Getting USGS Data

Free

• Most USGS data is available free, online, at USGS EarthExplorer
• You can also browse and directly download some raster data through the National Map Viewer and Download Platform
  • currently, this is limited to SRTM, NED (National Elevation Dataset), and some small areas of orthoimagery
• It helps to familiarize yourself first with all the kinds of data they provide, as well as units (see below).
• For many years, the web interfaces were painful, but as of 2012, they are actually quite usable.
• All DRGs are also online free at the Libre Map Project
• If you only need to get individual elevation points, the USGS Elevation Query Web Service return the elevation in feet or meters for a specific latitude/longitude (WGS 1984) point from the USGS Seamless Elevation datasets.
• LIDAR data are described at CLICK: the USGS Center for LIDAR Information Coordination and Knowledge

Quad Names

• Traditionally, most USGS data was on printed maps, 7.5 minutes by 7.5 minutes in size, and some of their digital data still follows this structure
• Each quadrilateral map ("Quad map") has a unique name, which is useful for locating and referencing the data
• Unfortunately, many download interfaces won't produce a map of these quads
• There used to be a USGS Map Finder until they took it offline; their new USGS Store doesn't yet have a geographic map locator
• There are data sources such as usgsquads KMZ 24k which can be viewed in Google Earth to quickly find a quad name, although their links all go to their company which sells the public-domain data

Arc vs. Meters

• The USGS likes to refer to spatial resolution (e.g. of elevation grids) in terms of degrees/minutes/seconds of arc, rather than meters or feet - even when the data itself is a grid in units of meters.  Here is the rough correspondence; keeping in mind that east-west meters decrease with lattitude: