Mobile mapping … cutting-edge technology for the development of digital maps
Description of the mobile mapping system
The mobile mapping system (MMS) used within Tele Atlas has the digital cameras mounted on a vehicle (van). At least two of the digital cameras are installed in stereoscopic pair, spaced apart so that the combined image can provide 3-dimensional information. The capturing speed of the images is linked to the speed of the vehicle up to 3 images per second to obtain a complete recording of the information along the road network.
Figure 1: Mobile Mapping van
The picture resolution which is used per camera is typically 1200 * 960. Although higher resolution cameras can be mounted, this resolution was experienced as a good balance between data-storage requirements and features and attributes to be recognized.
The camera configuration used is typically between 4-6 (2 forward, 2 right side, 1 back-ward and 1 left side looking camera), depending on the roads types to be surveyed.
Figure 2: Various camera configurations - Mobile Mapping van
With these configurations, >95% of the visual components of the required objects/features can be recognized. Specifically for house-numbers and street name signs, the accuracy is lower and specific camera mounting is needed per country and per situation.
The positioning devices calculate the co-ordinates of a specific point on the vehicle. Following positioning sensors are included:
- 5Hz differential GPS unit
- 100Hz single axis gyro
- Odometer with rear wheel instrumentation
Because orientation and distance of the cameras versus that point are known, the captured images can be used as photogrammetric source material.
To handle the high incoming data stream, 1-4 PC's (one PC is linked to 2 cameras) are mounted in the back of the van, including UPS back-up facilities.
Figure 3: Front & rear view of the MMS equipment
mounted in the mobile mapping van
The back-end of the mobile mapping system is an in-house processing unit that allows the visualisation of the imagery. It also allows photogrammetric measurements of objects on the images and the linking of the images and measurements to map data.
The Tele Atlas Mobile Mapping System (MMS) consists of two main components:
- MMS vehicle software & hardware to capture MMS data in the field;
- Post-process (PP) tools to handle & interpret MMS data (including vehicle position calculation).
The post-process tools have three major roles:
- Quality verification: tools that are used either in the vehicle or just after data arrival in the production offices, to determine integrity and quality of the session files and eliminate any issues before the data goes to production;
- Data streaming & converting: tools that are used to extract session positional information or convert images to different file format;
- Data interpretation & database operation software: This is carried out by a combination of licensed and proprietary Tele Atlas software.
The production environment is built up in a dual screen mode environment, and has a link between the displayed imagery on one side and the vehicle position and camera views translated on the map on the other side. One component of the software focuses on the measurement and interpretation of the imagery, the other component focuses on the mapping of the features and attributes in the navigable database.
Description of the mobile mapping process
The MMS vehicle is used in field to gather
- a sequence of images (taken by cameras situated on different vehicle sides),
- positional information and
- calibration parameters.
This information is stored in a session. A typical session is a record of 30-60 minutes of driving. This is the optimal balance between positional accuracy, time of work and post-proccess data handling. After the session is stored on a local hard drive it is backed-up, and send to the production office, where trained operators use post-processing tools for creating new or updating existing features in the navigable database.
Because of the high reliability and data stream demand, the MMS vehicle is only used for basic data streaming and data interpretation of positional information & camera frames. More sophisticated algorithms are handled in post-processing tools.
This approach gives the following advantages:
- The MMS vehicle can capture data at high frequencies. No data is lost in the capturing phase.
- If any error is detected in data interpretation and processing algorithms, the problem can be solved without a single need to redrive the area of interest (which gives a significant cost & time reduction).
The vehicle fleet operates to a carefully prepared schedule that aims to ensure all major roadways are checked frequently and that all other roads are checked regularly.
What do we capture on basis of mobile mapping?
The databases created and maintained by Tele Atlas are used for route calculation, route guidance, turn-by-turn navigation and other geospatial applications. They contain particular map features and attributes.
There are five main categories of features and attributes:
- Addressing features, such as street names;
- Routing features such as importance and type of road, direction of traffic flow direction;
- Turn-by-turn information such as accurate road geometry and manoeuvre information;
- Visual interface features e.g. land cover and usage, for cartographic representation;
- Guidance features e.g. signposts, lane information, traffic signs.
As digital map-based applications become more sophisticated, with systems such as ADAS, it increases the need to keep map databases up to date and to ensure complete accuracy. In addition, as new uses and use extensions are devised, such as 3-D mapping, the level of information and detail required also increases.
The imagery materials collected by the MMS do provide to a large extent the necessary reference material to create and/or maintain the features and attributes referred to above as illustrated in some example imagery in figure 4 below.
Figure 4: Examples of high quality images
collected by means of Mobile Mapping