GIS Frequently Asked Questions (FAQs)
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)?
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.
2. What is a Digital
Orthophoto Quadrangle (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)
3. What is a
Digital Line Graph (DLG) ?
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)
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
Last Revised: 17-Nov-2003