Elevation - 16 of 34

Elevation FAQs - 34 Found

What are the NED data sources?

NED source data are selected from an inventory of standard production USGS digital elevation models (DEMs), and from an increasing number of datasets that are project- or agency-specific. Quality and currency are primary factors in determining if a dataset will be ingested into the NED. General categories of NED source data are characterized below:


o  High-resolution data (3 meters or better), is typically derived from lidar or digital photogrammetry, and often with edited water bodies.


o  Moderate-resolution raster data (4 to 10 meters) is typically derived from digital photogrammetry and IFSAR 10-meter DEMs derived from 1:24,000-scale cartographic contours and hydrography. Such data were produced by the USGS as a standard elevation product, and though they are presently the predominant source in the NED, they continue to be replaced as better source data are acquired.


o  30-meter DEMs from 1:24,000-scale cartographic contours which are similar in most respects to their 10-meter counterparts, though usually of lower overall quality. These DEMs are only used as source for Puerto Rico and as void fill around the conterminous U.S.


o  30-meter photogrammetrically derived DEMs, which are the oldest DEMs in the 7.5-minute series. These data were derived directly from stereo photography, either by a human operator or by an early form of electronic image correlation. They are badly marred by production artifacts that are addressed to the greatest practical extent by digital filtering within the NED production process. These DEMs are the source for most of Canada and all of Mexico.


o  1 arc-second Shuttle Radar Topography Mission (SRTM) data are the NED source over the Aleutian Islands.


o  2 arc-second DEMs are a standard USGS product. They are derived from cartographic contours at a scale of 1:63,360 over the state of Alaska.


o  3 arc-second DEMs are another standard USGS product, and are generally only used within the NED as a source of fill values over large water bodies and small slivers at the borders with US and Canada/Mexico.


o  Bathymetric data for some coastal areas are collected using lidar, sonar, and other methods of subsurface terrain measurement, transformed to a common datum, and resampled to only 1/9 arc-second resolution within the NED.

Tags: Maps, Topographic, Data Products