Towards automatic near real-time traffic monitoring with an airborne wide angle camera system
© European Conference of Transport Research Institutes (ECTRI) 2008
Received: 19 September 2008
Accepted: 25 November 2008
Published: 7 December 2008
Large area traffic monitoring with high spatial and temporal resolution is a challenge that cannot be served by today available static infrastructure. Therefore, we present an automatic near real-time traffic monitoring approach using data of an airborne digital camera system with a frame rate of up to 3 fps.
By performing direct georeferencing on the obtained aerial images with the use of GPS/IMU data we are able to conduct near real-time traffic data extraction. The traffic processor consists mainly of three steps which are road extraction supported by a priori knowledge of road axes obtained from a road database, vehicle detection by edge extraction, and vehicle tracking based on normalized cross correlation.
Traffic data is obtained with a correctness of up to 79% at a completeness of 68%.
With this system we are able to perform area-wide traffic monitoring with high actuality independent from any stationed infrastructure which makes the system well suited for deployments on demand in case of disasters and mass events.
A society that relies on individual mobility day to day requires sufficient methods for traffic monitoring and guidance. Especially daily commuters want to know travel times for their way to work. Moreover, relief forces are interested in precise travel times for their routing in case of emergencies, mass events, and disasters. However, precise travel time prediction on road networks is one of the most important concerns and challenges in modern transportation and traffic sciences. In order to determine traffic flow on different road types automatically, several approaches are possible. In general, traffic monitoring is mainly based on data from conventional stationary ground measurement systems such as inductive loops, radar sensors or terrestrial cameras. All ground measurement systems embedded in road infrastructure deliver precise traffic data punctually with high temporal resolution, but their spatial distribution is still limited to selected motorways and main roads. The low spatial resolution of these systems makes area-wide traffic monitoring difficult. New approaches collect data by means of mobile measurement units which flow with the traffic. The so called floating car data (FCD, [4, 17]) obtained from taxicabs can deliver useful traffic information within cities, but they are only available in few big cities today. Furthermore, the traffic information available from this source depends on the routes taxicabs drive, but taxi drivers tend to avoid busy roads during rush hours. Hence, only few or no data will be available on roads burdened with commuter traffic. In order to contribute to area-wide traffic monitoring by remote sensing, several projects, based on airborne optical and SAR sensors as well as SAR satellite sensors are currently running at DLR or have already been concluded. In Reinartz et al.  the general suitability of image time series from airborne cameras for traffic monitoring was shown. Tests with several camera systems and various airborne platforms, as well as the development of an airborne traffic monitoring system and thematic image processing software for traffic parameters were performed within the projects “LUMOS” and “Eye in the Sky” [3, 8].
One of the actual projects is called “ARGOS” (AiRborne wide area hiGh altitude mOnitoring System). It aims on traffic monitoring in case of mass events and disasters. It is intended to support security authorities and organisations as well as rescue forces during these occasions. Collected traffic data will be provided to the relief forces via a traffic portal called “DELPHI” (e.g. ). Within the ARGOS project we are currently developing a system that will be able to deliver area-wide traffic data in near real-time by using airborne remote sensing technologies. It is mainly based on our newly developed 3 head digital frame sensor system, namely the “3K camera”. This sensor is capable of wide-angle imagery at a high repetition rate (up to 3 fps). The big advantage of the remote sensing techniques presented here is that the measurements can be applied nearly everywhere (exception: tunnel segments) and there are no dependencies on any third party infrastructure. Restrictions due to clouds and fog are overcome by using airborne SAR data, which will be implemented in the ARGOS project in future. First results on traffic monitoring based on remote sensing SAR systems have been already shown in e.g. Bethke et al. , or Suchandt et al. .
Up to now, there was also a restriction that optical data were not used during nights, but in our approach we show the capability of optical camera data to monitor traffic during nights.
Airborne imagery provides a high spatial resolution combined with acceptable temporal resolution depending on the flight repetition rate. However, automatic traffic monitoring from airborne optical imagery requires complex image analysis methods and traffic models. Moreover, estimates for travel times through the area of aerial surveillance can directly be determined from extracted traffic parameters . Although this prototype airborne traffic monitoring system is still deployed on demand during disasters and mass events, future continuous missions for traffic monitoring in congested urban areas may be possible based on future carriers like unmanned aerial vehicles (UAVs) or high altitude long endurance (HALE) aircrafts.
The publication is arranged as follows: Section 2 gives an overview of the sensor system and the obtained testing data, while Section 3 describes the developed algorithms for traffic monitoring in detail. In Section 4 the results from testing the algorithms are presented. Section 5 demonstrates the night shot capabilities of the system and Section 6 gives conclusions in brief.
2 System and database
The near real-time monitoring system consists of two parts. One part is installed onboard the aircraft, consisting of the 3K camera system, a real-time GPS/IMU unit, one PC for each single camera processing image data, one PC for traffic monitoring tasks, a downlink-antenna with a band width of 30 Mbit/s automatically tracking the ground station, and a PC for steering the antenna. The ground station mainly consists of a parabolic receiving antenna, which is automatically aligned with the antenna at the aircraft, and a PC system for visualization of the down-linked images and traffic data. Given an internet access at the place of the ground station, the obtained traffic data will be directly transferred to the DELPHI traffic portal.
2.1 The 3K-camera
2.2 The onboard system
For processing images acquired by the 3K-camera system in real time we are currently developing a distributed image processing system consisting of five PCs that will be on board of the plane. Each of the three cameras is connected via firewire to one PC. These PCs will be responsible for image acquisition, for orthorectification of images in real time (direct georeferencing) and for street segmentation. The fourth PC will perform vehicle detection and vehicle tracking. The fifth PC mosaikes images and sends them down via an S-Band microwave link. Thus, many image processing modules run concurrently on several PCs. Within the project ARGOS a new middleware called DANAOS1 (Distributed middlewAre for a Near reAl-time mOnitoring System) has been recently developed at DLR. In order to organize the real time modules this middleware is running on each PC. DANAOS handles inter-process communication over the network, provides name services, and synchronizes access to shared memory. The middleware also supports the integration of different time depending processes, which are distributed on a computer network. For direct georeferencing and traffic monitoring several image processing algorithms have been developed and have to be controlled in their time dependencies. This will be the main tasks of the middleware. For increased performance a shared memory access is implemented in DANAOS. This means, that modules are supported to exchange large data, especially image data without copying it explicitly. Thereby, the middleware administrates all shared memory access. For safety computation it monitors the running modules and is able to restart them.
2.3 Direct georeferencing
Direct georeferencing is performed by orthorectifying images using graphic processing units (GPUs) of the PCs. Orthorectification of images is the main process for all further processing steps like road segmentation and car tracking. Only if the subsequent images fit geometrically into the right coordinate system, the overlay with road databases can be achieved. Also it is necessary for integrating the image data into Geographic Information Systems (GIS). Onboard the GPS/IMU data are available in real time with 128 Hz, which are necessary for the orthorectification process. In order to rectify images Digital Surface Models (DSMs) are loaded from a database prior to flight. For holding the appropriate DSM available in memory, a Kalman-Filter is applied estimating the most probable area and triggering the DSM loading process. Then, the DSM covering this area is triangulated as fast as possible and loaded into the GPU. Beyond attitude and position, further parameters of interior and exterior orientation are required for orthorectification: The focal length, and the distortion parameters, as well as the distance from principle point to projection centre have been determined during a laboratory calibration. Up to now, we remove the radial distortion of images from the original image analytically, but we will accelerate the computation by adding an appropriate 3d-mesh to the triangulated DSM. The exterior parameters are estimated prior to traffic monitoring flight campaigns. This is done on-the-fly without ground control points based on automatically matched three-ray tie points in combination with GPS/IMU data .
2.4 Test site and 3K imagery
The processing chain was tested on data obtained at the motorways A95 and A96 near Munich, A 4 near Dresden, and the “Mittlere Ring” in Munich. The “Mittlere Ring” is a circular main road and serves as the backbone for the city traffic in Munich. It and the adjacent Motorways A95 and A96 are used to full capacity regularly on weekdays during rush hour, and are quite populated all day long. Therefore, these roads are good candidates to find a broad spectrum of traffic situations ranging from free flowing traffic to traffic jam. Hence, they are good targets for aerial images obtained for testing traffic monitoring applications. However, data were taken on 30 April 2007 at noon, which was not during rush hour at all. Data acquisition was performed on two flight strips, one flying ENE, covering the A96 and the western part of the “Mittlere Ring”, the other one flying WSW. Thereby, the southern part of the “Mittlere Ring” and the motorway A95 were imaged. The flight height was 1,000 m above ground for both strips which leads to a GSD of 15 cm in the nadir camera and up to 20 cm in the side-look cameras. After that, the flight track was repeated at a flight level of 2,000 m above ground. The data obtained at the motorway in Dresden was recorded during a flight campaign on 4 August 2008 at a flight level of 1,500 m. This campaign was performed in order to validate traffic data extracted from SAR satellite “TerraSAR-X” images, which were recorded at the same time and place.
For further traffic analysis, 3K images were orthorectified using onboard GPS/IMU measurements with an absolute position error of 3 m in nadir images and around one pixel relative. The relative georeferencing error between successive images mainly influences the accuracy of the derived vehicle velocities. Based on simulations and real data, the accuracy of the measured velocity was around 5 km/h depending on the flight height .
2.5 Road database
Data from a road database will be used as a priori information for the automatic detection of road area and vehicles. One of these road databases has been produced by the NAVTEQ Company. The roads are given by polygons which consist of piecewise linear “edges,” grouped as “lines” if the attributes of connected edges are identical. Up to 204 attributes are assigned to each polygon, including the driving direction on motorways, which is important for automated tracking. Recent validations of position accuracy of NAVTEQ road lines resulted in 5 m accuracies for motorways.
3 Processing chain
On the data obtained as described before, the processing chain for traffic monitoring was tested. This experimental processing chain, consisting of several modules can be roughly divided into three major steps. These are road extraction, car detection, and car tracking (see also Fig. 4).
3.1 Road extraction
For an effective real time traffic analysis, the road surface needs to be clearly determined. The road extraction starts by forming a buffer zone around the roads surfaces using a road database as described above as a basis for the buffer formation process. In the next step, two different methods for further feature analysis can be applied. Both modules automatically delineate the roadsides by two linear features. One module works as follows: Within the marked buffer zone, edge detection and feature extraction techniques are used. The edge detection is based on an edge detector proposed by Phillipe Paillau for noisy SAR images . Derived from Deriche filter  and proposed for noisy SAR images, we found this edge detector after ISEF filtering  efficient for our purpose of finding edges along the roadsides and suppressing any other kind of surplus edges and noise present. With this method, mainly the edge between the tarry road area and the vegetation is found. The alternative module searches for the roadside markings by extracting lines on a dynamic threshold image. In this module, only the longest lines are kept representing the drawn through roadside marking lines. As a side effect, the dashed midline markings are detected in this module, too. These markings often cause confusion in the car detection, since they resemble white cars. However, these false alarms can be deleted from car detection, since the module for roadside marking detection finds the dashed midline markings and stores them in a separate class.
3.2 Vehicle detection
With the information of the roadside obtained in the processing step described before, it is possible to restrict vehicle detections and tracking only to the well determined road areas. This increases performance and enhances the accuracy of vehicle detection. Based on this, we developed an algorithm for the detection of vehicles which is described in the following.
A Canny edge algorithm  is applied and a histogram on the edge steepness is calculated. Then, a k-means algorithm is used to split edge steepness statistics into three parts which represent three main classes. These three classes are namely edges belonging to vehicles, edges belonging to roads, and edges within road and vehicle edges, and therefore not yet classifiable.
Edges in the class with lowest steepness are ignored, while edges in the highest steepness class are directly assumed to be due to vehicles. For the histogram part with medium steepness a hysteresis threshold is applied examining neighbourhood in order to assign edges in this class either to the vehicle or the road class. In the next step, the edges belonging to the roadside markings still contaminating the vehicle class are eliminated from the histogram.
3.3 Vehicle tracking
Vehicle tracking is based on matching by normalized cross correlation (e.g. ). Tracking is performed on each consecutive image pair within an exposure burst. With the vehicle detection done on the first image of the burst, vehicle tracking starts with the image pair consisting of the first and second image of an image sequence. For each vehicle detected in the first image, a circular template image of a certain radius (e.g. r = 3 m for cars) is generated at the position of the vehicle detection in the first image. The vehicle position is transferred into the second image. There, a rectangular search window is opened aligned into driving direction starting at the vehicle position obtained from the detection in the first window. Thereby, driving direction is obtained from the road database.
The length of the search window depends on the maximum expected velocity for the road and the time difference between the two images. Then, the normalized cross correlation between the template image and second image is calculated while the template image is shifted all along the search window. The calculated correlation value gives a score for a possible hit. This value obtained lies between 0.0 and 1.0. We store the maximum score and the corresponding position in the second image. Furthermore we require the score to exceed a certain value for keeping it as a hit. We reached maximum correctness with an acceptable completeness in tracking by setting this score threshold to a value of 0.9. A vehicle detection that does not reach this threshold during correlation at any position in the search window is not tracked anymore. The program for tracking can be restarted with the second and the third image (and with further consecutive pairs of the exposure burst in succession) in order to track the vehicles through a whole image sequence. For vehicles that disappear at image borders or below bridges during an exposure of the sequence (but have been detected or tracked in the image before) the tracking algorithm does not dump a match. This means that disappeared vehicles are normally not confused with other vehicles or objects, because of the high matching threshold of 0.9. Vehicles occluded by bridges or other objects may be detected again after reappearance by a new vehicle detection performed on a further exposure sequence. However, they appear as new detections and loose their identification relation, but this is irrelevant on our application. Due to illumination invariance vehicles normally can be tracked if they shift from fully illuminated regions into shadow regions. In order to increase correctness, cross correlation is performed as matching in RGB color space. Here, the average score obtained from cross correlation in each of the three channels is calculated and stored. This helps since vehicles are varicoloured objects. For vehicle tracking on motorways, rotations of the template vehicle image are neglected. This is valid, since the lane change angles on typical velocities obtained on motorway is quite low due to physical reasons, and hence the change in course in between two exposures (at a frame rate of 3 fps) can be neglected. However, for city regions, rotation of the template during correlation can be switched on, but this will rise in calculation time linear with the number of rotation steps during correlation. We accelerate normalized cross correlation by an estimation of the normalization, since calculating the full norm at each position in the search window costs quite a lot of calculation time. Assuming that the illumination situation does not change a lot between two images, an upper limit of the correlation score is estimated for each correlation position in the search window.
Only if this upper limit exceeds the score threshold the exact normalized cross correlation is calculated at that position. For the estimate of the score only the first channel of the RGB-image is used. These arrangements decrease calculation time by a factor of at least four.
Since vehicle tracking based on normalized cross correlation in RGB color space itself works fine at high resolutions, it is sensitive to false vehicle detections. Although several false vehicle detections can be eliminated during tracking as outliers in direction or velocity space, other false alarms still remain in tracking. Especially objects from the dashed lane markings that were detected as vehicles erroneously, may still remain in tracking. This is due to the fact, that the object shape of the dashed markings reappears periodically within a search window and the fact that all of these markings have almost exactly the same shape and intensity. Hence, the focus for improving our traffic monitoring algorithms will be placed in future on improving the vehicle detection module.
We tested our processing chain based on the data take from 30 April 2007 as described in Section 2. For that, the completeness and correctness of vehicle detection and tracking are determined on data of several resolutions, obtained from different flight levels.
4.1 Road detection
Road detection was performed using two different modules. It turned out, that detecting roadside markings for determining the road area is a good strategy on images taken at a lower flight height of 1,000 to 1,500 m resulting in a resolution of 15 to 21 cm GSD. Nevertheless, at higher flight levels (for instance at 2,000 m) road extraction works well with the module searching the edge between blacktop and vegetation. Figure 3 shows typical results of road extraction using roadside markings. Top image shows the line extraction, whereas in the image in the middle the finally extracted roadsides after smoothing and closing gaps are shown. Bottom image shows road extraction on a nadir image taken from a flight height of 1,500 m.
4.2 Vehicle detection
Results on testing vehicle detection on data obtained at several test sites (from different flight heights)
Motorway (1,000 m)
Motorway (1,500 m)
Motorway (2,000 m)
City (1,000 m)
4.3 Vehicle tracking
On images with a resolution of 15 cm GSD, vehicle tracking on motorways performs perfectly well, with a correctness of better than 95% and a completeness of almost 100% on each image pair. On images obtained from higher flight levels (≥30 cm GSD) tracking still works fine with a completeness of 90% while having a correctness of 75%. We attribute the good tracking performance on low flight heights to the fact that with a resolution of 15 cm GSD vehicle details like sunroof, windscreen and backlight, and body type go into the correlation which simplifies the search for the correct match. However, these details are not anymore seen at higher flight levels.
Traffic monitoring requires actual traffic parameters. Thus, we are planning to execute the extraction of traffic parameters on the 3 × 16 Mpix RGB-images in near real-time with high performance. Till now, tests on road and vehicle detection as well as vehicle tracking were performed on actual standard hardware consisting of a dual-core PC with a CPU frequency of 1.86 GHz and 2 GB RAM. The first generation of research programs for road extraction, vehicle detection, and vehicle tracking was developed within DLR in-house image processing software XDibias (X-Window DIgital Bavarian Image Analysis System), based on C code. With this XDibias based prototype of the processing chain, computing times of about 2 min for images covering an area of 1 km2 were achieved for a whole traffic extraction. In order to guarantee high actuality of traffic information and to enable near-real time traffic data extraction, the research modules were accelerated using the Machine Vision Library “HALCON” . This library provides fast implementations of image processing operators due to the use of extended processor instruction sets like MMX and SSE(2), as well as due to parallel processing on multi-core CPUs. By replacing the operators used in the first generation of the traffic processor with the fast HALCON operators, we are now able to extract traffic data from images covering an area of 1 km2 within less than 1 min.
Road extraction on a typical motorway takes less than 10 s for one nadir and two side-look exposures in total. Vehicle detection on these three images needs 20 s of calculation time on the present system. In comparison, car tracking is quite fast, consuming only 15 s for a tracking of 3 × 15 cars over an image sequence consisting of 3 × 4 images. Moreover, the pure calculation time for cross correlation is 30 ms per vehicle for a tracking through the whole sequence. In total, it costs less than 60 s to analyse the traffic within one image sequence. However, the onboard computer system for traffic monitoring will possess a multi-core CPU with at least four cores. By sufficient parallelization of the processes that will be managed by the middleware DANAOS, we expect to be able reducing the processing time by a factor of 2. That means, assuming a break of 7 s between each image “burst” (which would result in a overlap of 10% between two image “bursts” at a flight speed of 60 m/s and a flight height of 1,000 m), we will have a time overhead in the processing chain for traffic monitoring of a factor of 4. However, the prototype of our processing chain is built up still modular, which means that each module in the chain reads an image from hard disk into memory, performs an operation, and at the end writes a new image to hard disk. We estimate to halve the overhead by reducing hard disk read/write. Nevertheless we are already able to perform automatic traffic data extraction on a large amount of data in near real-time. Therefore, the system already provides area wide traffic data with a high actuality, with capacities of increasing performance in near future.
5 Night shot capability
During a test flight on 6 May 2008 from 9:40 pm until 10:28 pm near the city Rosenheim and the motorway junction “Inntaldreieck” we were able to show the capabilities of the 3K camera system for traffic monitoring applications at night. Two strips which cover a part of the city (strip A) of Rosenheim and the motorway (strip B) were acquired repeatedly with different camera configurations. Flight height was 2,000 m above ground, flight speed was 65 m/s. For this test flight, the sensor was set into a special configuration called along-track modus. In order to increase the chance for recording car headlights the camera platform was rotated with an angle of 90° azimuthally. Hence, one of the former side-looking cameras was aligned in flight direction, the other side-looking camera was now looking in backward direction. With an off nadir angle of 35° and a flight direction along the motorways we expected the forward camera being able to detect headlights of forthcoming vehicles, and the backward camera to record the headlights of cars travelling in flight direction. In this configuration, a nadir image covers an area of 1.4 × 1.0 km; the ground pixel size is around 29 cm.
Figure 7 shows two orthorectified images (A-1 and A-2) from the city of Rosenheim taken with different camera configuration. As the exposure time in A-2 with 1/512 s is double than in A-1, more lights from the city of Rosenheim are visible, but also the motion blurring is more visible.
Gray values of vehicle head- and taillights and maximum total blurring
Vehicle headlights R/G/B
Max. blurringa (m)
∅ 50/–/– Max R = 109
0.06 + 0.04
∅ 102/80/61 Max R = 192
Max R = 48
0.13 + 0.08
∅ 59/46/35 Max R = 67
0.13 + 0.08
∅ 82/–/– Max R = 182
0.08 + 0.05
∅ 46/–/– Max R = 82
0.08 + 0.05
The accuracy of speed determination is influenced not only by the accuracy of georeferencing but also from blurring effects caused by the exposure time, as the distance measurement is not so precise. In the examples in Fig. 8 it could be seen, that vehicles are detected by head- and taillights from the front as well as from the side, i.e. traffic in different directions can be detected. Information about completeness and correctness of vehicle visibility are not available as no ground truth data were acquired. Furthermore, no algorithms for automatic traffic data extraction on night shots are available at this time and have to be developed in future. Hence, we could show that using this optical sensor system for traffic monitoring under night conditions is basically possible, which might be an interesting field for future research and development.
Despite the large amount of incoming data from the wide angle camera system, we are able to perform traffic data extraction with high actuality in near real-time. This means that the processing chain is capable to perform a complete traffic data extraction on an area of 1 km2 within few 10 s. Thereby, high accuracies for velocities (5 km/h), good correctness in vehicle detection (79%) and in vehicle tracking (90% of detected vehicles) is reached. Furthermore, the system performs image orthorectification in real-time using GPU computing power. Although algorithms for automatic traffic monitoring at night have not yet been developed the capability of the system to provide traffic information at night has been demonstrated successfully during a test flight.
Hence, the investigations show the high potential using aerial wide angle image time series for traffic monitoring and similar applications, like the estimation of travel times or the derivation of other relevant traffic parameters. In future, the data processing speed will be further improved by converting the modules of the processing chain into tasks that share memory access to image data stored in the RAM instead of reading and writing them on hard disk, as done by our prototype. We further plan to evaluate the performance of the system in case of difficult scenes such as large cities with high buildings occluding parts of the roads and under various weather conditions (e.g. snow, wet roads) during three campaigns in 2009. Moreover it is planned to use an additional radar sensor providing traffic data in case of bad visibility conditions (e.g. clouds, fog) where remote sensing traffic monitoring based on optical sensors fails.
The whole system is thought to be a technology test bed for future traffic monitoring applications and it is in operation at a DLR research aircraft. This limits the operations at the moment only to campaigns on demand, like mass events or in case of disasters. However, this prototype of a traffic monitoring system or a successor version of this system could be mounted to any other carrier such as UAV or HALE in future. This would enable continuous and area-wide traffic monitoring in metropolitan areas at high actuality without the use of stationary infrastructure.
DANAOS was king of ARGOS in the fifteenth century before Chr.
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