
| 1/60
1/10000
|
| Lens Mount
| C mount
|
Table 2. Configuration of CCD Camera
