perspectivesheader.jpg

A digital elevation model (DEM) represents the elevation of Earth’s surface, including features (vegetation, buildings, etc.). A digital terrain model (DTM) provides a bare earth representation of terrain or surface topography. Both are highly useful data sets for visualizing our planet for scientific and commercial landscape study.

Traditionally elevation was derived by direct survey of the land, then photogrammetry techniques allowed for terrain extraction with a pair of images with stereoscopic techniques. Today, highly accurate digital elevation can be achieved with light detection and ranging (LIDAR) and interferometric synthetic aperture radar (IfSAR). LIDAR transmits laser pulses that bounce off the Earth’s surface and are measured. IFSAR is a radar technique that uses stereo pairs of radar images and photogrammetric processing to derive the Earth’s surface.

With each technological advancement, the digital elevation models have improved in accuracy, resulting in a much more useful model of the Earth.

Data Sources and Resolution

To date, global data is available in approximately 1 km resolution from GTOPO30 and 90 m accuracy from the Shuttle Radar Topography Mission (SRTM).

A number of government IfSAR satellites are in orbit, the European Space Agency has ERS-2 (30 m) and more recently ASAR aboard Envisat (30 m), and the Canadian Space Agency has Radarsat-1 (10 m) and will soon launch Radarsat-2 (3 m). The new public/private partnership between the German government and Infoterra launched the TerraSAR-X satellite and is already capable of delivery elevation data in 1-2 meter accuracy. These space-based platforms are crucial for wide-area coverage and their frequent revisit rates means that a valuable archive will be built for scientific observation.

Commercial airborne-based elevation collection is ongoing as well. Intermap Technologies began an aggressive collection of IFSAR elevation data back in the late 90’s, and has delivered entire country data sets. Their NEXTMap program is active in Europe (with projected completion by the end of 2007) and the United States (with completion slated for the end of 2008). Intermap delivers elevation accuracy of .5 meter.

LIDAR service companies collect data on a project basis at an elevation resolution of roughly 15 cm. There are a number of LIDAR Services companies with headquarters in the United States that are conducting large-scale projects — EarthData has a project underway to collect the entire state of North Carolina, Merrick & Company has completed a number of large-scale projects including a project in Colombia, South America, 3001 and MJ Harden (now a part of GeoEye) are also very active.

While LIDAR delivers a more precise measurement, the advantage of radar technologies is that they cut through clouds and can be flown at night. Therefore, IFSAR is a much more efficient process for capturing large geographies.

Applications

Detailed elevation models are used for a wide variety of scientific and commercial purposes. Hydrology or water modeling is one of the premier applications as water flows downward and the greater the elevation accuracy the better the water model. Water models also help determine flood extent and assess flood damage.

Marine observations of wave heights and ocean characteristics can be determined by satellite elevation sensors, and highly accurate elevation in coastal zones greatly help the study of coastal change and storm impacts. Observing forests with elevation sensors returns forest characteristics such as crown height and canopy cover.

Geological land observations are also dependent on accurate elevation to determine such things as land subsidence, landslides or avalanches, and extracting geomorphological information.

Elevation data is critical for creating 2D relief maps, 3D flythroughs, and physical raised-relief models. Elevation in a city provides lie-of-sight information and an understanding of light and shadow that are important for energy consumption studies. A growing interest in city model collection will depend on accurate DEM and DTM data.

Elevation Projects of Note

In the United States there’s a large multi-year project underway to modernize the Federal Emergency Management Association (FEMA) flood maps. The National Flood Insurance Program relies heavily on accurate flood maps to reduce risk and limit payouts when flood disasters occur. Much of the current map products are paper based. The effort underway aims for modern Web-based mapping products with much greater accuracy by 2010.

The National Geodetic Survey has also been testing a variety of different elevation technologies in order to map the 95,000 miles of U.S. coastline that they’re responsible for.

The Federal Aviation Administration is also in the process of modernizing with a GPS-assisted Next Generation Air Transportation System (NGATS). Billions of dollars will be spent on this upgrade, and elevation data will likely be a key component.

Future Uses

Intermap Technologies has been one of the most active companies in exploring and exploiting new opportunities in the elevation data space. They have a transportation group that has tapped into intelligent transportation and automated driver assistance initiatives.

With accurate elevation data and know position, vehicles can respond to their surroundings. The use of data and systems in the long-haul trucking industry could allow the engine to respond in advance to terrain changes, saving both on maintenance and on fuel consumption. There’s also talk about adaptive lighting with beams from headlights following road changes. Driver safety becomes a huge benefit of this added awareness.

Infrastructure projects have just begun to exploit large-scale and accurate elevation data. Most design projects don’t take the earth surface into account. As this mindset changes, we’ll see an increasing opportunity for high-level accuracy for the design of more efficient systems.

Elevation is a critical component of any earth observation, whether you’re using a bare-earth digital terrain model or a detailed digital elevation model with ground features. As the accuracy of elevation observations have increases on an ever-broadening scale, this data has grown in importance and utility. I’m certain we’ll see a growing number of interesting applications for elevation data in the next few years as more and better data becomes available.

Read Jeff Thurston’s thoughts here.

Most Commented Posts