Glossary of Terms

Compiled by users of the Turnkey_APRS list
(Please feed me more!)


APRS = Automatic Position Reporting System (Trademarked/Invented by Bob Bruninga, WB4APR). Some of the available APRS programs: Xastir, DosAPRS, WinAPRS, APRS+SA, MacAPRS, XAPRS, pocketAPRS, APRSCE, UI-View.

CAP = Civil Air Patrol.

Conic Projection = A map projection where the parallels of latitude are still parallel, but curved. The meridians are straight but not parallel, being closer together at the top (northern hemisphere). Set a cone on a transparent globe, with the tip on the earth's axis (north or south). The cone will touch the globe along a line of constant latitude. Adjust the cone so that it touches at your latitude. Turn on a light at the center of the globe and the surface features will project on the cone. The further away from your latitude, the greater the distortion. http://biology.usgs.gov/geotech/documents/datum.html

Datum = The mathematical model of the earth that your map data is based on. Very compact and relatively easy to use. As your models become more precise, the size of the description increases. Examples are: North American Datum of 1927 (NAD27), NAD83, World Geodetic System 1984 (WGS84). NAD83 and WGS84 are close enough to be called the same for non-geodetic applications like Search and Rescue (the difference is about 2 meters). USGS maps published after about 1992 use NAD83, those published in the 1980's use NAD27 with reference info for NAD83, and those published before 1980 are NAD27 without reference to NAD83. Aviation instruments and maps now use WGS84 and generally don't provide any alternatives. Since most topo maps used for SAR will be NAD27 for a long time (the USGS is not in any hurry!) we will need to be aware of the datum difference in our areas of interest. In general, the grid shifts east by about 100m, and varies little in any general area. Look at any 7.5' map published since 1980 and the difference will be noted, or there will be reference points near the map corners for the datum other than the one that map uses. http://biology.usgs.gov/geotech/documents/datum.html

DEM = Digital Elevation Model. A data file from the USGS containing only altitude points for a given area. Useful for drawing 3-D images of an area. The files are relatively small and available from several internet sites. http://rmmcweb.cr.usgs.gov/elevation/

DEM = Department of Emergency Management.

DLG = Digital Line Graph. A vector image map format from the USGS. Used for roads, water, power lines, railroad tracks, etc.

DRG = Digital Raster Graphic. http://mcmcweb.er.usgs.gov/drg/ The files are relatively large (10 to 30 Mb), and some are available from internet sites. Otherwise CD-ROM's can be purchased commercially. If the CD's use a format similar to the USGS, the maps cover one "square" degree, including 64 maps at 1:24,000, 2 maps at 1:100,000, and 1 map at 1:250,000 (covers double the area as the rest of the maps). Some of the one-degree sections require two CD's to hold the data (Seattle area is a good example). The file format is geoTIFF, which is listed in the TIFF 6.0 specification. These are USGS topo maps, scanned in from paper maps and then calibrated and geo-referenced. They look like normal TIFF files to programs that don't understand the geoTIFF tags. To programs that do understand the tags, the maps are automatically calibrated. http://www.map1.com/ $18 each, with coverate of NV, UT, CO, AZ, NM (there are a wide variety of prices, and some think that $50 is low, so look around!). On WE7U's main web page are links to sites where you can download the map files for free. Find someone with a CD-writer and burn your own CD's.

EOC = Emergency Operations Center.

MARS = Military Affiliate Radio System. http://www.asc.army.mil/mars/

Mercator Projection = A map projection where the meridians are parallel and the parallels of lattitude are straight lines whose distance from each other increases with distance from the equator. http://biology.usgs.gov/geotech/documents/datum.html

OziExplorer = Map program that understands DRG's files natively.

Raster Image = Bitmapped image. Every pixel must have a value. Slower to draw, larger files, and less scaleable than vector images.

SAR = Search And Rescue.

Topo = Topographic Map. A map showing contour intervals (lines of equal altitude). Common USGS maps and scales are:
7.5' covering 7.5' of latitude and longitude at a scale of 1:24,000 or 1"=2000'.
15' covering 15' of lat and long at a scale of 1:62,500 (1:63,360 in Alaska) or 1"=1 mile.
1:100,000 or 1"=4 miles covering 30' of latitude and 60' of longitude. Contours are in METERS!
1:250,000 or 1"=8 miles covering 1 degree of latitude and 2 degrees of longitude.
http://mcmcweb.er.usgs.gov/topomaps/

USGS = United States Geological Survey. They are responsible for much more than mapping. http://www.usgs.gov/

UTM = Universal Transverse Mercator. A grid system displayed on USGS topo maps, sometimes as a grid over the map, otherwise marked in the margin. The grid is in meters and displayed as a rectangular grid on USGS topo maps. The units increase towards the east (right) and north (up), measured from an imaginary point on the equator and far to the west (no negative numbers!). The grid is related to the military MGRS, with different terminology. http://biology.usgs.gov/geotech/documents/datum.html

Vector Image = An image created from vectors and points. Only those lines/points need be drawn. Typically much more compact than bitmapped images, faster to draw, and more scaleable.