Digital Spatial Data

Digital Elevation Model

Overview

A digital elevation model (DEM) is a digital file consisting of terrain elevations for ground positions at regularly spaced horizontal intervals. Various levels of digital elevation products are available. Although all are identical in the manner the data are structured, each varies in sampling interval, geographic reference system, areas of coverage, and accuracy; with the primary differing characteristics being the spacing, or sampling interval, of the data.
(Picture: watersheds polygon over shaded DEM)

USGS NED (National Elevation Dataset)

• Description: Seamless & best available US Geological Survey elevation data. The USGS NED has been developed by merging the highest-resolution, best quality elevation data available across the United States into a seamless raster format. NED is the result of the maturation of the USGS effort to provide 1:24,000-scale (or 1 arc second) DEM data for the conterminous US and 1:63,360-scale DEM data for Alaska. As more 1/3 arc second (10m) data covers the US, then this will also be a seamless dataset.
• Coverage: US 
• Spatial resolution/spacing: 1 arc second (30 meters - seamless dataset) or 1/3 arc second (10 meters - being developed for about half of the US as of August 2004)
• Dates: N/A
• Format: ESRI Grid
• Projection: Geographic, decimal degrees projection, horizontal datum - NAD83, vertical datum NADV88,elevation unit is meters

(Shuttle Radar Topography Mission) - US

• Description: Digital raster elevation data developed by USGS in cooperation with NASA.
• Coverage: US
• Spatial resolution/spacing: 1 arc second (30 meters)
• Dates: N/A
• Format: ESRI Grid
• Projection: Geographic, decimal degrees projection, horizontal datum - NAD83, vertical datum NADV88, elevation unit is meters

USGS 7.5 minutes DEM 

• Description: 7.5 minute (1:24,000-scale) tile digital  elevation data based on USGS topographic quads. This dataset is being superceded by USGS NED (seamless dataset).
• Coverage: AGSL archives most of WI 7.5-minutes DEM
• Spatial resolution/spacing: 1 arc second (30 meters)
• Dates: Varies
• Format: STDS (Spatial Data Transfer Standard)
• Projection: UTM Zone 15 or 16 (Horizontal datum - NAD27, Vertical datum -varies, either feet or meters; check accompanying metadata for the vertical datum information)

SRTM (Shuttle Radar Topography Mission) - World

Description: Digital elevation data developed by USGS and NASA.
Coverage: Global (land area) - AGSL also archives all available SRTM
Spatial resolution/spacing: 3 arc second (90 meters)
Dates:2000
Format: SRTM raster format (see our ArcGIS tip for converting SRTM to grid)
Projection: Geographic, decimal degrees projection, WGS 84 (image at left by Aaron Michelsen)

GTOPO30 - Global 30 Arc-second Elevation Dataset  

• Description: Seamless & best available public domain elevation data. Global1-km digital elevation model derived from a variety of sources.
• Coverage: Global 
• Spatial resolution/spacing: 30 arc second (roughly 1 km)
• Dates: N/A
• Format: STDS (Spatial Data Transfer Standard)
• Projection: Geographic, decimal degrees projection, WGS84
• Access constraints: None

For assistance in finding and obtaining DEMs, please send email to gisdata@uwm.edu. Or please check our Wisconsin data page to check data availability of the Wisconsin counties.

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Comments for the AGSL? Send them to agsl@uwm.edu.