Executive compass | June 2008

Ground-breaking Ortho On-the-Fly tool orthorectifies images in near-real-time

Kurt de Venecia

Kurt de Venecia

It wasn’t long ago that pre-processing of imagery as an advanced or extended step was required for the simplest viewing solutions. One example of pre-processing that is nearly obsolete is epipolar rectification for stereo visualization. An on-the-fly solution for epipolar rectification in the late 1980s to early 1990s required custom hardware and software solutions, affordable only by large government agencies such as the Defense Mapping Agency (DMA). In 1990, these early solutions cost the DMA over $1 million per workstation.

In the early 1990s, commercial photogrammetry software applications, such as SOCET SET®, running on commercial hardware platforms, i.e., Sun®, SGI®, or PC workstations, required pre-processing of left/right stereo pairs for epipolar rectification. The requirements for epipolar rectification produced copies of the original images with a simple affine transformation applied to both left/right images to correct for offsets, rotation, and scales. The resulting image pairs were stored as separate files, which were loaded into the display for comfortable stereo viewing. The minifications, or image pyramids, had to be computed for these derived images as well as the raw ones. There are still products today that create left/right stereo pairs, enabling software applications to work with the imagery. However, on-the-fly epipolar processing and smart photogrammetry software solutions could make these products obsolete.

At the 2008 BAE Systems GXP International User Conference in San Diego last month, we demonstrated ground-breaking new tools for SOCET GXP® v3.0, scheduled for release later this year. One of these tools is Ortho On-the-Fly, which orthorectifies images in near-real-time with minimal mathematical compromises when compared to similar functionality in SOCET SET’s Ortho module. This innovative development is a sign of the times. The number-crunching power of today’s desktop and laptop PCs has increased to remarkable levels. Powerful computers, coupled with the availability of low-cost terrain and raw imagery with fairly accurate metadata, allows tools like Ortho On-the-Fly to be used immediately.

Ortho On-the-Fly orthorectifies raw imagery, and mosaics the component images together to produce a continuous image, which can cover large areas. The large area coverage mosaic can be used for immediate product generation or further analysis. This functionality makes SOCET GXP unique in the marketplace. It offers image analysts, geospatial analysts and mapping professionals an easy- to-use application, as well as a visual conduit into advanced capabilities within the software.

Throughout the GXP User Conference, the Ortho On-the-Fly tool exemplified a recurring theme — that product processing of raw imagery and data sources should be done by the user at the workstation level. Keynote presentations echoed the necessity for analysts to have access to raw data, such as complex radar data, as part of the workflow. Tools like Ortho On-the-Fly allow first responders to view and analyze imagery in a corrected form (orthorectified) or revert back to the raw data.

There are advantages to having the original data in this convenient, new way. For example, mosaicking imagery without orthorectification will result in discontinuities at the join lines between component images, which are likely be confusing or annoying to users. Mosaicking with the Ortho On-the-Fly tool allows for image correction; consequently, these discontinuities are minimized.

On the other hand, raw projective imagery allows true 3D measurement, whereas measurement in a 2D orthorectified mosaic with interpolation in the underlying elevation data to estimate Z is not as good or accurate. Therefore, Ortho On-the-Fly gives analysts the benefit of an orthorectified mosaic from the raw images, thereby ensuring that these raw images remain available for precise measurement or targeting. Finally, the raw imagery is required for triangulation with error propagation, data extraction such as automatic terrain generation, visualization, and other advanced products.

eXtreme Analysis with Ortho-On-the-Fly: a closer look

The example below illustrates how easy it is for terrain and imagery to be combined for an Ortho On-the-Fly result. This example uses SRTM DTED level 1, which has near worldwide coverage that can be purchased for $60 per DVD (region), with all 13 DVDs covering nearly the entire globe, and Digital Globes’ WorldView-1 rigorous imagery in NCDRD format with a horizontal and vertical error of a few meters in this sample. The displacement of the imagery is clearly visible in the non-orthorectified display, whereas the displacement is only a couple of meters after pressing the Ortho On-the-Fly button on the SOCET GXP Multiport. The amount of displacement can be reduced further using SOCET GXP v3.0′s advanced tools for triangulation and automatic terrain generation.

Mosaic with SRTM DTED level 1 and Ortho On-the-Fly.

Mosaic with SRTM DTED level 1 and Ortho On-the-Fly.


Mosaic without terrain and without Ortho On-the-Fly.

Mosaic without terrain and without Ortho On-the-Fly.

The ease of use and real-time processing of raw imagery allows analysts and mapping professionals to produce a variety of products that were once created in advanced or extended processes. SOCET GXP v3.0 provides tools for “do-it-yourself” users, from data analysis to product generation. A do-it-yourself user can improve the accuracy of an Ortho On-the-Fly solution when the georeferencing information of the input imagery and the underlying terrain model is inaccurate. The do-it-yourself user has access to tools such as triangulation and terrain generation/edit that allow higher levels of accuracy when the input data needs improvement. These processes are still part of advanced or extended batch solutions, but they are easily accessible from the Ortho On-the-Fly view or from the SOCET GXP Workspace Manager.

Today’s faster computer processors and data pipes allow on-the-fly process advances to include near-real-time orthorectification and mosaicking for hundreds of images as seen in the SOCET GXP v3.0 Multiport. With tools such as Ortho On-the-Fly, orthorectified products stored as separate files may become as obsolete as storing separate epipolar rectified images files for stereo visualization. Before that happens, there are many places where batch processes remain necessary to allow fully productive image exploitation. One such process is the creation of the image pyramids required to accomplish real-time image zooming.

Eventually, image zooming for viewing, automatic tie point measurement for triangulation, and automatic terrain generation may all be on-the-fly. The benefit of not creating image pyramids would yield a savings of one-third of the disk space consumed by the full-resolution image, as well as the burden of housekeeping and data management for image pyramid files. It’s only a matter of time before on-the-fly processing is standard for many analysis and geospatial product generation workflows.

Sincerely,
Kurt de Venecia
Kurt de Venecia
Director of GXP Product Management
BAE Systems GXP

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