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Elevation FAQs - 39 Found

How does NED compare to SRTM?

The Shuttle Radar Topography Mission (SRTM) digital elevation dataset was collected by a modified radar system that flew on board the Space Shuttle Endeavor in February 2000, using a technique known as Interferometric Synthetic Aperture Radar (IfSAR) to generate the most complete high-resolution digital topographic dataset of the earth. SRTM data were collected for approximately 80% of the earth's surface. Read more about the mission at the SRTM home page.

SRTM data were captured at a nominal 30-meter posting over the United States and provided in 1x1 degree blocks. SRTM data were later edited to remove significant spikes and delineate water bodies and coastlines, but certain issues persist. Water features depicted in the SRTM data were derived from ancillary source imagery resulting in significant temporal differences between water features and the SRTM data. Significant voids and above-ground elevations exist in areas such as tree canopy due to the sensor's measurement of reflective surfaces.

A key difference between NED and SRTM data is that source data used to produce the NED are processed to represent bare ground readings and do not contain elevated surfaces. NED is developed from a variety of source data, including photogrammetrically derived terrain data, digitized contours, lidar, and IfSAR. Source data undergo rigorous quality assurance to qualify as input to the NED layers (see the FAQ on "How are the highest quality elevation data sources selected for deriving the NED seamless layers?").

Many of these source data that contribute to the NED are collected at much higher resolutions and have much greater inherent accuracies than the SRTM data. While source data are continually collected and used to update the NED layers, some source datasets pre-date the SRTM data. Therefore, localized variations in elevations and geographic features are evident when comparing the NED and SRTM data. 

The chart below shows differences between the NED and SRTM data collections:




Source Data

Photogrammetry/Aerial Data/Digitized Contours/Lidar/IfSAR Data

IfSAR data

Source Resolution

High resolution, 10m & 30m DEMs


Source Dates

1924 - Present

February, 2000 Space Shuttle Endeavour

Source Type

Digital Elevation Model

Digital Surface Model

Vertical Accuracy

1.55m RMSE (root mean square error)*

10m RMSE (Mission Specification)

*Accuracy Assessment of the U.S. Geological Survey National Elevation Dataset, and Comparison with Other Large-Area Elevation Datasets—SRTM and ASTER: U.S. Geological Survey Open-File Report 2014–1008

Tags: Maps, Topographic, Data Products