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  1. What is magnetic declination and what is the value for a particular spot in the world ?
2. What is a Digital Orthophoto Quadrangle (DOQ) ?
3. What is a Digital Line Graph (DLG) ?
4. What is a Digital Elevation Model (DEM) ?
5. What is a Digital Raster Graphic (DRG)?

1. What is magnetic declination and what is the value for a particular spot in the world ?

The earth's magnetic field has a dipole component that is roughly coincident with the spin axis. However, the north geographic pole (the spin axis) and the "north magnetic pole" are not found in the same place. This is because the Earth's magnetic field is a complex phenomenon, and the location where the inclination of the field is 90 degrees (the "poles") wander constantly due to fluctuations in the magnetic field.

A compass points in the directions of the horizontal component of the magnetic field where the compass is located, and not to any single point. Knowing the magnetic declination (angle between true north and the horizontal trace of the magnetic field) for your location allows you to correct your compass for the magnetic field in your area. A mile or two away the magnetic declination may be considerably different, requiring a different correction. The figure above illustrates the variation in declination around the world.

To calculate the current declination for a particular location on the Earth, use the convenient form from the National Geophysical Data Center.

For more information on geomagnetism, see the NGDC and their Frequently Asked Questions page.

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2. What is a Digital Orthophoto Quadrangle (DOQ)?
A Digital Orthophoto Quadrangle (DOQ) is a digital, uniform-scale image created from aerial photos. It is a photographic map in which ground features are displayed in their true ground position, because relief displacements caused by the camera and terrain of an aerial photograph have been removed. It combines the image characteristics of a photograph with the geometric qualities of a map, thus it is possible to get direct measurements of distances, areas, angles, and positions from a DOQ.

An aerial photograph and an orthophoto or orthoimage may look alike but there are several important differences that allow an orthophoto to be used like a map. A conventional perspective aerial photograph contains image distortions caused by the tilting of the camera and terrain relief (topography). It does not have a uniform scale. You cannot measure distances on an aerial photograph like you can on a map. An aerial photo is not a map. The effects of tilt and relief are removed from the aerial photograph by a mathematical process called rectification. An orthophoto is a uniform-scale image. Since an orthophoto has a uniform scale, it is possible to measure directly on it like other maps. An orthophoto may serve as a base map onto which other map information may be overlaid.

A DOQ can be used on-screen to collect, review, and revise other digital data, especially digital line graphs (DLG) and topographic maps. When the DOQ is combined with other digital products, such as digital raster graphics (DRG) or digital elevation models (DEM), the resulting image provides additional visual information for the extraction and revision of base cartographic information. (From USGS factsheet)


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3. What is a Digital Line Graph (DLG) ?


A Digital Line Graph (DLG) is digital vector data representing cartographic information. DLGs contain a wide variety of information depicting geographic features (for example, hypsography, hydrography, boundaries, roads, utility lines, etc). DLGs are derived from hypsographic data (contour lines) using USGS 7.5-minute, 15-minute, 2-arc-second (30- by 60-minute), and 1:2 million-scale topographic quadrangle maps. (From USGS factsheet)



4. What is a Digital Elevation Model (DEM) ?

A Digital Elevation Model (DEM) is digital cartographic/geographic data in raster form. The terrain elevations for ground positions are sampled at regularly spaced horizontal intervals. DEMs are derived from hypsographic data (contour lines) and/or photogrammetric methods using USGS 7.5-minute, 15-minute, 2-arc-second (30- by 60-minute), and 1-degree (1:250,000-scale) topographic quadrangle maps. (From USGS factsheet)



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5. What is a Digital Raster Graphic (DRG)?

A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) topographic map. The scanned image includes all map collar information. The image inside the map neatline is georeferenced to the surface of the Earth. The DRG can be used to collect, review, and revise other digital data, especially digital line graphs (DLG). When the DRG is combined with other digital products, such as digital orthophoto quadrangles (DOQ) or digital elevation models (DEM), the resulting image provides additional visual detail for the extraction and revision of base cartographic information. (From USGS Factsheet)


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