There are several methods used today to obtain high accuracy digital elevation models (DEMs), including LiDAR point clouds, photogrammetric analysis of aerial photos, and satellite and aerial stereo imagery. The geographic location, size of the area and type of terrain all contribute to the selection of a preferred approach on a case-by-case basis. Satellite stereo imagery is a cost effective way to obtain access to a high precision DEM, especially over remote and large areas. In locations that have flying restrictions or where there could be security risks for a site survey team on the ground, satellite imaging might be the only option. High resolution satellite imagery can support a host of applications around engineering and design for transportation, infrastructure, telecom networks, utilities, and land use management.

GeoEye, a provider of geospatial information and services based in Herndon, Va., works with clients to obtain DEMs for 3D modeling. “We are seeing increasing demand for high resolution stereo satellite imagery where there is a need for three-dimensional viewing and feature recognition, such as DEM extraction, stereo visualization of natural features and building height extraction,” said Andy Stephenson, GeoEye senior regional director of sales in Asia. “Applications include a variety of construction and planning projects that require accurate mapping of buildings, airports, ports, harbor, railways and entire cities. We also see demand increasing for large remote areas where timely access is important.”

As the use of 3D modeling becomes more common for a variety of applications, the need for affordable automated feature extraction software tools will also grow, Stephenson said. “Historically, costs have been very high for labor intensive feature extraction services that can take weeks or months, depending on the size of the area being processed,” he said. “The software technology is being pushed to keep up with the high resolution imagery that is now available in various forms to expedite feature extraction.”

GeoEye-1 multispectral stereo imagery of Russia
Multispectral stereo imagery from the high resolution GeoEye-1 satellite is already being used in Russia to support ongoing monitoring and support services for a railway project. Image courtesy of GeoEye.

Railway mapping is a particularly appropriate application for satellite stereo imagery because the geographic location may encompass very remote areas and dense population centers, the area of interest may be corridors thousands of miles long, and the terrain may vary from mountains to plains. The DEM itself is needed to get a precise understanding of the underlying terrain at the planning and design phase of the railway track so planners can take into consideration the changes in slope and elevation and optimize the route, from both a cost perspective (i.e., the shortest path) as well as the safety aspects.

A railroad network, like any strategic transportation infrastructure, is characterized by its sheer size and intense 24/7 operations, often in challenging landscapes and severe climate conditions. Development of Earth remote sensing techniques makes it possible to use imagery as a source of up-to-date and reliable geospatial information, which is necessary to provide efficient management of a complex geographically distributed system. Railways may use GeoEye data in combination with other sources to originally plan and design a railway and to maintain existing railway infrastructure.

GeoEye recently announced an order from the Government of India for 50-centimeter pan-sharpened multispectral stereo imagery from the high resolution GeoEye-1 satellite to support development of India's dedicated freight railway corridors. The cost of stereo imagery is approximately 60 percent more than for mono images. RITES, a Government of India Enterprise, is purchasing a total of 6,000 square kilometers in a 3-kilometer-wide strip along two railway corridors in India. The National Remote Sensing Centre (NRSC), GeoEye's Master Reseller for India, is facilitating the fulfillment of the contract.

GeoEye is tasking GeoEye-1 specifically for this project, and collections have already commenced. The project is expected to be complete by May or June 2013, before the start of the monsoons in India. GeoEye-1 is able to collect a single stereo image in one pass using bi-directional scanning capability. The satellite collects panchromatic and multispectral images simultaneously, which are then merged to create the 50-centimeter pan-sharpened multispectral product. This capability is ideal for large-area mapping projects.

“High resolution imagery remotely sensed by satellites fills a need in some locations that cannot be met by other methods of data collection, particularly in countries that restrict flying permits or present other challenges to airplanes and flight crews on the ground,” concluded Stephenson. “In the future it would not be inconceivable to see instances in Asia and other parts of the world where satellite and aerial collection is used together to provide complementary product offerings, to provide clients comparable results at lower costs more quickly.”