| Airborne/Modeling: New Technology Attacks High-Cost DEMs
By Mark E. Romano Producing low cost, high-accuracy, high-resolution Digital Elevation Models (DEMs) and digital imagery requires new remote sensing techniques to eliminate the labor intensive tasks of traditional aerial and ground survey methods.
In a project being conducted under a NASA Earth Observations Commercial Applications Program (EOCAP) contract, EagleScan Inc. (ESI) of Boulder, Colo. has applied new processes and hardware that eliminate the need for high resolution photography in generating DEMs. Other factors such as fast turn around for timely data, and cost are key success criteria for ESI's EOCAP partnership.
EagleScan commenced operations in 1994, concentrating primarily on the environmental, mining, forestry, construction, and urban planning markets. Applications within those areas include terrain modeling, multilayer vegetation mapping, dimensioning of buildings, stock pile surveillance, and cut and fill planning. ESI's approach stresses cost effectiveness as compared to traditional methods where high resolution can be synonymous with high cost. To increase productivity in remote sensing techniques, the Digital Airborne Topographic Imaging System (DATIS) was developed. The DATIS consists of a high-accuracy scanning Laser Detection and Ranging (LADAR) system coupled with an onboard GPS, an inertial measurement unit, and a high-resolution digital camera. System parameters such as laser pulse rate, swath width, and image size are adjustable to meet end-user needs. The high-accuracy DEM (AccuDEM) and DEM Draped Images (AccuDRAPE) products are provided in a wide variety of available formats, including digital files for contour maps, planimetric views, and most common GIS data bases. High-resolution imagery can be orthorectified, mosaicked, or DEM draped as customer needs warrant. Attacking the Cost Problem
Several affordability issues are associated with high-resolution DEMs and imagery: 1) the cost to produce quality DEMs and imagery for delivery, 2) the increased cost in remote sensing products as accuracy and resolution increases, and 3) the intangible yet important costs associated with timely delivery of current DEM and image products.
The cost for generating DEMs using conventional approaches can become significant for increased resolution, accuracy, and especially number of elevation postings. Costs associated with various methods of generating DEMs are directly reflective of the labor-intensive and time-consuming processes currently used. Bringing new technology to bear and implementing new concepts to generate terrain elevation data addresses these issues head-on. By directly measuring topography with a LADAR and by collecting imagery in digital form, the DATIS process improves the productivity of producing data files for workstation use. The result of the DATIS process is higher density, directly measured DEMs and digital imagery that can result in reduced cost and quicker product delivery. This new approach is estimated to reduce cost by 20-50 percent over current methods.
As the accuracy and resolution needs for photogrammetric DEM generation increase by a factor of two, the cost typically increases by a factor of four. The stereo photogrammetric approach is directly related to image resolution considerations: as accuracy increases by 2x, area resolution increases by 4x, data points increase by 4x and so on. This increase factor has a direct impact on the end product costs of the photogrammetry. The cost and schedule savings achieved using the DATIS approach therefore become even more significant with increasing requirements for higher resolutions and better accuracy. Producing High-Resolution DEMS
Nominal DATIS sample spacing for elevation posting is 3-5 meters over a 1-kilometer swath width. This spacing results in a measured point saturation that can reach 100,000 points in a typical 1-square-kilometer image. Even higher densities can be achieved by using narrower swath widths, higher laser pulse rates, reduced air speeds, and multiple overflights. Achieving this type of measured point density from a typical photogrammetric approach can be extremely costly. Imagine manually generating 100,000 points per square kilometer! Current elevation accuracies exceed 15 cm elevation with better than 1-meter geolocation.
These new techniques were recently applied in The Gray Mountain Project. One of ESI's first EOCAP projects, it was selected as a test site because of its dramatic mix of topography including extremely steep terrain, deep canyons, trees, boulders, roads, houses, and other unique features. The amount of structure produced in multiple types of terrain was a key performance acceptance criteria for the DATIS system. The results met initial design criteria and yielded extremely detailed laser imagery. Figure 4 shows a perspective vies of a nominal 1 kilometer gray shade AccuDEM generated using the DATIS workstation less than 6 hours after collection. The data used to produce the surface-shaded terrain model is of sufficient detail and resolution to produce 1-foot contour interval maps of the area, contoured perspective views of the terrain, image-draped DEMs, and realistic automated fly-throughs of the model.
Producing a high-accuracy, high resolution orthophoto requires a high-resolution DEM. The better the quality of the DEM, the better and more accurate the orthophoto or orthoimage. But achieving a high-resolution DEM no longer requires high-resolution photography. EagleScan has separated that process and can acquire a DEM directly, so end users requiring only DEMs pay for only DEMs. Assessing Complementary Digital Imagery
Will ESI's new methodology replace the need for imagery generated using aerial photography ? Not currently. In many cases, aerial photography still has benefits over the current digital camera technology. However, photographic data must converted to digital format which causes image quality and dynamic range degradation. The EagleScan panchromatic digital camera contains a 2024 x 2044 pixel array with Ground Sample Distance (GSD) of 0.25 to 0.5 meters, so for higher spatial resolutions and large collection areas, photography still has an advantage. However, digital camera arrays are fast approaching the capability to achieve scanned photographic spatial resolutions. Figure 2 shows a typical 1-square-kilometer image at 0.5 meter GSD. Delivering Timely Data
More often than not, data for large-area projects, such as urban planning and change detection, take months to collect, process, and deliver using traditional methods. Many project files can become obsolete before the user ever sees them. Another part of ESI's EOCAP program is demonstrating quick turn-around and currency of delivered remote sensing products. Since imagery is collected and stored digitally, EagleScan imagery is literally available for viewing immediately after landing. The Gray Mountain Project panchromatic image was viewed by the pilot and test team in the aircraft hanger using a PC workstation less than 10 minutes after landing. The DEM for the same project was processed and ready for viewing just a few hours later.
End-user costs associated with obtaining DEM and image products in a short time cycle can be considerable. Generation of digital end products using digital collection and direct measurement methods have a direct, significant impact on the product cycle time. Since availability of DEMs can significantly constrain a user's production and delivery schedule, the EagleScan process offers considerable time leverage in this important area. Quick turnaround of imagery and DEMs can mean the difference in winning jobs and increasing productivity an profitability. Defeating Adverse Conditions to Collect DEMS
Many times shadowing effects due to sensor angular displacement can reduce or prevent acquiring accurate ground observations. High-density urban and metropolitan areas are sometimes extremely difficult to map due to obstructions and shadowing effects. Figure 3, a surface-shaded model image of metropolitan Denver, Colo., shows how the effects of shadowing have been minimized using direct DATIS measurements. The direct measurement technique also permits high-accuracy results on extremely flat and low-contrast surfaces, a situation that can present significant difficulties for traditional approaches for generating DEMs.
The weather has also long been a problem when collecting data by traditional aerial methods. ESI's concept has implemented new techniques to deal with more severe aircraft attitude motion, hence attacking this limitation as well. ESI's second EOCAP project was conducted to acquire data over the Denver metropolitan district. The Denver metro data was collected in severe weather conditions for photography, including wind gusts exceeding 85 knots, yet excellent results were achieved because ESI's process determines precise aircraft position, attitude, and accelerations during the entire data collection procedure. Each data point is then corrected automatically to yield data products void of effects from platform motion and accurately geopositioned without ground control. By eliminating the need for imagery in the DEM creation process, high wind, overcast skies, and hazy days are no longer limiting factors to acquiring data. DEM data can even be acquired at night if imagery is not required. Another area hampering DEM generation in the past has been deciduous growth areas. Typically, topographic generation has been a very seasonal process in many areas. Very narrow photographic time windows exist in many areas between leaf off, snowfall, and leaf on. For good stereo photogrammetry contrast and true penetration to the ground, leaves and snow have always been obstacles. Except for multiple canopy rain forests, true ground heights can usually be achieved with the DATIS system during the majority of the year. In fact, snow depth can be accurately tracked on a very large-scale area with ESI's high sample densities, opening new possible applications for snow pack and snow depth monitoring. Anticipating the Future
The future is the information age. The computer revolution and more RS/GIS users with new ideas are bringing increased requirements and demand for digital remote sensing data. Today's computer literacy is growing exponentially. This increased knowledge, coupled with affordable, high-powered PC platforms and software, will fuel tomorrow's GIS user base. The need for more affordable, timely, high-resolution data products will be the result. Implementing new techniques, concepts and technology, such as EagleScan's DATIS sensor coupled with AccuDEM and AccuDRAPE processes, can address these requirements and demands in a timely, cost-effective manner. Future development of aerial and space based methods will bring costs down and open up a much greater range of products to the GIS community. These changes will benefit all market segments and their suppliers. As digital sensor technology increases and platform memory costs decrease, GIS users will benefit from greatly enhanced lower cost digitally collected data with unique capabilities. About the Author:
Mark Romano is manager, applications engineering at EagleScan Inc. in Boulder, Colo. Back |
