A New Photogrametric Workflow
Vexcel Imaging’s Ultramap Platform

Figure 1: UltraCam X digital aerial camera system with the Sensor Unit (right) and the airborne Computing Unit including two removable Data Units (left).

Digital large format aerial cameras have been used for aerial survey and mapping operations for several years. One example is the UltraCamX, developed by Vexcel Imaging GmbH, Graz, Austria. UltraCamX was introduced into the market in May 2006 and is currently the largest digital frame camera for photogrammetric applications. Since May 2006, Vexcel has been operating under the ownership of Microsoft Corp. and as such contributes to the Microsoft Virtual Earth Initiative. This huge project is based on high quality aerial images for fully automated, three dimensional, digital re-construction of urban areas. Vexcel now introduces a new processing software called
UltraMap Platform which implements a complete photogrammetric workflow including quality control and visualization.

By Alexander Wiechert and Michael Gruber

Vexcel Imaging GmbH is headquartered in Graz, Austria and known for its manufacture of photogrammetric devices. The company’s first product was the precision film scanner UltraScan5000. In May 2003, the digital large format camera system UltraCamD was presen­ted at the ASPRS conference and exhibition in Anchorage, Alaska, and three years later the new large format digital aerial camera system, the UltraCamX was introduced to the international mapping market at ASPRS 06 in Reno, Nevada.

Tab. 1: Technical Data and Specifications of the UltraCamX Senor Unit

Both, UltraCamD and UltraCamX were successfully introduced into the global mapping market and almost 100 Units have been sold up to May of this year. Since Microsoft has acquired Vexcel this camera is used as the sole airborne optical mapping sensor for the Virtual Earth Initiative. With the success of its imaging systems, Vexcel has now introduced a new processing software called UltraMap Platform, which implements a complete photogramme­tric workflow.

UltraCamX, the Largest Digital Frame Camera

UltraCamX is a digital large format frame ca­mera and at the moment offers the largest image format among mapping sensors. Even if the size of each frame is 136 Megapixels the camera will allow a short frame interval of 1.35 seconds and therefore is able to produce stereo overlaps for large scale applications. This feature is very much appreciated when undertaking large scale mapping projects in urban areas.

Figure 2: The UltraCamX sensor head (left) consists of 8 camera heads, 4 of them contributing to the large format panchromatic image. These 4 heads are equipped with 9 CCD sensors in their 4 focal planes. The focal plane of the so called Master Cone (M) carries 4 CCDs (right).

 

Figure 3: Aerial image of very high intra scene contrast (snow and ice vs. shadows and dark rocks). The vertical profile shows more than 7000 intensity levels (12.9 bit). The lower image series show digitally enhanced image portion and demonstrates show the high contrast scene at different gamma setting. Bright areas are displayed best on the left, structure in the dark shadows are better visible on the right.

The most considerable advantages of UltraCam X are:

• large image format of 14430 pixels cross track and 9420 pixels along track
• excellent optical system with 100 mm focal length for the panchromatic camera heads and 33 mm for the multi spectral camera heads
• best in class geometric accuracy >12 bit radiometric performance
• 1.35 sec. frame interval
• The cam image storage capacity of 4700 frames for one single data storage unit
• almost unlimited image harvest due to exchangeable data storage units
• instant data download from the airplane by removable data storage units
• fast data transfer to the post processing system by the new docking station

The camera consists of the sensor unit, the onboard storage and data capture system, the operator interface panel and two removable data storage units. Software to operate the camera and process the image data after the flight mission completes the system (figure 1).

The UltraCam X Sensor Head

The basic design of the UltraCamX sensor is unique (figure 2). It consists of eight independent camera cones, four contributing to the large format panchromatic image, and four contributing to the multi spectral image. The sensor head of the UltraCamX is equipped with 13 FTF5033 high performance CCD sensor units, each producing 16 mega pixels of image information at a radiometric bandwidth of more than 12 bit.

The transition to 7.2 µm CCD sensors caused a redesign of the optical system which is able to resolve the 70 lp/mm of the CCD pixel grid. In cooperation with LINOS/Rodenstock, a high performance optical system was developed with a focal length of 100 mm for the panchromatic cones and a focal length of 33 mm for the multi spectral cones. This set of two lenses supports the pan sharpening ratio of 1:3.

The image format of 14430 pixels cross track and 9420 pixels in flight direction contributes to productivity in the air. With a 25 % side overlap between strips the UltraCamX covers more than one mile, or 1650 m, at 6 inch pixel size.

Figure 4: Results of the Automatic Aerotriangulation from several UltraCamX testflights at 10 cm GSD (1380 m AGL).Sigma_o values are in the range of 0.7 µm to 1.6 µm. The average value of sigma_o is less than 1 µm.

 The UltraCamX on Board Storage System

The new data storage system of the UltraCamX improves the end-to-end workflow of the aerial mission and vastly improves the operational activities of the aerial crew. The system contains two independent data units for redundant image capture. The data units are able to capture up to 4700 images of 136 mega pixels each and – most valuable for large scale missions – can be replaced by spare units within a few minutes. Thus one can increase the entire number of images for one single mission by a factor of two or three, and enjoy practically unlimited onboard image storage capacity. Disconnecting the data units from the camera system after the completion of a flight mission and shipping the raw data to the office becomes a very simple task.

The downloading of the image data is suppor­ted by a docking station, which allows complete data transfer of 4000 images within 8 hours, through four parallel data transfer channels. Therefore, a 24 hour cycle of flying, copying and QC can be achieved.

UltraCamX exploits the radiometric quality of the high performance CCD sensor FTF5033 manufactured by DALSA. Not much less then 13 bit of radiometric information can be extracted via the 14 bit analog/digital converter. Such broad bandwidth will resolve the dark and bright areas in the same scene, such as those found in a city area on a bright sunny day, with dark shadows in the streets and almost white roofs. Similarly, images of mountainous areas with bright snow and dark rocks and shadows

Geometric Accuracy at the 1 Micrometer Level

After the geometric laboratory calibration the performance of every Ultra­CamX is verified by a flight mission over a well known test area. A flight pattern with high overlap (80% endlap, 60% sidelap) and cross strips offers a redundant dataset which allows detailed investigation of the interior geometry of the camera.

The automatic tie point matching is done using INPHO’s aerial triangulation software package, Match AT. The sigma_o value reflects the quality level of image coordinate measurements of an aerial triangulation project. Such values have been computed for several UltraCamX image datasets. The sigma_o values of the flight missions shown in figure 4 are close to, or smaller than, 1 µm at that huge redundancy of high overlaps and additional cross strips. In addition, cross check and additional self calibration options were applied using BINGO. 

UltraMap Platform

Digital aerial frame cameras are able to produce enormous volumes of image data. One can imagine that handling large amounts of images can be a problem in the office. After each processing step, the operator requires a QC (quality control) check to ensure that all processed data fulfil the required quality expectations. At best, this is done not on an image-by-image manner, but should be done within the whole block. Since aerial cameras are now producing imagery with increased numbers of pixels (~130 Mpix, UltraCamX), handling aerial imagery is becoming a problem for the operator. The situation gets worse if the QC requires the visua­lization of geo-located high resolution images, in order to validate whole projects (usually >2,000 images, about 253 Gpix). In most cases, quick views (down sampled to 1/100 of the ori­ginal size) are used to carry out a quick quality check. The downside of this approach is obvious. Neighbouring information cannot be taken into account if a block of images should be evaluated. In addition, the quick views may not reflect the actual image content since they are also radiometrically reduced (8 bits instead of the original radiometric resolution). 

The UltraMap Platform (UMP) has been designed to provide a complete photogrammetric workflow to the UltraCam user, which includes interactive visualization and QC tools:

•           Complete and integrated photogrammetric workflow

•           Data download and ship packs

•           Level 0-2 and level 2-3 processing

•           Complete aero triangulation

•           Photogrammetric bundle adjustment

•           Automated tie point collection

•           Guided ground control point measurements

•           Camera auto calibration

•           Quality control

            o          Interactive visualization

            o          Overlap

            o          GPS/IMU accuracy

            o          Crab angle

            o          Radiometry

UMP eases the handling of large amounts of data by using cutting-edge technologies based on Microsoft’s .NET and Seadragon technology. It provides:

•           Complete 16 bit image pipeline

•           Supports distributed processing

            o          Level 0-2 and level 2-3

            o          Central system management

            o          Load balancing

•           And fully integrates UltraCam specific hardware such as the DX and DKS docking station

To handle the huge quantity of images UMP utilises a new technical-based approach called “Image Pyramids”. This allows fast access to multi-resolution image data. During visualization, the required information is retrieved from the various images and used for fast display. With this approach, the visualization performance only depends on the screen resolution and not on the resolution of the images or the number of images. Another advantage of this approach is that it will maintain the high dynamic range of images (>12 bit) within the pyramid data. The 8 bit conversion is done directly on the graphics card and can therefore easily be changed interactively for the whole block.

Consistent Model

All images are bundled to one consistent model. The model allows fast access to image-related information such as position, orientation, camera parameters, and so forth. The application uses this information to apply the required parameters to the visualization mo­dule. The model also has the capability to recover missing GPS positions, automatically detect strips, and check for consistency. 

Visualization Modes

UMP supports multiple visualization modes. During runtime of the application, the user can switch between different modes on-line. The supported modes are as follows:

• Footprint view: This shows the image outline projected down to the ground level using the available GPS and/or IMU data.
• Indexmap: The indexmap shows a block of images using the position and orientation of each image. By using a slider the scale of the footprints can be changed interactively
• Heatmap: The heatmap is a visualization type for showing the degree of overlap of the image block. The color-coded regions allow immediate visual recognition of flight patterns.
• Thumbnail: The thumbnail mode allows a more semantic image clustering. For instance, images may be grouped by their strip number or histogram statistics.
• Overlays: The overlay concept is designed for visualizing additional meta information on top of image data. This may include image IDs as text, footprints, projection centers, ground control points, tie points, and so forth.
  
  

Figure 6: Close-up view of individual high resolution images.
Left images shows a high crab angle of the inspected image. The right image gives a 100% view.

Interaction

Besides the visualization, the interaction capability is also a very important aspect. Easy pan and zoom functionality is done by using the mouse. Image selection is important for grouping or removing individual images. High level interaction is required when it comes to measuring (ground) control points. Especially for the last task, the multi-image block visualization eases tremendously the amount of time for interaction (figure 5).        

We tested our method with block sizes of 2,000 high resolution UltraCamX images, with given GPS information, and track over ground angle. Our approach allows for seamless browsing through the whole image collection, beginning with an overview of the image, to a close-up view (100%) of individual images. Radiometric inconsistency, missing images, and high crab angles can be seen in one view (left image of figure 5). The right image of figure 5 shows a heatmap. A heatmap gives the information about the degree of overlap. Green regions show high overlap, whereas red regions lower overlap.

The user can then browse seamlessly from the overall block overview to the individual image as shown in figure 6. Every single detail of the original image can be explored. In addition, heavy crab angles are made obvious at once.
           

Conclusion

Vexcel Imaging GmbH has announced its new UltraMap Platform, a new photogrammetric workflow for distributed processing, quality control and visualization of large amounts of UltraCam images. UltraMap is based on the

latest Microsoft technologies, namely .NET and Seadragon. It provides an unparalleled set of functionality to support the end user with efficient image processing, quality control and image handling. UltraMap Platform leverages the productivity of the UltraCam and defines a new industry standard for the processing and handling of large projects. UMP clearly demonstrates the synergy achieved by Vexcel as a Microsoft company and also shows the strong innovation momentum of Microsoft Vexcel.

Alexander Weichert is General Manager of Vexcel Imaging GmbH. Michael Gruber is Chief Scientist Photogrammetry at Vexcel. For more information, have a look at www.ultracamx.com www.microsoft.com/ultracam