What are DEM's, DRG's, and DOQQ's
DEM's are sampled arrays of elevation values representing ground positions at regularly spaced intervals. Digital Elevation Model (DEM) is the terminology adopted by the USGS to describe terrain elevation data sets in a digital raster form. The standard DEM consists of a regular array of elevations cast on a designated coordinate projection system. The DEM data are stored as a series of profiles in which the spacing of the elevations along and between each profile is in regular whole number intervals. The normal orientation of data is by columns and rows. Each column contains a series of elevations ordered from south to north with the order of the columns from west to east.
The DEM is formatted as one ASCII header record (A-record), followed by a series of profile records (B-records) each of which include a short B-record header followed by a series of ASCII integer elevations per each profile. The last physical record of the DEM is an accuracy record (C-record). 7.5-minute DEM (30- by 30-meter data spacing, cast on Universal Transverse Mercator (UTM) projection). Provides coverage in 7.5- by 7.5-minute blocks. Each product provides the same coverage as a standard USGS 7.5-minute quadrangle without over edge. Coverage is for the Contiguous United States, Hawaii, and Puerto Rico. 7.5-minute DEM's have rows and columns which vary in length and are staggered. The UTM bounding coordinates form a quadrilateral (no two sides are parallel to each other), rather than a rectangle. The user will need to pad out the uneven rows and columns with blanks or flagged data values, if a rectangle is required for the user's application. Some software vendors have incorporated this function into their software for input of standard formatted USGS DEMs.
Figure 1. USGS Digital Elevation Model (DEM)
For the conterminous U.S and Hawaii, primary series DEM's are prepared at two levels: Level 1-derived from manual profiling or image correlation of aerial photographic stereomodels; Level 2-derived from hypsography, either by digitizing USGS primary map contour separates or by stereomodel digitizing. The unit of coverage for DEM's in the conterminous U.S. is the 7.5-minute quadrangle. For Alaska, Level 2 DEM's are prepared from the hypsography overlays of the 1:63,360-scale Alaska primary series map, and the unit of DEM coverage is the Alaska primary series map. All DEM's adhere to the USGS NMPTI Standards for Digital Elevation Models.
Digital Raster Graphics are raster images of standard quadrangle format USGS topographic maps. A DRG is a raster image of a scanned USGS topographic or planimetric map including the collar information, georeferenced to the UTM grid. A DRG is useful as a source or background layer in a GIS as a means to perform quality assurance on other digital products, and as a source for the collection and revision of vector data.
Figure 2. USGS Digital Raster Graphic (DRG)
Within the 1:24,000 and 1:25,000 series are included Provisional (P) maps as well as Orthophotomaps. DRG's retain the geometric and visual qualities of the source maps in all respects with two exceptions; DRG's are referenced to the UTM grid, and the colors of the DRG may differ slightly from the original printed map. They serve a variety of purposes, from use as backgrounds in a geographic information system (GIS) to source for collecting or revising USGS base cartographic data. The DRG shows potential as a tool for validating and assessing other non-USGS digital data. DRG's can be combined with DOQ's, DEM's, and DLG's, or combinations of each to create several "hybrid" products, each with its own unique applications. DRG prototypes have been utilized as "browse images" on USGS data management systems, and may serve similar functions in production systems and sales databases. While a DRG can be made from any map, the DRG program is oriented primarily toward the production of digital raster graphics from 1:24,000 and 1:25,000 USGS topographic maps to be used in the validation and revision of digital line graph data. DRG's adhere to the USGS NMPTI Standards for Digital Raster Graphics.
A DOQQ is a digitized image of an aerial photograph, corrected for displacements caused by camera angle and relief. The result is a spatially accurate image with planimetric features appearing in their true geographic positions. Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map. The primary digital orthophoto quarter quad (DOQQ) is a 1-meter ground resolution, quarter-quadrangle (3.75-minutes of latitude by 3.75-minutes of longitude) image cast on the Universal Transverse Mercator Projection (UTM) on the North American Datum of 1983 (NAD83).
The geographic extent of the DOQQ is equivalent to a quarter-quad plus a slight amount over the quarter-quad boundary. The overedge ranges from a minimum of 50 meters to a maximum of 300 meters beyond the extremes of the primary and secondary corner points. The overedge is included to facilitate tonal matching for mosaicking and for the placement of the NAD83 and secondary datum corner ticks. The normal orientation of data is by lines (rows) and samples (columns). Each line contains a series of pixels ordered from west to east with the order of the lines from north to south. The standard, archived digital orthophoto is formatted as four ASCII header records, followed by a series of 8-bit binary image data records. The radiometric image brightness values are stored as 256 gray levels ranging from 0 to 255. The metadata provided in the digital orthophoto contain a wide range of descriptive information including format source information, production instrumentation and dates, and data to assist with displaying and georeferencing the image. The standard primary series USGS DOQQ is a quarter-quadrangle image (3.75- by 3.75-minutes). USGS DOQQ's are derived from photography meeting National Aerial Photography Program (NAPP) specifications, and meet National Map Accuracy Standards for 1:12,000-scale quarter quadrangles. DOQQ's adhere to the USGS NMPTI Standards for Digital Orthophotos.
Figure 3. USGS Digital Orthophoto Quarter Quadrangle (DOQQ)
This material is based on information excerpted from the USGS website.