Accurate Mapping from the Comfort of Your Desk

A walking survey can produce very detailed information of the transportation features but it is time consuming with numerous safety concerns. Arial photography can be used to cover large areas but cannot provide the detail required for many survey objectives. New technologies offer opportunities to greatly improve the effectiveness of mass and incremental data collection. One of the newly developed technologies for collecting infrastructure data is mobile mapping. When mobile mapping is combined with walking and aerial surveys accuracy, detail and cost efficiencies can be realized. Mobile mapping systems utilizes Global Positioning Systems (GPS) integrated with an Inertial Navigation Systems (INS) and state-of-the-art cameras.

By integrating GPS and INS, the accurate GPS position is used to update the INS, and the INS then produces the high rate of accurate position and attitude data of mobile mapping system. The INS is needed for continuously measuring camera location and orientation. Combining GPS, INS and Distance Measurement Indicator (DMI) data is a very efficient and accurate method to determine position (lat/long/height), azimuth, pitch and roll angles of the system cameras. The measurements of the INS come from two sensor triads, an accelerometer block and a gyro block. They are defined as three components of the specific force vector and three components of the body rotation rate. Integrated with GPS data, the system geometry data are calculated using the Kalman method.

The integrated GPS/INS solution produces continuous, smooth position and orientation of the system even when the GPS signals are lost due to obstructions such as bridges, trees, tunnels, mountains, high-rise buildings or limited and sporadic satellite coverage.

Another advancement in mobile mapping is the stereovision system that consists of not one but two cameras facing in each direction as stereo pairs of high speed, high-resolution progressive scan cameras. The stereo systems, such as the GPSVision^TM system offered by Lambda Tech International (www.lambdatech.com), captures stereo image pairs while the system drives at posted road speeds.

The images are taken according to an operator-defined distance interval to provide full coverage of the route and its surroundings. By applying sophisticated photogrammetric triangulation technology, any point that appears in any set of two images can be located in a global coordinate system during digitization with Lambda Tech’s Feature Extraction software.

The GPSVision^TM stereo imaging is more than video logging. Stereo imaging allows for determining absolute positions of features such as signs in latitude and longitude to sub-meter accuracy and also allows very accurate relative measurements of all visible roadside attributes, such as the width, height and offset of a sign. Stereo imagery allows for multiple views of the same object with 3D capabilities and the ability to recreate image views where the original cameras never took a picture.

In order to perform precise positioning, it is necessary to calibrate the entire system. The camera geometry, lens distortions, the relative orientation parameters and the offset between the location of the stereovision system and the location of the GPS/INS system on the vehicle must be determined. The camera geometry may be constant for a long time, but the relative and rotation offsets may change between different missions. Therefore, the entire system calibration is divided into two parts. The first part is the calculation of camera parameters using known control points in a test-field. The second is the calculation of the relative orientation and rotation offset based on a co-planarity equation and known constraints. In the second part, the position and rotation parameters of an image pair are used. These calibrations are performed prior to the system collecting data.

Accurate positioning and referencing is the cornerstone of the GPSVision™ technology.

GPSVision^TM specified absolute accuracy for terrestrial data positions is one meter or less depending on the distance the feature to be extracted is from the camera lens. Depending on the image spacing this accuracy can be increased. The GPSVision™ system was designed to deliver sub-meter RMS positions when visible features are within the camera field of view of both image pairs and no farther than 30 meters in front of the camera lenses.

From a photogrammetric perspective, GPSVision^TM is a fix-based stereovision system with known position and attitude provided by the GPS/INS component. Just as a person uses two eyes to determine the distance of an object, every infrastructure feature that is “seen” by the cameras can be triangulated into a three-dimensional coordinate and then transferred into a global coordinate system (e.g., latitude, longitude, height).

GPSVision^TM Feature Extraction software is executable on Microsoft Windows operating systems. It is driven by an external rule base and is language neutral. The user interacts with the software by pointing at features of interest seen in the stereo image pairs with the mouse or stylus. The software then triangulates the relative position of the selected feature and transfers it into the global coordinate system and positioned to within one meter or less of their actual location.

Lambda Tech Group’s One Click Centerline Feature

From an application perspective, the GPSVision^TM system is used to collect digital images along highways, state roads, residential streets, alleys, and railroads while traveling at posted speed limits. These geo-referenced digital images are used for video log applications but most importantly, software is used to position visible physical features, such as poles, curb lines, traffic signs, manholes, pedestals and building locations. In addition, the GPS/INS positioning component creates base maps of the route network for Geographic Information Systems (GIS) base map and Computer Aided Drafting and Design (CADD) applications.

A typical client can use this data along with limited walking surveys and aerial photography to accurately position items of infrastructure and develop detailed base maps complete with image data directly from an operator’s personal workstation in the vehicle or in the office. The geo-referenced digital image data and the position and attribute data of the features are stored in a simple format that can be loaded into the target GIS or other application. Data is easily displayed in map format, analyzed and/or manipulated utilizing GIS database query functions. The data can be supplied to CADD systems to create detailed engineering drawings of pole locations and type, span lengths and attachment heights that can be used to greatly assist in construction and maintenance work and overall more efficient management of the clients’ infrastructure networks.

For more information, contact Lambda Tech International on (001) 260 407 1750 (Fax 260 407 6448) or visit www.lambdatech.com.