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Elevation FAQs - 34 Found

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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 homepage: http://srtm.usgs.gov/index.php. 


The SRTM data were captured at a nominal 30-meter posting over the United States and provided in 1x1 degree blocks. The 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 the 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 data 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 datasets are 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 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. Therefore, localized variations in elevations and geographic features are evident when comparing the NED data and SRTM data. 
 


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





















 



NED



SRTM



Source Data



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



IfSAR data



Source Resolution



High resolution, 10m & 30m DEMs



30-m



Source Dates



1924 - Present



February, 2000 Space Shuttle Endeavour



Source Type



Digital Elevation Model



Digital Surface Model



Vertical Accuracy



2.44m RMSE (root mean square error) Measured



10-m RMSE (Mission Specification)


 

Tags: Maps, Topographic, Data Products