Developing better digital terrain elevation models

By on 27 March, 2023

Artist’s impression of the TanDEM-X synthetic aperture radar satellite. Courtesy DLR.

With a 50-year legacy, Digital Terrain Elevation Data inches towards its next goal: 10-metre post spacing.

When US National Geospatial-Intelligence Agency (NGA) partners plan operations, the agency provides them with products that identify the latitude and longitude of various locations, so that they can pinpoint the path to their objective. Yet, sometimes a partner needs more than latitude and longitude to get the job done.

That’s where digital terrain elevation data comes in.

“It gives the Z value of every location on the Earth,” David Bidinger, a Digital Terrain Elevation Data (DTED) lead in the Source Office of Geomatics, explained. “If you want to know how far a location is above sea level, what the slope of an area is or surface roughness you need a Z value, and you get that from DTED.”

Digital terrain elevation data is a standard of digital datasets which consists of a matrix of terrain elevation values that are essential to creating a wide range of GEOINT products for disaster relief, humanitarian aid, international cooperation, scientific research and other national security objectives.

For example, analysts in Analysis’ Office of Science and Methodologies — which applies leading scientific technologies to GEOINT applications — have used DTED to help both civil and military partners to find areas of significant flooding while estimating its impact on local facilities and infrastructure.

Meanwhile, NGA’s International Affairs directorate uses DTED during its engagements with foreign partners to support host-nation or combined military operations training. This elevation data is key to help tanks and other military equipment navigate varied terrain effectively; providing military planners a better idea of where they can conduct cross-country transit of both light and heavy equipment.

“When you are moving ordinance [supplies] through the slopes in Afghanistan, anything more than a 10 to 30% slope would make it difficult to move equipment,” Bidinger said.

Ups and downs

Digital terrain elevation data has been in production since 1968 when the US Air Force’s Aeronautical Chart and Information Center, the oceanographic and charting services of the US Naval Hydrographic Office, and the US Army Map Service used it to support plastic relief mapping. These organisations later merged into the Defense Mapping Agency — an NGA predecessor agency — in 1972.

By the early 1980s, DTED had proved its value to Defense Mapping Agency (DMA) and its partners in a wide range of mapping, charting and geodesy applications. But DMA leaders quickly found that gathering and processing DTED data was expensive.

A massive 36-bit UNIVAC 1108 computer system like the ones that DMA used to process DTED data could cost up US$3 million each, depending on which accessories and service package the purchaser bought. The UNIVAC 1108 was an early computer used in the 1960s, 1970s and even in the early 1980s. Yet, the computer system operated inefficiently and was prone to errors and memory overload, as a 1978 government report noted.

Meanwhile, as the calendar turned from the 1980s to the 1990s — and computer technologies improved — DMA likewise boosted its engineering abilities in the digital terrain elevation data arena. As the tools and techniques became available, the resolution of DTED products improved.

By the early 1990s, NGA’s military partners could use DTED to help it plan a variety of military operations — such as air navigation — while the disaster responders could use data to prepare for floods, hurricanes and other natural disasters.

But even higher resolution digital terrain elevation data — at 30-metre post spacing [the horizontal measurement between two elevation readings measured in arc-seconds] — wasn’t high enough resolution for targeting, said Bidinger. And despite the increased processing power of computers in the late 1990s, obtaining and storing higher resolution DTED data remained expensive. Much of that elevation data was collected manually, using stereo imagery on a plotter, or collected off a map source.

“It was very labour-intensive,” Tyler Shipp, senior GEOINT officer for elevation in Source’s Foundation GEOINT Group, said of the DTED collection efforts.

Through this data acquisition process, NGA obtained 2,063 one-degree by one-degree cells of traditional digital terrain elevation data level-2 that comprise 8% of the Earth. Those 2,063 cells take up 42 gigabytes of disk space, making it impractical to put that data on the servers that most NGA partners own. Even today, DTED data is sent to NGA partners on compact disk, instead of being uploaded to a shared server or being sent by email.

Traditional DTED level-0, level-1 and level-2 products were captured using what is known as ‘bare earth’ data. That is to say, traditional DTED contains elevation data from the ground surface, and does not capture information about treetops and building heights, said Bidinger.

A Shuttle Radar Topography Mission-generated image showing New Zealand’s impressive terrain. Courtesy NASA/JPL/NGA.

To make it easier to capture elevation data, in the early 2000s, NGA shifted to the Shuttle Radar Topographic Mission. SRTM was an international research effort that obtained digital elevation models from 57 degrees south of the equator to 60 degrees north.

SRTM — a February 2000 mission of the space shuttle Endeavour — is a reflective surface digital elevation model or digital surface model, a DSM, meaning that the elevation readings from the first radar return indicates treetops and tops of buildings. While SRTM is distinct from DTED, it can be saved in the same file format as traditional DTED. The shuttle mission obtained reflective elevation data, meaning that the data showed ‘bumps’ that indicated treetops and building heights.

The resolution of SRTM elevation data was leaps and bounds better than previously captured DTED data. In fact, a November 2008 article noted: “The improved DTED contributed incomparable accuracy to everything from geophysics to earthquake research to the best location for cell towers to missile targeting.”

Yet, NGA still didn’t have a global coverage for elevation data.

Data for DTED

To fix the problem, NGA shifted again — this time to TanDEM-X, a reflective elevation data set that a German satellite collected, between 2010 and 2015. TanDEM-X data (which NGA obtained starting in 2016) is more current than legacy DTED data and has global coverage. Additionally, TanDEM-X is more accurate than the legacy digital terrain elevation data because it was collected at 10- to 12-metre post spacing, compared with DTED level-2, which was collected at 30-metre post spacing.

TanDEM-X fills the elevation data void seen in legacy DTED and SRTM data — and does it at a higher resolution than NGA previously had.

NGA’s US government partners are using TanDEM-X data to support their missions, even though the raw, unprocessed data has voids and anomalies that still need to corrected. As a temporary fix, those voids are filled in with legacy DTED and SRTM data, before the TanDEM-X data is put into GeoTIFF format.

These GeoTIFF files, in turn, are put on NGA’s Map of the World — a tool that enables NGA and its partners to visualise and access integrated intelligence content tied to accurate and reliable geographic features on Earth. The GeoTIFFs are also loaded on the Geospatial Repository and Data Management System, a database for the storage, processing, visualisation and dissemination of three-dimensional point cloud data. These GeoTIFFs can likewise be put on compact disks and other storage devices.

But processing the huge volumes of TanDEM-X data and correcting the anomalies takes a lot of work. To help complete the work, NGA has joined forces with 31 other nations to finish a 12-metre resolution elevation dataset that has global coverage.

The partnership, known as the TanDEM-X High Resolution Data Exchange is working to edit the raw data and stitch it together into a seamless global elevation dataset. The goal of TREx is to have a finished TanTEM-X dataset by early 2026. But even as NGA works with the TREx consortium, traditional DTED still has value… and it isn’t going away.

For example, organisations that require elevation data, but which don’t have access to TanDEM-X data, will continue to use older DTED. TanDEM-X data, on the other hand, is unclassified, but still LIMDIS. LIMDIS is short for limited dissemination, a measure the US government takes to protect information that is not publicly releasable, but isn’t at the secret level.

“What’s most impressive about DTED is that it’s been around for almost 50 years, and is still the most requested elevation product by our military and other partners,” said Bidinger. “There is an extensive range of functional use for DTED such as aeronautical, maritime, topographical, controlled imagery, mission planning and analysis.”

Article courtesy of NGA

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